alpha perfomance fix

assets/embed.go

@@ -0,0 +1,8 @@
+package assets
+
+import _ "embed"
+
+// JetBrainsMono contains the font used for video text overlays.
+//
+//go:embed fonts/JetBrainsMonoNLNerdFontMono-Regular.ttf
+var JetBrainsMono []byte

cmd/mxl-pattern/main.go

@@ -5,7 +5,6 @@ package main
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -19,14 +18,15 @@ import (
 	"github.com/qvest-digital/go-mxl/mxl"
 	"github.com/spf13/pflag"
 
+	"mxl-pattern-generator/internal/app"
 	"mxl-pattern-generator/internal/audio"
 	"mxl-pattern-generator/internal/flowdef"
 	"mxl-pattern-generator/internal/video"
 )
 
 const (
-	APP_NAME = "MXL pattern generator"
+	APP_NAME = app.Name
-	APP_VER  = "0.1.0"
+	APP_VER  = app.Version
 )
 
 type appArgs struct {
@@ -43,10 +43,13 @@ type appArgs struct {
 	overlayY    int
 	overlayPos  string
 
-	videoWidth  uint
+	videoWidth   uint
-	videoHeight uint
+	videoHeight  uint
-	videoFPS    string
+	videoFPS     string
-	videoUUID   string
+	videoUUID    string
+	noVideo      bool
+	videoAlpha   bool
+	videoBackend string
 
 	audioChannels     uint8
 	audioSamplingFreq string
@@ -59,16 +62,6 @@ type parseResult struct {
 	shouldRun bool
 }
 
-type namedRunner struct {
-	name string
-	run  func(context.Context) error
-}
-
-type runnerResult struct {
-	name string
-	err  error
-}
-
 var frameRates = map[string]mxl.Rational{
 	"23.97": {Num: 24000, Den: 1001},
 	"24":    {Num: 24, Den: 1},
@@ -94,6 +87,7 @@ func printHelp(w io.Writer, fs *pflag.FlagSet) {
 	fmt.Fprintln(w, "Usage: mxl-gen -d <domain> [-v <flowDef.json>] [-a <flowDef.json>] [options]")
 	fmt.Fprintln(w, "   or: mxl-gen -d <domain> [--width <width px>] [--height <height px>] [--fps <framerate>] \\")
 	fmt.Fprintln(w, "   [-c <channels amount>] [-f <sample rate>]")
+	fmt.Fprintln(w, "   or: mxl-gen -d <domain> --no-video (-c <channels amount> | -a <flowDef.json>)")
 	fmt.Fprintln(w, "   or: mxl-gen -d <domain> with default params")
 	fmt.Fprintln(w, "Video and audio feed parameters are ignored when a flow definition file is provided.")
 	fmt.Fprintln(w)
@@ -102,7 +96,7 @@ func printHelp(w io.Writer, fs *pflag.FlagSet) {
 }
 
 func printUsage(w io.Writer) {
-	fmt.Fprintln(w, "Usage: mxl-gen -d <domain> [-v <flowDef.json>] [-a <flowDef.json>] [options]")
+	fmt.Fprintln(w, "Usage: mxl-gen -d <domain> [--no-video] [-v <flowDef.json>] [-a <flowDef.json>] [options]")
 	fmt.Fprintln(w, "Try 'mxl-gen -h' for more information.")
 }
 
@@ -141,15 +135,20 @@ func validateFlowDefPath(label, path string) error {
 }
 
 func validateVideoArgs(args *appArgs) error {
+	if args.noVideo {
+		return nil
+	}
+	if _, err := parseVideoBackend(args.videoBackend); err != nil {
+		return err
+	}
 	if !video.HasPattern(args.pattern) {
 		return fmt.Errorf("unknown video pattern %q (use --list-patterns to see available patterns)", args.pattern)
 	}
 	if args.videoFlowDefFile != "" {
 		return nil
 	}
-	if args.videoWidth == 0 || args.videoWidth%6 != 0 {
+	if args.videoWidth == 0 || args.videoWidth%2 != 0 {
-		// v210 stores 6 pixels in each 16-byte block.
+		return fmt.Errorf("video width must be greater than zero and even for 4:2:2 video")
-		return fmt.Errorf("video width must be greater than zero and divisible by 6")
 	}
 	if args.videoHeight == 0 {
 		return fmt.Errorf("video height must be greater than zero")
@@ -168,6 +167,22 @@ func validateVideoArgs(args *appArgs) error {
 	return nil
 }
 
+func validateMediaSelection(args appArgs) error {
+	if args.noVideo && args.videoFlowDefFile != "" {
+		return fmt.Errorf("--no-video cannot be used with --video")
+	}
+	if args.noVideo && args.audioFlowDefFile == "" && args.audioChannels == 0 {
+		return fmt.Errorf("--no-video requires audio enabled with --channel or --audio")
+	}
+	if args.videoAlpha && args.noVideo {
+		return fmt.Errorf("--alpha cannot be used with --no-video")
+	}
+	if args.videoAlpha && args.videoFlowDefFile != "" {
+		return fmt.Errorf("--alpha cannot be used with --video; set media_type to %q in the flow definition", flowdef.MediaTypeV210A)
+	}
+	return nil
+}
+
 func validateAudioArgs(args *appArgs) error {
 	if args.audioFlowDefFile == "" && args.audioChannels == 0 {
 		return nil
@@ -199,6 +214,7 @@ func validateAudioArgs(args *appArgs) error {
 func validateArgs(args *appArgs) error {
 	checks := []func() error{
 		func() error { return validateDomain(args.domain) },
+		func() error { return validateMediaSelection(*args) },
 		func() error { return validateFlowDefPath("video", args.videoFlowDefFile) },
 		func() error { return validateFlowDefPath("audio", args.audioFlowDefFile) },
 		func() error { return validateVideoArgs(args) },
@@ -273,10 +289,13 @@ func addFlags(fs *pflag.FlagSet, args *appArgs) {
 			"br - bottom-right corner",
 	)
 
-	fs.UintVar(&args.videoWidth, "width", 1920, "Video pattern width. Zero = no video [TODO: 0 width case]")
+	fs.UintVar(&args.videoWidth, "width", 1920, "Video pattern width")
 	fs.UintVar(&args.videoHeight, "height", 1080, "Video pattern height")
 	fs.StringVar(&args.videoFPS, "fps", "25", "Video pattern FPS")
 	fs.StringVar(&args.videoUUID, "video-id", "", "Video UUID. Will be created, if not provided")
+	fs.BoolVar(&args.noVideo, "no-video", false, "Disable video generation; audio must be enabled")
+	fs.BoolVar(&args.videoAlpha, "alpha", false, "Generate video/v210a with a moving transparent square")
+	fs.StringVar(&args.videoBackend, "backend", string(video.BackendAuto), "Video generator backend: auto, gpu or cpu")
 	// Audio pattern flags
 	fs.Uint8VarP(&args.audioChannels, "channel", "c", 0, "Amount of audio channels. Each channel: num * 1kHz")
 	fs.StringVarP(&args.audioSamplingFreq, "freq", "f", "48", "Sampling frequency of test audio feed in kHz")
@@ -312,21 +331,41 @@ func parseArgs(argv []string, stdout, stderr io.Writer) (parseResult, error) {
 	return parseResult{args: args, shouldRun: true}, nil
 }
 
-func buildVideoConfig(args appArgs) (video.Config, error) {
+func parseVideoBackend(value string) (video.Backend, error) {
+	if value == "" {
+		return video.BackendAuto, nil
+	}
+	backend := video.Backend(value)
+	switch backend {
+	case video.BackendAuto, video.BackendGPU, video.BackendCPU:
+		return backend, nil
+	default:
+		return "", fmt.Errorf(
+			"unsupported video backend %q (supported: auto, gpu, cpu)",
+			value,
+		)
+	}
+}
+
+func buildVideoConfig(args appArgs) (*video.Config, error) {
+	if args.noVideo {
+		return nil, nil
+	}
+
 	var definition flowdef.Video
 	if args.videoFlowDefFile != "" {
 		data, err := os.ReadFile(args.videoFlowDefFile)
 		if err != nil {
-			return video.Config{}, fmt.Errorf(
+			return nil, fmt.Errorf(
 				"read video flow definition %q: %w",
 				args.videoFlowDefFile,
 				err,
 			)
 		}
 
-		definition, err = flowdef.ParseV210Video(data)
+		definition, err = flowdef.ParseVideo(data)
 		if err != nil {
-			return video.Config{}, fmt.Errorf(
+			return nil, fmt.Errorf(
 				"parse video flow definition %q: %w",
 				args.videoFlowDefFile,
 				err,
@@ -335,11 +374,15 @@ func buildVideoConfig(args appArgs) (video.Config, error) {
 	} else {
 		rate, ok := frameRates[args.videoFPS]
 		if !ok {
-			return video.Config{}, fmt.Errorf("unsupported video FPS %q", args.videoFPS)
+			return nil, fmt.Errorf("unsupported video FPS %q", args.videoFPS)
 		}
 
 		var err error
-		definition, err = flowdef.NewV210Video(
+		newVideo := flowdef.NewV210Video
+		if args.videoAlpha {
+			newVideo = flowdef.NewV210AVideo
+		}
+		definition, err = newVideo(
 			args.videoUUID,
 			args.videoWidth,
 			args.videoHeight,
@@ -349,17 +392,22 @@ func buildVideoConfig(args appArgs) (video.Config, error) {
 			},
 		)
 		if err != nil {
-			return video.Config{}, fmt.Errorf(
+			return nil, fmt.Errorf(
 				"build video flow definition: %w",
 				err,
 			)
 		}
 	}
 	if !video.HasPattern(args.pattern) {
-		return video.Config{}, fmt.Errorf("unknown video pattern %q", args.pattern)
+		return nil, fmt.Errorf("unknown video pattern %q", args.pattern)
 	}
 
-	return video.Config{
+	backend, err := parseVideoBackend(args.videoBackend)
+	if err != nil {
+		return nil, err
+	}
+
+	return &video.Config{
 		Definition: definition,
 		Pattern:    args.pattern,
 		Overlay: video.OverlayConfig{
@@ -368,6 +416,7 @@ func buildVideoConfig(args appArgs) (video.Config, error) {
 			Y:        args.overlayY,
 			Position: args.overlayPos,
 		},
+		Backend: backend,
 	}, nil
 }
 
@@ -433,7 +482,7 @@ func main() {
 	}
 }
 
-func run(ctx context.Context, args appArgs) (runErr error) {
+func run(ctx context.Context, args appArgs) error {
 	videoCfg, err := buildVideoConfig(args)
 	if err != nil {
 		return fmt.Errorf("video configuration: %w", err)
@@ -443,69 +492,9 @@ func run(ctx context.Context, args appArgs) (runErr error) {
 		return fmt.Errorf("audio configuration: %w", err)
 	}
 
-	log.Printf("%s %s", APP_NAME, APP_VER)
+	return app.Run(ctx, app.Config{
-	log.Printf("Domain: %s", args.domain)
+		Domain: args.domain,
-	log.Printf("Video: %dx%d %d/%d",
+		Video:  videoCfg,
-		videoCfg.Width(), videoCfg.Height(), videoCfg.Rate().Num, videoCfg.Rate().Den)
+		Audio:  audioCfg,
-	log.Printf("Video ID: %s", videoCfg.ID())
+	})
-	if audioCfg != nil {
-		log.Printf("Audio: %d channels %d/%d Hz %.0f dBFS",
-			audioCfg.Channels(), audioCfg.Rate().Num, audioCfg.Rate().Den, audioCfg.LevelDBFS)
-		log.Printf("Audio ID: %s", audioCfg.ID())
-	}
-
-	inst, err := mxl.NewInstance(args.domain, "")
-	if err != nil {
-		return fmt.Errorf("initialize MXL domain %q: %w", args.domain, err)
-	}
-	defer func() {
-		if err := inst.Close(); err != nil {
-			runErr = errors.Join(runErr, fmt.Errorf("close MXL instance: %w", err))
-		}
-	}()
-
-	runners := []namedRunner{
-		{
-			name: "video",
-			run: func(ctx context.Context) error {
-				return video.Run(ctx, inst, videoCfg)
-			},
-		},
-	}
-	if audioCfg != nil {
-		runners = append(runners, namedRunner{
-			name: "audio",
-			run: func(ctx context.Context) error {
-				return audio.Run(ctx, inst, *audioCfg)
-			},
-		})
-	}
-
-	return runConcurrent(ctx, runners...)
-}
-
-func runConcurrent(ctx context.Context, runners ...namedRunner) error {
-	if len(runners) == 0 {
-		return nil
-	}
-
-	ctx, cancel := context.WithCancel(ctx)
-	defer cancel()
-	results := make(chan runnerResult, len(runners))
-	for _, runner := range runners {
-		runner := runner
-		go func() {
-			results <- runnerResult{name: runner.name, err: runner.run(ctx)}
-		}()
-	}
-
-	var resultErr error
-	for range runners {
-		result := <-results
-		if result.err != nil {
-			resultErr = errors.Join(resultErr, fmt.Errorf("%s flow: %w", result.name, result.err))
-			cancel()
-		}
-	}
-	return resultErr
 }

cmd/mxl-pattern/main_test.go

@@ -2,18 +2,30 @@ package main
 
 import (
 	"bytes"
-	"context"
 	"encoding/json"
-	"errors"
 	"os"
 	"strings"
 	"testing"
-	"time"
 
 	"mxl-pattern-generator/internal/audio"
 	"mxl-pattern-generator/internal/flowdef"
+	"mxl-pattern-generator/internal/video"
+
+	"github.com/spf13/pflag"
 )
 
+func TestAlphaFlag(t *testing.T) {
+	var args appArgs
+	flags := pflag.NewFlagSet("test", pflag.ContinueOnError)
+	addFlags(flags, &args)
+	if err := flags.Parse([]string{"--alpha"}); err != nil {
+		t.Fatalf("Parse: %v", err)
+	}
+	if !args.videoAlpha {
+		t.Fatal("videoAlpha = false, want true")
+	}
+}
+
 func TestParseArgsHelpStopsBeforeValidation(t *testing.T) {
 	var stdout, stderr bytes.Buffer
 
@@ -98,6 +110,119 @@ func TestValidateAudioArgsSkipsDisabledAudio(t *testing.T) {
 	}
 }
 
+func TestValidateVideoArgsSkipsDisabledVideo(t *testing.T) {
+	args := appArgs{
+		noVideo:    true,
+		pattern:    "not-a-pattern",
+		videoWidth: 1,
+		videoFPS:   "unsupported",
+		videoUUID:  "not-a-uuid",
+	}
+
+	if err := validateVideoArgs(&args); err != nil {
+		t.Fatalf("validateVideoArgs: %v", err)
+	}
+	if args.videoUUID != "not-a-uuid" {
+		t.Fatalf("video UUID changed while video is disabled: %q", args.videoUUID)
+	}
+}
+
+func TestParseVideoBackend(t *testing.T) {
+	tests := []struct {
+		name    string
+		value   string
+		want    video.Backend
+		wantErr bool
+	}{
+		{name: "zero value defaults to auto", want: video.BackendAuto},
+		{name: "auto", value: "auto", want: video.BackendAuto},
+		{name: "gpu", value: "gpu", want: video.BackendGPU},
+		{name: "cpu", value: "cpu", want: video.BackendCPU},
+		{name: "unknown", value: "other", wantErr: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got, err := parseVideoBackend(tc.value)
+			if tc.wantErr {
+				if err == nil || !strings.Contains(err.Error(), "unsupported video backend") {
+					t.Fatalf("error = %v, want unsupported backend error", err)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("parseVideoBackend: %v", err)
+			}
+			if got != tc.want {
+				t.Fatalf("backend = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestValidateVideoArgsRejectsUnknownBackend(t *testing.T) {
+	args := appArgs{videoBackend: "other"}
+	if err := validateVideoArgs(&args); err == nil || !strings.Contains(err.Error(), "unsupported video backend") {
+		t.Fatalf("error = %v, want unsupported backend error", err)
+	}
+}
+
+func TestValidateMediaSelection(t *testing.T) {
+	tests := []struct {
+		name       string
+		args       appArgs
+		wantErrSub string
+	}{
+		{
+			name:       "no video with video definition",
+			args:       appArgs{noVideo: true, videoFlowDefFile: "video.json", audioChannels: 2},
+			wantErrSub: "cannot be used with --video",
+		},
+		{
+			name:       "no flows",
+			args:       appArgs{noVideo: true},
+			wantErrSub: "requires audio enabled",
+		},
+		{
+			name: "generated audio only",
+			args: appArgs{noVideo: true, audioChannels: 2},
+		},
+		{
+			name: "external audio only",
+			args: appArgs{noVideo: true, audioFlowDefFile: "audio.json"},
+		},
+		{
+			name: "video enabled by default",
+			args: appArgs{},
+		},
+		{
+			name:       "alpha without video",
+			args:       appArgs{noVideo: true, videoAlpha: true, audioChannels: 2},
+			wantErrSub: "cannot be used with --no-video",
+		},
+		{
+			name:       "alpha with custom video definition",
+			args:       appArgs{videoAlpha: true, videoFlowDefFile: "video.json"},
+			wantErrSub: "cannot be used with --video",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			err := validateMediaSelection(tc.args)
+			if tc.wantErrSub == "" {
+				if err != nil {
+					t.Fatalf("validateMediaSelection: %v", err)
+				}
+				return
+			}
+			if err == nil || !strings.Contains(err.Error(), tc.wantErrSub) {
+				t.Fatalf("error = %v, want substring %q", err, tc.wantErrSub)
+			}
+		})
+	}
+}
+
 func TestListPatternsIsSorted(t *testing.T) {
 	var output bytes.Buffer
 	listPatterns(&output)
@@ -127,6 +252,60 @@ func TestBuildVideoConfigFromArgs(t *testing.T) {
 	if cfg.Rate().Num != 30000 || cfg.Rate().Den != 1001 {
 		t.Fatalf("rate = %d/%d, want 30000/1001", cfg.Rate().Num, cfg.Rate().Den)
 	}
+	if cfg.Backend != video.BackendAuto {
+		t.Fatalf("backend = %q, want %q", cfg.Backend, video.BackendAuto)
+	}
+}
+
+func TestBuildVideoConfigWithAlpha(t *testing.T) {
+	cfg, err := buildVideoConfig(appArgs{
+		videoUUID:   "5fbec3b1-1b0f-417d-9059-8b94a47197ed",
+		videoWidth:  1280,
+		videoHeight: 720,
+		videoFPS:    "50",
+		pattern:     "gray-ramp",
+		videoAlpha:  true,
+	})
+	if err != nil {
+		t.Fatalf("buildVideoConfig: %v", err)
+	}
+	if cfg.Definition.MediaType != flowdef.MediaTypeV210A {
+		t.Fatalf("media type = %q, want %q", cfg.Definition.MediaType, flowdef.MediaTypeV210A)
+	}
+	if !cfg.HasAlpha() {
+		t.Fatal("HasAlpha() = false, want true")
+	}
+}
+
+func TestBuildVideoConfigBackends(t *testing.T) {
+	for _, backend := range []video.Backend{video.BackendGPU, video.BackendCPU} {
+		t.Run(string(backend), func(t *testing.T) {
+			cfg, err := buildVideoConfig(appArgs{
+				videoUUID:    "5fbec3b1-1b0f-417d-9059-8b94a47197ed",
+				videoWidth:   1920,
+				videoHeight:  1080,
+				videoFPS:     "25",
+				pattern:      "ebu75",
+				videoBackend: string(backend),
+			})
+			if err != nil {
+				t.Fatalf("buildVideoConfig: %v", err)
+			}
+			if cfg.Backend != backend {
+				t.Fatalf("backend = %q, want %q", cfg.Backend, backend)
+			}
+		})
+	}
+}
+
+func TestBuildVideoConfigDisabled(t *testing.T) {
+	cfg, err := buildVideoConfig(appArgs{noVideo: true})
+	if err != nil {
+		t.Fatalf("buildVideoConfig: %v", err)
+	}
+	if cfg != nil {
+		t.Fatalf("config = %+v, want nil for disabled video", cfg)
+	}
 }
 
 func TestBuildVideoConfigFromFile(t *testing.T) {
@@ -253,67 +432,3 @@ func TestValidateAudioArgsRejectsUnknownLevelForFlowDefinition(t *testing.T) {
 		t.Fatalf("error = %v, want unsupported audio level error", err)
 	}
 }
-
-func TestRunConcurrentCancelsSiblingAndWaitsForCleanup(t *testing.T) {
-	wantErr := errors.New("writer failed")
-	peerStarted := make(chan struct{})
-	peerStopped := make(chan struct{})
-
-	err := runConcurrent(context.Background(),
-		namedRunner{
-			name: "video",
-			run: func(ctx context.Context) error {
-				<-peerStarted
-				return wantErr
-			},
-		},
-		namedRunner{
-			name: "audio",
-			run: func(ctx context.Context) error {
-				close(peerStarted)
-				<-ctx.Done()
-				close(peerStopped)
-				return nil
-			},
-		},
-	)
-
-	if !errors.Is(err, wantErr) {
-		t.Fatalf("error = %v, want wrapped %v", err, wantErr)
-	}
-	if !strings.Contains(err.Error(), "video flow") {
-		t.Fatalf("error = %q, want runner name", err)
-	}
-	select {
-	case <-peerStopped:
-	default:
-		t.Fatal("runConcurrent returned before the sibling completed cleanup")
-	}
-}
-
-func TestRunConcurrentParentCancellationIsGraceful(t *testing.T) {
-	ctx, cancel := context.WithCancel(context.Background())
-	started := make(chan struct{})
-	done := make(chan error, 1)
-	go func() {
-		done <- runConcurrent(ctx, namedRunner{
-			name: "video",
-			run: func(ctx context.Context) error {
-				close(started)
-				<-ctx.Done()
-				return nil
-			},
-		})
-	}()
-
-	<-started
-	cancel()
-	select {
-	case err := <-done:
-		if err != nil {
-			t.Fatalf("runConcurrent: %v", err)
-		}
-	case <-time.After(time.Second):
-		t.Fatal("runConcurrent did not stop after parent cancellation")
-	}
-}

internal/app/app.go

@@ -0,0 +1,79 @@
+package app
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+
+	"mxl-pattern-generator/internal/audio"
+	"mxl-pattern-generator/internal/video"
+
+	"github.com/qvest-digital/go-mxl/mxl"
+)
+
+const (
+	Name    = "MXL pattern generator"
+	Version = "0.2.0"
+)
+
+type Config struct {
+	Domain string
+	Video  *video.Config
+	Audio  *audio.Config
+}
+
+func Run(ctx context.Context, cfg Config) (runErr error) {
+	if err := validateConfig(cfg); err != nil {
+		return err
+	}
+
+	log.Printf("%s %s", Name, Version)
+	log.Printf("Domain: %s", cfg.Domain)
+	if cfg.Video != nil {
+		log.Printf("Video: %dx%d %d/%d",
+			cfg.Video.Width(), cfg.Video.Height(), cfg.Video.Rate().Num, cfg.Video.Rate().Den)
+		log.Printf("Video ID: %s", cfg.Video.ID())
+	}
+	if cfg.Audio != nil {
+		log.Printf("Audio: %d channels %d/%d Hz %.0f dBFS",
+			cfg.Audio.Channels(), cfg.Audio.Rate().Num, cfg.Audio.Rate().Den, cfg.Audio.LevelDBFS)
+		log.Printf("Audio ID: %s", cfg.Audio.ID())
+	}
+	inst, err := mxl.NewInstance(cfg.Domain, "")
+	if err != nil {
+		return fmt.Errorf("initialize MXL domain %q: %w", cfg.Domain, err)
+	}
+	defer func() {
+		if err := inst.Close(); err != nil {
+			runErr = errors.Join(runErr, fmt.Errorf("close MXL instance: %w", err))
+		}
+	}()
+
+	runners := make([]Runner, 0, 2)
+	if cfg.Video != nil {
+		runners = append(runners, Runner{
+			Name: "video",
+			Run: func(ctx context.Context) error {
+				return video.Run(ctx, inst, *cfg.Video)
+			},
+		})
+	}
+	if cfg.Audio != nil {
+		runners = append(runners, Runner{
+			Name: "audio",
+			Run: func(ctx context.Context) error {
+				return audio.Run(ctx, inst, *cfg.Audio)
+			},
+		})
+	}
+
+	return RunConcurrent(ctx, runners...)
+}
+
+func validateConfig(cfg Config) error {
+	if cfg.Video == nil && cfg.Audio == nil {
+		return fmt.Errorf("at least one media flow must be enabled")
+	}
+	return nil
+}

internal/app/app_test.go

@@ -0,0 +1,15 @@
+package app
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestValidateConfigRejectsNoFlows(t *testing.T) {
+	err := validateConfig(Config{
+		Domain: "/unused",
+	})
+	if err == nil || !strings.Contains(err.Error(), "at least one media flow") {
+		t.Fatalf("error = %v, want no-flow validation error", err)
+	}
+}

internal/app/runner.go

@@ -0,0 +1,43 @@
+package app
+
+import (
+	"context"
+	"errors"
+	"fmt"
+)
+
+type Runner struct {
+	Name string
+	Run  func(context.Context) error
+}
+
+type runnerResult struct {
+	name string
+	err  error
+}
+
+func RunConcurrent(ctx context.Context, runners ...Runner) error {
+	if len(runners) == 0 {
+		return nil
+	}
+
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	results := make(chan runnerResult, len(runners))
+	for _, runner := range runners {
+		runner := runner
+		go func() {
+			results <- runnerResult{name: runner.Name, err: runner.Run(ctx)}
+		}()
+	}
+
+	var resultErr error
+	for range runners {
+		result := <-results
+		if result.err != nil {
+			resultErr = errors.Join(resultErr, fmt.Errorf("%s flow: %w", result.name, result.err))
+			cancel()
+		}
+	}
+	return resultErr
+}

internal/app/runner_test.go

@@ -0,0 +1,74 @@
+package app_test
+
+import (
+	"context"
+	"errors"
+	"mxl-pattern-generator/internal/app"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestRunConcurrentCancelsSiblingAndWaitsForCleanup(t *testing.T) {
+	wantErr := errors.New("writer failed")
+	peerStarted := make(chan struct{})
+	peerStopped := make(chan struct{})
+
+	err := app.RunConcurrent(context.Background(),
+		app.Runner{
+			Name: "video",
+			Run: func(ctx context.Context) error {
+				<-peerStarted
+				return wantErr
+			},
+		},
+		app.Runner{
+			Name: "audio",
+			Run: func(ctx context.Context) error {
+				close(peerStarted)
+				<-ctx.Done()
+				close(peerStopped)
+				return nil
+			},
+		},
+	)
+
+	if !errors.Is(err, wantErr) {
+		t.Fatalf("error = %v, want wrapped %v", err, wantErr)
+	}
+	if !strings.Contains(err.Error(), "video flow") {
+		t.Fatalf("error = %q, want runner name", err)
+	}
+	select {
+	case <-peerStopped:
+	default:
+		t.Fatal("runConcurrent returned before the sibling completed cleanup")
+	}
+}
+
+func TestRunConcurrentParentCancellationIsGraceful(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	started := make(chan struct{})
+	done := make(chan error, 1)
+	go func() {
+		done <- app.RunConcurrent(ctx, app.Runner{
+			Name: "video",
+			Run: func(ctx context.Context) error {
+				close(started)
+				<-ctx.Done()
+				return nil
+			},
+		})
+	}()
+
+	<-started
+	cancel()
+	select {
+	case err := <-done:
+		if err != nil {
+			t.Fatalf("runConcurrent: %v", err)
+		}
+	case <-time.After(time.Second):
+		t.Fatal("runConcurrent did not stop after parent cancellation")
+	}
+}

internal/flowdef/flowdef.go

@@ -12,6 +12,7 @@ const (
 	FormatVideo      = "urn:x-nmos:format:video"
 	FormatAudio      = "urn:x-nmos:format:audio"
 	MediaTypeV210    = "video/v210"
+	MediaTypeV210A   = "video/v210a"
 	MediaTypeFloat32 = "audio/float32"
 
 	InterlaceProgressive = "progressive"
@@ -75,7 +76,12 @@ type VideoComponent struct {
 	BitDepth uint   `json:"bit_depth"`
 }
 
-func NewV210Video(id string, width, height uint, rate Rational) (Video, error) {
+func newVideo(
+	id string,
+	width, height uint,
+	rate Rational,
+	mediaType string,
+) (Video, error) {
 	definition := Video{
 		Common: Common{
 			Description: "go-mxl-pattern-gen generated video",
@@ -86,7 +92,7 @@ func NewV210Video(id string, width, height uint, rate Rational) (Video, error) {
 			Format:    FormatVideo,
 			Label:     "go-mxl-pattern-gen generated video",
 			Parents:   []string{},
-			MediaType: MediaTypeV210,
+			MediaType: mediaType,
 		},
 		GrainRate:     rate,
 		FrameWidth:    width,
@@ -105,7 +111,15 @@ func NewV210Video(id string, width, height uint, rate Rational) (Video, error) {
 	return definition, nil
 }
 
-func ParseV210Video(data []byte) (Video, error) {
+func NewV210Video(id string, width, height uint, rate Rational) (Video, error) {
+	return newVideo(id, width, height, rate, MediaTypeV210)
+}
+
+func NewV210AVideo(id string, width, height uint, rate Rational) (Video, error) {
+	return newVideo(id, width, height, rate, MediaTypeV210A)
+}
+
+func ParseVideo(data []byte) (Video, error) {
 	var definition Video
 	if err := json.Unmarshal(data, &definition); err != nil {
 		return Video{}, fmt.Errorf("decode video flow definition: %w", err)
@@ -123,8 +137,10 @@ func (v Video) Validate() error {
 	if v.Format != FormatVideo {
 		return fmt.Errorf("format must be %q, got %q", FormatVideo, v.Format)
 	}
-	if v.MediaType != MediaTypeV210 {
+	switch v.MediaType {
-		return fmt.Errorf("media_type must be %q, got %q", MediaTypeV210, v.MediaType)
+	case MediaTypeV210, MediaTypeV210A:
+	default:
+		return fmt.Errorf("media_type must be %q or %q, got %q", MediaTypeV210, MediaTypeV210A, v.MediaType)
 	}
 	if v.InterlaceMode != InterlaceProgressive {
 		return fmt.Errorf("interlace_mode must be %q, got %q", InterlaceProgressive, v.InterlaceMode)
@@ -132,8 +148,8 @@ func (v Video) Validate() error {
 	if v.ColorSpace != ColorSpaceBT709 {
 		return fmt.Errorf("colorspace must be %q, got %q", ColorSpaceBT709, v.ColorSpace)
 	}
-	if v.FrameWidth == 0 || v.FrameWidth%6 != 0 {
+	if v.FrameWidth == 0 || v.FrameWidth%2 != 0 {
-		return fmt.Errorf("frame_width must be greater than zero and divisible by 6, got %d", v.FrameWidth)
+		return fmt.Errorf("frame_width must be greater than zero and even for 4:2:2 video, got %d", v.FrameWidth)
 	}
 	if v.FrameHeight == 0 {
 		return fmt.Errorf("frame_height must be greater than zero")

internal/flowdef/flowdef_test.go

@@ -28,14 +28,37 @@ func TestNewV210Video(t *testing.T) {
 	}
 }
 
-func TestNewV210VideoRejectsInvalidWidth(t *testing.T) {
+func TestNewV210AVideo(t *testing.T) {
-	_, err := NewV210Video(testVideoID, 1919, 1080, Rational{Numerator: 25, Denominator: 1})
+	definition, err := NewV210AVideo(testVideoID, 1280, 720, Rational{Numerator: 50, Denominator: 1})
-	if err == nil || !strings.Contains(err.Error(), "divisible by 6") {
+	if err != nil {
-		t.Fatalf("error = %v, want width divisibility error", err)
+		t.Fatalf("NewV210AVideo: %v", err)
+	}
+	if definition.MediaType != MediaTypeV210A {
+		t.Fatalf("media type = %q, want %q", definition.MediaType, MediaTypeV210A)
+	}
+	wantComponents := []VideoComponent{
+		{Name: "Y", Width: 1280, Height: 720, BitDepth: 10},
+		{Name: "Cb", Width: 640, Height: 720, BitDepth: 10},
+		{Name: "Cr", Width: 640, Height: 720, BitDepth: 10},
+	}
+	if len(definition.Components) != len(wantComponents) {
+		t.Fatalf("component count = %d, want %d", len(definition.Components), len(wantComponents))
+	}
+	for i, want := range wantComponents {
+		if definition.Components[i] != want {
+			t.Fatalf("component %d = %+v, want %+v", i, definition.Components[i], want)
+		}
 	}
 }
 
-func TestParseV210Video(t *testing.T) {
+func TestNewV210VideoRejectsOddWidth(t *testing.T) {
+	_, err := NewV210Video(testVideoID, 1919, 1080, Rational{Numerator: 25, Denominator: 1})
+	if err == nil || !strings.Contains(err.Error(), "even") {
+		t.Fatalf("error = %v, want even-width error", err)
+	}
+}
+
+func TestParseVideo(t *testing.T) {
 	want, err := NewV210Video(testVideoID, 1920, 1080, Rational{Numerator: 25, Denominator: 1})
 	if err != nil {
 		t.Fatalf("NewV210Video: %v", err)
@@ -45,28 +68,58 @@ func TestParseV210Video(t *testing.T) {
 		t.Fatalf("json.Marshal: %v", err)
 	}
 
-	got, err := ParseV210Video(data)
+	got, err := ParseVideo(data)
 	if err != nil {
-		t.Fatalf("ParseV210Video: %v", err)
+		t.Fatalf("ParseVideo: %v", err)
 	}
 	if got.ID != want.ID || got.FrameWidth != want.FrameWidth || got.GrainRate != want.GrainRate {
 		t.Fatalf("parsed definition = %+v, want %+v", got, want)
 	}
 }
 
-func TestParseV210VideoRejectsAudio(t *testing.T) {
+func TestParseVideoAcceptsV210A(t *testing.T) {
+	want, err := NewV210AVideo(testVideoID, 1920, 1080, Rational{Numerator: 25, Denominator: 1})
+	if err != nil {
+		t.Fatalf("NewV210AVideo: %v", err)
+	}
+	data, err := json.Marshal(want)
+	if err != nil {
+		t.Fatalf("json.Marshal: %v", err)
+	}
+
+	got, err := ParseVideo(data)
+	if err != nil {
+		t.Fatalf("ParseVideo: %v", err)
+	}
+	if got.MediaType != MediaTypeV210A {
+		t.Fatalf("media type = %q, want %q", got.MediaType, MediaTypeV210A)
+	}
+}
+
+func TestParseVideoRejectsAudio(t *testing.T) {
 	data := []byte(`{
 		"id":"5fbec3b1-1b0f-417d-9059-8b94a47197ed",
 		"format":"urn:x-nmos:format:audio",
 		"media_type":"audio/float32"
 	}`)
 
-	_, err := ParseV210Video(data)
+	_, err := ParseVideo(data)
 	if err == nil || !strings.Contains(err.Error(), "format must be") {
 		t.Fatalf("error = %v, want video format error", err)
 	}
 }
 
+func TestVideoRejectsUnknownMediaType(t *testing.T) {
+	definition, err := NewV210Video(testVideoID, 1920, 1080, Rational{Numerator: 25, Denominator: 1})
+	if err != nil {
+		t.Fatalf("NewV210Video: %v", err)
+	}
+	definition.MediaType = "video/unknown"
+	if err := definition.Validate(); err == nil || !strings.Contains(err.Error(), "media_type") {
+		t.Fatalf("error = %v, want media_type error", err)
+	}
+}
+
 func TestNewFloat32Audio(t *testing.T) {
 	definition, err := NewFloat32Audio(testAudioID, 2, Rational{Numerator: 48000, Denominator: 1})
 	if err != nil {

internal/generator/alpha.go

@@ -0,0 +1,92 @@
+package generator
+
+import (
+	"encoding/binary"
+	"fmt"
+	"math"
+)
+
+const (
+	alphaTransparent uint32 = 64
+	alphaOpaque      uint32 = 940
+)
+
+func packAlphaBlock(dst []byte, samples [3]uint32) {
+	const mask uint32 = 0x3ff
+
+	word := samples[0]&mask |
+		(samples[1]&mask)<<10 |
+		(samples[2]&mask)<<20
+
+	binary.LittleEndian.PutUint32(dst, word)
+}
+
+func fillAlphaPlane(
+	dst []byte,
+	width, height int,
+	value uint32,
+) error {
+	need := AlphaFrameSize(width, height)
+	if len(dst) < need {
+		return fmt.Errorf(
+			"alpha: destination is too small: got %d bytes, need %d",
+			len(dst),
+			need,
+		)
+	}
+
+	stride := AlphaLineSize(width)
+	for y := 0; y < height; y++ {
+		row := dst[y*stride : (y+1)*stride]
+		for x := 0; x < width; x += 3 {
+			var samples [3]uint32
+			for i := range samples {
+				if x+i < width {
+					samples[i] = value
+				}
+			}
+			packAlphaBlock(row[x/3*4:], samples)
+		}
+	}
+	return nil
+}
+
+func patchAlphaMovingSquare(dst []byte, width, height, frameIndex int) error {
+	need := AlphaFrameSize(width, height)
+	if len(dst) < need {
+		return fmt.Errorf(
+			"alpha: destination is too small: got %d bytes, need %d",
+			len(dst),
+			need,
+		)
+	}
+
+	bounds := movingSquareBounds(width, height, frameIndex)
+	firstPixelX := max(0, int(math.Floor(bounds.minX)))
+	lastPixelX := min(width, int(math.Ceil(bounds.maxX)))
+	firstBlockX := firstPixelX / 3 * 3
+	lastBlockX := min(width, (lastPixelX+2)/3*3)
+	firstY := max(0, int(math.Floor(bounds.minY)))
+	lastY := min(height, int(math.Ceil(bounds.maxY)))
+	stride := AlphaLineSize(width)
+
+	for y := firstY; y < lastY; y++ {
+		for blockX := firstBlockX; blockX < lastBlockX; blockX += 3 {
+			var samples [3]uint32
+			for i := range samples {
+				x := blockX + i
+				switch {
+				case x >= width:
+					samples[i] = 0
+				case bounds.contains(x, y):
+					samples[i] = alphaTransparent
+				default:
+					samples[i] = alphaOpaque
+				}
+			}
+			offset := y*stride + blockX/3*4
+			packAlphaBlock(dst[offset:], samples)
+		}
+	}
+	return nil
+}

internal/generator/alpha_test.go

@@ -0,0 +1,161 @@
+package generator
+
+import (
+	"encoding/binary"
+	"strings"
+	"testing"
+)
+
+func TestPackAlphaBlock(t *testing.T) {
+	var dst [4]byte
+	packAlphaBlock(dst[:], [3]uint32{64, 512, 940})
+
+	word := binary.LittleEndian.Uint32(dst[:])
+
+	if got := word & 0x3ff; got != 64 {
+		t.Errorf("sample 0 = %d, want 64", got)
+	}
+	if got := (word >> 10) & 0x3ff; got != 512 {
+		t.Errorf("sample 1 = %d, want 512", got)
+	}
+	if got := (word >> 20) & 0x3ff; got != 940 {
+		t.Errorf("sample 2 = %d, want 940", got)
+	}
+	if got := word >> 30; got != 0 {
+		t.Errorf("unused bits = %d, want 0", got)
+	}
+}
+
+func TestPackAlphaBlockMasksSamples(t *testing.T) {
+	var dst [4]byte
+	packAlphaBlock(dst[:], [3]uint32{0x401, 0x802, 0xc03})
+
+	word := binary.LittleEndian.Uint32(dst[:])
+
+	if got := word & 0x3ff; got != 1 {
+		t.Errorf("sample 0 = %d, want 1", got)
+	}
+	if got := (word >> 10) & 0x3ff; got != 2 {
+		t.Errorf("sample 1 = %d, want 2", got)
+	}
+	if got := (word >> 20) & 0x3ff; got != 3 {
+		t.Errorf("sample 2 = %d, want 3", got)
+	}
+}
+
+func TestFillAlphaPlaneCompleteBlocks(t *testing.T) {
+	const width, height = 6, 2
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			if got := sampleAlpha(dst, width, x, y); got != alphaOpaque {
+				t.Errorf("sample (%d,%d) = %d, want %d", x, y, got, alphaOpaque)
+			}
+		}
+	}
+}
+
+func TestFillAlphaPlaneZerosPartialBlockPadding(t *testing.T) {
+	const width, height = 4, 2
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaTransparent); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+
+	stride := AlphaLineSize(width)
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			if got := sampleAlpha(dst, width, x, y); got != alphaTransparent {
+				t.Errorf("sample (%d,%d) = %d, want %d", x, y, got, alphaTransparent)
+			}
+		}
+		lastWord := binary.LittleEndian.Uint32(dst[y*stride+4:])
+		if got := (lastWord >> 10) & 0x3ff; got != 0 {
+			t.Errorf("row %d padding sample 1 = %d, want 0", y, got)
+		}
+		if got := (lastWord >> 20) & 0x3ff; got != 0 {
+			t.Errorf("row %d padding sample 2 = %d, want 0", y, got)
+		}
+		if got := lastWord >> 30; got != 0 {
+			t.Errorf("row %d unused bits = %d, want 0", y, got)
+		}
+	}
+}
+
+func TestFillAlphaPlaneRejectsSmallDestination(t *testing.T) {
+	const width, height = 6, 2
+	dst := make([]byte, AlphaFrameSize(width, height)-1)
+	err := fillAlphaPlane(dst, width, height, alphaOpaque)
+	if err == nil || !strings.Contains(err.Error(), "destination is too small") {
+		t.Fatalf("error = %v, want destination size error", err)
+	}
+}
+
+func TestPatchAlphaMovingSquare(t *testing.T) {
+	const width, height = 304, 200
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+	if err := patchAlphaMovingSquare(dst, width, height, 0); err != nil {
+		t.Fatalf("patchAlphaMovingSquare: %v", err)
+	}
+
+	if got := sampleAlpha(dst, width, width/2, height/2); got != alphaTransparent {
+		t.Errorf("square center = %d, want transparent %d", got, alphaTransparent)
+	}
+	if got := sampleAlpha(dst, width, 10, height/2); got != alphaOpaque {
+		t.Errorf("outside square = %d, want opaque %d", got, alphaOpaque)
+	}
+
+	// At frame zero the square begins at x=77. Its first three-sample word
+	// therefore contains two opaque samples followed by one transparent sample.
+	for x, want := range []uint32{alphaOpaque, alphaOpaque, alphaTransparent} {
+		if got := sampleAlpha(dst, width, 75+x, height/2); got != want {
+			t.Errorf("boundary sample x=%d = %d, want %d", 75+x, got, want)
+		}
+	}
+}
+
+func TestPatchAlphaMovingSquarePreservesPartialBlockPadding(t *testing.T) {
+	const width, height = 100, 200
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+	if err := patchAlphaMovingSquare(dst, width, height, 0); err != nil {
+		t.Fatalf("patchAlphaMovingSquare: %v", err)
+	}
+
+	lastWordOffset := height/2*AlphaLineSize(width) + (width/3)*4
+	lastWord := binary.LittleEndian.Uint32(dst[lastWordOffset:])
+	if got := lastWord & 0x3ff; got != alphaTransparent {
+		t.Errorf("last visible sample = %d, want %d", got, alphaTransparent)
+	}
+	if got := lastWord >> 10; got != 0 {
+		t.Errorf("partial-block padding bits = %#x, want 0", got)
+	}
+}
+
+func TestPatchAlphaMovingSquareRejectsSmallDestination(t *testing.T) {
+	const width, height = 100, 200
+	err := patchAlphaMovingSquare(
+		make([]byte, AlphaFrameSize(width, height)-1),
+		width,
+		height,
+		0,
+	)
+	if err == nil || !strings.Contains(err.Error(), "destination is too small") {
+		t.Fatalf("error = %v, want destination size error", err)
+	}
+}
+
+func sampleAlpha(buf []byte, width, x, y int) uint32 {
+	offset := y*AlphaLineSize(width) + x/3*4
+	word := binary.LittleEndian.Uint32(buf[offset:])
+	return (word >> uint(x%3*10)) & 0x3ff
+}

internal/generator/cpu.go

@@ -0,0 +1,84 @@
+package generator
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+type YCbCr10 struct {
+	Y  uint32
+	Cb uint32
+	Cr uint32
+}
+
+// FrameRenderer writes a complete frame or patches part of an existing frame.
+type FrameRenderer func(dst []byte, width, height, frameIndex int) error
+
+type CPUGenerator struct {
+	width  int
+	height int
+	base   []byte
+	patch  FrameRenderer
+}
+
+func NewCPUGenerator(
+	width, height uint,
+	baseRenderer FrameRenderer,
+	patch FrameRenderer,
+) (*CPUGenerator, error) {
+	if width == 0 || height == 0 {
+		return nil, fmt.Errorf("cpu: width and height must be greater than zero, got %dx%d", width, height)
+	}
+	if width%2 != 0 {
+		return nil, fmt.Errorf("cpu: width must be even for 4:2:2 video, got %d", width)
+	}
+	if baseRenderer == nil {
+		return nil, fmt.Errorf("cpu: base renderer is nil")
+	}
+
+	g := &CPUGenerator{
+		width:  int(width),
+		height: int(height),
+		base:   make([]byte, V210FrameSize(int(width), int(height))),
+		patch:  patch,
+	}
+	if err := baseRenderer(g.base, g.width, g.height, 0); err != nil {
+		return nil, fmt.Errorf("cpu: render base frame: %w", err)
+	}
+	return g, nil
+}
+
+func (g *CPUGenerator) GenerateFrame(dst []byte, frameIndex int) error {
+	if len(dst) < len(g.base) {
+		return fmt.Errorf(
+			"cpu: destination is too small: got %d bytes, need %d",
+			len(dst),
+			len(g.base),
+		)
+	}
+
+	copy(dst, g.base)
+
+	if g.patch != nil {
+		if err := g.patch(dst[:len(g.base)], g.width, g.height, frameIndex); err != nil {
+			return fmt.Errorf("cpu: patch frame %d: %w", frameIndex, err)
+		}
+	}
+
+	return nil
+}
+
+func (g *CPUGenerator) Close() error { return nil }
+
+func packV210Block(dst []byte, pixels [6]YCbCr10) {
+	const mask uint32 = 0x3ff
+	word0 := pixels[0].Cb&mask | (pixels[0].Y&mask)<<10 | (pixels[0].Cr&mask)<<20
+	word1 := pixels[1].Y&mask | (pixels[2].Cb&mask)<<10 | (pixels[2].Y&mask)<<20
+	word2 := pixels[2].Cr&mask | (pixels[3].Y&mask)<<10 | (pixels[4].Cb&mask)<<20
+	word3 := pixels[4].Y&mask | (pixels[4].Cr&mask)<<10 | (pixels[5].Y&mask)<<20
+
+	binary.LittleEndian.PutUint32(dst, word0)
+	binary.LittleEndian.PutUint32(dst[4:], word1)
+	binary.LittleEndian.PutUint32(dst[8:], word2)
+	binary.LittleEndian.PutUint32(dst[12:], word3)
+}

internal/generator/cpu_moving_square.go

@@ -0,0 +1,74 @@
+package generator
+
+import "math"
+
+const movingSquareSize = 150
+
+type baseColorFunc func(x, y, width, height int) YCbCr10
+
+type squareBounds struct {
+	minX float64
+	maxX float64
+	minY float64
+	maxY float64
+}
+
+func movingSquareBounds(width, height, frameIndex int) squareBounds {
+	const half = float64(movingSquareSize) / 2
+	centerX := float64(width) / 2
+	centerY := float64(height) / 2
+	time := float64(frameIndex) / 25.0
+	offset := math.Sin(time*0.5) * (centerX - half)
+	return squareBounds{
+		minX: centerX - half + offset,
+		maxX: centerX + half + offset,
+		minY: centerY - half,
+		maxY: centerY + half,
+	}
+}
+
+func (b squareBounds) contains(x, y int) bool {
+	return float64(x) >= b.minX && float64(x) < b.maxX &&
+		float64(y) >= b.minY && float64(y) < b.maxY
+}
+
+func patchMovingSquare(
+	dst []byte,
+	width, height, frameIndex int,
+	baseColor baseColorFunc,
+) error {
+	bounds := movingSquareBounds(width, height, frameIndex)
+
+	firstPixelX := max(0, int(math.Floor(bounds.minX)))
+	lastPixelX := min(width, int(math.Ceil(bounds.maxX)))
+	firstBlockX := firstPixelX / 6 * 6
+	lastBlockX := min(width, (lastPixelX+5)/6*6)
+	firstY := max(0, int(math.Floor(bounds.minY)))
+	lastY := min(height, int(math.Ceil(bounds.maxY)))
+	stride := V210LineSize(width)
+
+	for y := firstY; y < lastY; y++ {
+		for x := firstBlockX; x < lastBlockX; x += 6 {
+			var pixels [6]YCbCr10
+			for i := range pixels {
+				px := x + i
+				color := baseColor(px, y, width, height)
+				if bounds.contains(px, y) {
+					color = invertStudioRange(color)
+				}
+				pixels[i] = color
+			}
+			offset := y*stride + x/6*16
+			packV210Block(dst[offset:offset+16], pixels)
+		}
+	}
+	return nil
+}
+
+func invertStudioRange(color YCbCr10) YCbCr10 {
+	return YCbCr10{
+		Y:  1004 - color.Y,
+		Cb: 1024 - color.Cb,
+		Cr: 1024 - color.Cr,
+	}
+}

internal/generator/cpu_patterns.go

@@ -0,0 +1,245 @@
+package generator
+
+import (
+	"fmt"
+)
+
+var ebu75Colors = [...]YCbCr10{
+	{Y: 721, Cb: 512, Cr: 512},
+	{Y: 674, Cb: 176, Cr: 543},
+	{Y: 581, Cb: 589, Cr: 176},
+	{Y: 534, Cb: 253, Cr: 207},
+	{Y: 251, Cb: 771, Cr: 817},
+	{Y: 204, Cb: 435, Cr: 848},
+	{Y: 111, Cb: 848, Cr: 481},
+	{Y: 64, Cb: 512, Cr: 512},
+}
+
+var ebu100Colors = [...]YCbCr10{
+	{Y: 940, Cb: 512, Cr: 512},
+	{Y: 877, Cb: 64, Cr: 553},
+	{Y: 754, Cb: 615, Cr: 64},
+	{Y: 691, Cb: 167, Cr: 105},
+	{Y: 313, Cb: 857, Cr: 919},
+	{Y: 250, Cb: 409, Cr: 960},
+	{Y: 127, Cb: 960, Cr: 471},
+	{Y: 64, Cb: 512, Cr: 512},
+}
+
+var smpte75Colors = [...]YCbCr10{
+	{Y: 721, Cb: 512, Cr: 512},
+	{Y: 674, Cb: 176, Cr: 543},
+	{Y: 581, Cb: 589, Cr: 176},
+	{Y: 534, Cb: 253, Cr: 207},
+	{Y: 251, Cb: 771, Cr: 817},
+	{Y: 204, Cb: 435, Cr: 848},
+	{Y: 111, Cb: 848, Cr: 481},
+}
+
+var grayBarsColors = [...]YCbCr10{
+	{Y: 64, Cb: 512, Cr: 512},
+	{Y: 137, Cb: 512, Cr: 512},
+	{Y: 210, Cb: 512, Cr: 512},
+	{Y: 283, Cb: 512, Cr: 512},
+	{Y: 356, Cb: 512, Cr: 512},
+	{Y: 429, Cb: 512, Cr: 512},
+	{Y: 502, Cb: 512, Cr: 512},
+	{Y: 575, Cb: 512, Cr: 512},
+	{Y: 648, Cb: 512, Cr: 512},
+	{Y: 721, Cb: 512, Cr: 512},
+	{Y: 794, Cb: 512, Cr: 512},
+	{Y: 867, Cb: 512, Cr: 512},
+	{Y: 940, Cb: 512, Cr: 512},
+}
+
+var (
+	ebu75BaseColor    = colorBars(ebu75Colors[:])
+	ebu100BaseColor   = colorBars(ebu100Colors[:])
+	grayBarsBaseColor = colorBars(grayBarsColors[:])
+)
+
+func NewCPUPatternGenerator(width, height uint, pattern string) (*CPUGenerator, error) {
+	var baseColor baseColorFunc
+	var dynamic bool
+	switch pattern {
+	case "ebu75":
+		baseColor = ebu75BaseColor
+	case "ebu75-move":
+		baseColor, dynamic = ebu75BaseColor, true
+	case "ebu100":
+		baseColor = ebu100BaseColor
+	case "ebu100-move":
+		baseColor, dynamic = ebu100BaseColor, true
+	case "smpte":
+		if err := validateRP219Size(width, height); err != nil {
+			return nil, err
+		}
+		baseColor = rp219BaseColor
+	case "smpte-move":
+		if err := validateRP219Size(width, height); err != nil {
+			return nil, err
+		}
+		baseColor, dynamic = rp219BaseColor, true
+	case "gray-bars":
+		baseColor = grayBarsBaseColor
+	case "gray-bars-move":
+		baseColor, dynamic = grayBarsBaseColor, true
+	case "gray-ramp":
+		baseColor = grayRampBaseColor
+	case "gray-ramp-move":
+		baseColor, dynamic = grayRampBaseColor, true
+	default:
+		return nil, fmt.Errorf("cpu pattern %q is not implemented", pattern)
+	}
+
+	var patch FrameRenderer
+	if dynamic {
+		patch = movingSquarePatch(baseColor)
+	}
+	return NewCPUGenerator(width, height, baseRenderer(baseColor), patch)
+}
+
+func grayRampBaseColor(x, _, width, _ int) YCbCr10 {
+	return YCbCr10{
+		Y:  uint32(64 + (x*876)/width),
+		Cb: 512,
+		Cr: 512,
+	}
+}
+
+func colorBars(colors []YCbCr10) baseColorFunc {
+	return func(x, _, width, _ int) YCbCr10 {
+		bar := min(x*len(colors)/width, len(colors)-1)
+		return colors[bar]
+	}
+}
+
+func baseRenderer(baseColor baseColorFunc) FrameRenderer {
+	return func(dst []byte, width, height, _ int) error {
+		return renderBasePattern(dst, width, height, baseColor)
+	}
+}
+
+func movingSquarePatch(baseColor baseColorFunc) FrameRenderer {
+	return func(dst []byte, width, height, frameIndex int) error {
+		return patchMovingSquare(dst, width, height, frameIndex, baseColor)
+	}
+}
+
+func renderBasePattern(dst []byte, width, height int, baseColor baseColorFunc) error {
+	stride := V210LineSize(width)
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x += 6 {
+			var pixels [6]YCbCr10
+			for i := range pixels {
+				pixels[i] = baseColor(x+i, y, width, height)
+			}
+			offset := y*stride + x/6*16
+			packV210Block(dst[offset:offset+16], pixels)
+		}
+	}
+	return nil
+}
+
+var (
+	gray40 = YCbCr10{Y: 414, Cb: 512, Cr: 512}
+	gray15 = YCbCr10{Y: 195, Cb: 512, Cr: 512}
+	black  = YCbCr10{Y: 64, Cb: 512, Cr: 512}
+	white  = YCbCr10{Y: 940, Cb: 512, Cr: 512}
+)
+
+func rp219BaseColor(x, y, width, height int) YCbCr10 {
+	barsWidth := (height / 3) * 4
+	barsStart := (width - barsWidth) / 2
+	barsEnd := barsStart + barsWidth
+	oneBarWidth := barsWidth / 7
+
+	unitH := height / 12
+	section1End := unitH * 7
+	section2End := section1End + unitH
+	section3End := section2End + unitH
+
+	rampStart := barsStart + oneBarWidth
+	rampEnd := barsEnd
+	rampWidth := rampEnd - rampStart
+
+	switch {
+	case y < section1End:
+		if x < barsStart || x >= barsEnd {
+			return gray40
+		}
+
+		bar := min(
+			(x-barsStart)*len(smpte75Colors)/barsWidth,
+			len(smpte75Colors)-1,
+		)
+		return smpte75Colors[bar]
+
+	case y < section2End:
+		switch {
+		case x < barsStart:
+			return YCbCr10{Y: 754, Cb: 615, Cr: 64} // 100% cyan
+		case x < rampStart:
+			return YCbCr10{Y: 244, Cb: 612, Cr: 395} // -I
+		case x >= barsEnd:
+			return YCbCr10{Y: 127, Cb: 960, Cr: 471} // 100% blue
+		default:
+			return YCbCr10{Y: 721, Cb: 512, Cr: 512} // 75% white
+		}
+
+	case y < section3End:
+		switch {
+		case x < barsStart:
+			return YCbCr10{Y: 877, Cb: 64, Cr: 553} // 100% yellow
+		case x < rampStart:
+			return YCbCr10{Y: 141, Cb: 697, Cr: 606} // +Q
+		case x >= barsEnd:
+			return YCbCr10{Y: 250, Cb: 409, Cr: 960} // 75% red
+		default:
+			rampY := 64 + ((x-rampStart)*876)/rampWidth
+			return YCbCr10{Y: uint32(rampY), Cb: 512, Cr: 512}
+		}
+
+	default:
+		if x < barsStart || x >= barsEnd {
+			return gray15
+		}
+
+		bar := (x - barsStart) / oneBarWidth
+		switch {
+		case bar == 1:
+			return white
+		case bar == 3:
+			offset := (x - barsStart) % oneBarWidth
+			subBar := offset * 3 / oneBarWidth
+
+			switch subBar {
+			case 0:
+				return YCbCr10{Y: 46, Cb: 512, Cr: 512}
+			case 1:
+				return black
+			default:
+				return YCbCr10{Y: 82, Cb: 512, Cr: 512}
+			}
+
+		default:
+			return black
+		}
+	}
+}
+
+func validateRP219Size(width, height uint) error {
+	if height < 12 {
+		return fmt.Errorf("cpu RP 219 requires frame height of at least 12, got %d", height)
+	}
+	barsWidth := (height / 3) * 4
+	if barsWidth < 7 || width < barsWidth {
+		return fmt.Errorf(
+			"cpu RP 219 requires frame width %d to fit a 4:3 pattern area for height %d, got %d",
+			barsWidth,
+			height,
+			width,
+		)
+	}
+	return nil
+}

internal/generator/cpu_patterns_test.go

@@ -0,0 +1,202 @@
+package generator
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestCPUEBU75Static(t *testing.T) {
+	const width, height = 1920, 1080
+	g, err := NewCPUPatternGenerator(width, height, "ebu75")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, width*height*8/3)
+	if err := g.GenerateFrame(frame, 99); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	for _, tc := range []struct {
+		x                     int
+		wantY, wantCb, wantCr uint32
+	}{
+		{x: 0, wantY: 721, wantCb: 512, wantCr: 512},
+		{x: 300, wantY: 674, wantCb: 176, wantCr: 543},
+		{x: 960, wantY: 251, wantCb: 771, wantCr: 817},
+		{x: 1800, wantY: 64, wantCb: 512, wantCr: 512},
+	} {
+		y, cb, cr := sampleV210(frame, width, tc.x, 100)
+		if y != tc.wantY || cb != tc.wantCb || cr != tc.wantCr {
+			t.Fatalf("x=%d: got %d/%d/%d, want %d/%d/%d",
+				tc.x, y, cb, cr, tc.wantY, tc.wantCb, tc.wantCr)
+		}
+	}
+}
+
+func TestCPUEBU75MovingSquare(t *testing.T) {
+	const width, height = 1920, 1080
+	g, err := NewCPUPatternGenerator(width, height, "ebu75-move")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, width*height*8/3)
+
+	if err := g.GenerateFrame(frame, 0); err != nil {
+		t.Fatalf("GenerateFrame(0): %v", err)
+	}
+	if y, cb, cr := sampleV210(frame, width, 960, 540); y != 753 || cb != 253 || cr != 207 {
+		t.Fatalf("tick 0 center = %d/%d/%d, want inverted magenta 753/253/207", y, cb, cr)
+	}
+
+	if err := g.GenerateFrame(frame, 79); err != nil {
+		t.Fatalf("GenerateFrame(79): %v", err)
+	}
+	if y, cb, cr := sampleV210(frame, width, 960, 540); y != 251 || cb != 771 || cr != 817 {
+		t.Fatalf("old square position = %d/%d/%d, want restored magenta 251/771/817", y, cb, cr)
+	}
+	if y, cb, cr := sampleV210(frame, width, 1840, 540); y != 940 || cb != 512 || cr != 512 {
+		t.Fatalf("shifted square = %d/%d/%d, want inverted black 940/512/512", y, cb, cr)
+	}
+}
+
+func TestCPUStaticPatterns(t *testing.T) {
+	const width, height = 1920, 1080
+	tests := []struct {
+		name                  string
+		x                     int
+		wantY, wantCb, wantCr uint32
+	}{
+		{name: "ebu100", x: 0, wantY: 940, wantCb: 512, wantCr: 512},
+		{name: "ebu100", x: 300, wantY: 877, wantCb: 64, wantCr: 553},
+		{name: "ebu100", x: 960, wantY: 313, wantCb: 857, wantCr: 919},
+		{name: "gray-bars", x: 0, wantY: 64, wantCb: 512, wantCr: 512},
+		{name: "gray-bars", x: 960, wantY: 502, wantCb: 512, wantCr: 512},
+		{name: "gray-bars", x: 1800, wantY: 940, wantCb: 512, wantCr: 512},
+		{name: "gray-ramp", x: 0, wantY: 64, wantCb: 512, wantCr: 512},
+		{name: "gray-ramp", x: 6, wantY: 66, wantCb: 512, wantCr: 512},
+		{name: "gray-ramp", x: 960, wantY: 502, wantCb: 512, wantCr: 512},
+		{name: "gray-ramp", x: 1918, wantY: 939, wantCb: 512, wantCr: 512},
+	}
+
+	frames := make(map[string][]byte)
+	for _, tc := range tests {
+		frame, ok := frames[tc.name]
+		if !ok {
+			g, err := NewCPUPatternGenerator(width, height, tc.name)
+			if err != nil {
+				t.Fatalf("NewCPUPatternGenerator(%q): %v", tc.name, err)
+			}
+			frame = make([]byte, width*height*8/3)
+			if err := g.GenerateFrame(frame, 0); err != nil {
+				t.Fatalf("GenerateFrame(%q): %v", tc.name, err)
+			}
+			frames[tc.name] = frame
+		}
+
+		y, cb, cr := sampleV210(frame, width, tc.x, 100)
+		if y != tc.wantY || cb != tc.wantCb || cr != tc.wantCr {
+			t.Errorf("%s x=%d: got %d/%d/%d, want %d/%d/%d",
+				tc.name, tc.x, y, cb, cr, tc.wantY, tc.wantCb, tc.wantCr)
+		}
+	}
+}
+
+func TestCPUMovingPatternUsesOwnBaseColor(t *testing.T) {
+	const width, height = 1920, 1080
+	g, err := NewCPUPatternGenerator(width, height, "ebu100-move")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, width*height*8/3)
+	if err := g.GenerateFrame(frame, 0); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+	if y, cb, cr := sampleV210(frame, width, 960, 540); y != 691 || cb != 167 || cr != 105 {
+		t.Fatalf("center = %d/%d/%d, want inverted EBU100 magenta 691/167/105", y, cb, cr)
+	}
+}
+
+func TestCPURP219Pattern(t *testing.T) {
+	const width, height = 1920, 1080
+	g, err := NewCPUPatternGenerator(width, height, "smpte")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, width*height*8/3)
+	if err := g.GenerateFrame(frame, 0); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	tests := []struct {
+		name                  string
+		x, y                  int
+		wantY, wantCb, wantCr uint32
+	}{
+		{"top left gray flank", 100, 100, 414, 512, 512},
+		{"top white bar", 300, 100, 721, 512, 512},
+		{"top green bar", 960, 100, 534, 253, 207},
+		{"top right gray flank", 1800, 100, 414, 512, 512},
+		{"section 2 cyan flank", 100, 650, 754, 615, 64},
+		{"section 2 minus I", 300, 650, 244, 612, 395},
+		{"section 2 white", 600, 650, 721, 512, 512},
+		{"section 2 blue flank", 1800, 650, 127, 960, 471},
+		{"section 3 yellow flank", 100, 750, 877, 64, 553},
+		{"section 3 plus Q", 300, 750, 141, 697, 606},
+		{"section 3 red flank", 1800, 750, 250, 409, 960},
+		{"bottom gray flank", 100, 900, 195, 512, 512},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			y, cb, cr := sampleV210(frame, width, tc.x, tc.y)
+			if y != tc.wantY || cb != tc.wantCb || cr != tc.wantCr {
+				t.Fatalf("pixel (%d,%d): got %d/%d/%d, want %d/%d/%d",
+					tc.x, tc.y, y, cb, cr, tc.wantY, tc.wantCb, tc.wantCr)
+			}
+		})
+	}
+}
+
+func TestCPURP219MovingSquare(t *testing.T) {
+	const width, height = 1920, 1080
+	g, err := NewCPUPatternGenerator(width, height, "smpte-move")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, width*height*8/3)
+	if err := g.GenerateFrame(frame, 0); err != nil {
+		t.Fatalf("GenerateFrame(0): %v", err)
+	}
+	if y, cb, cr := sampleV210(frame, width, 960, 540); y != 470 || cb != 771 || cr != 817 {
+		t.Fatalf("tick 0 center = %d/%d/%d, want inverted green 470/771/817", y, cb, cr)
+	}
+	if err := g.GenerateFrame(frame, 79); err != nil {
+		t.Fatalf("GenerateFrame(79): %v", err)
+	}
+	if y, cb, cr := sampleV210(frame, width, 1840, 540); y != 590 || cb != 512 || cr != 512 {
+		t.Fatalf("tick 79 shifted square = %d/%d/%d, want inverted gray 590/512/512", y, cb, cr)
+	}
+}
+
+func TestCPURP219RejectsInvalidGeometry(t *testing.T) {
+	for _, tc := range []struct {
+		name          string
+		width, height uint
+	}{
+		{name: "height too small", width: 1920, height: 11},
+		{name: "canvas too narrow", width: 600, height: 1080},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := NewCPUPatternGenerator(tc.width, tc.height, "smpte")
+			if err == nil || !strings.Contains(err.Error(), "RP 219") {
+				t.Fatalf("error = %v, want RP 219 geometry error", err)
+			}
+		})
+	}
+}
+
+func TestNewCPUPatternGeneratorRejectsUnknownPattern(t *testing.T) {
+	_, err := NewCPUPatternGenerator(1920, 1080, "unknown")
+	if err == nil || !strings.Contains(err.Error(), "not implemented") {
+		t.Fatalf("error = %v, want unsupported pattern error", err)
+	}
+}

internal/generator/cpu_test.go

@@ -0,0 +1,147 @@
+package generator
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func fillFrame(value byte) FrameRenderer {
+	return func(dst []byte, _, _, _ int) error {
+		for i := range dst {
+			dst[i] = value
+		}
+		return nil
+	}
+}
+
+func TestNewCPUGeneratorValidation(t *testing.T) {
+	renderer := fillFrame(0)
+	tests := []struct {
+		name       string
+		width      uint
+		height     uint
+		renderer   FrameRenderer
+		wantErrSub string
+	}{
+		{name: "zero width", height: 1, renderer: renderer, wantErrSub: "greater than zero"},
+		{name: "zero height", width: 6, renderer: renderer, wantErrSub: "greater than zero"},
+		{name: "odd width", width: 7, height: 1, renderer: renderer, wantErrSub: "even"},
+		{name: "nil renderer", width: 6, height: 1, wantErrSub: "renderer is nil"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := NewCPUGenerator(tc.width, tc.height, tc.renderer, nil)
+			if err == nil || !strings.Contains(err.Error(), tc.wantErrSub) {
+				t.Fatalf("error = %v, want substring %q", err, tc.wantErrSub)
+			}
+		})
+	}
+}
+
+func TestCPUGeneratorCopiesBaseAndRestoresBeforePatch(t *testing.T) {
+	patch := func(dst []byte, _, _, frameIndex int) error {
+		switch frameIndex {
+		case 1:
+			dst[0] = 0x22
+		case 2:
+			dst[16] = 0x33
+		}
+		return nil
+	}
+	g, err := NewCPUGenerator(12, 1, fillFrame(0x11), patch)
+	if err != nil {
+		t.Fatalf("NewCPUGenerator: %v", err)
+	}
+
+	frame := make([]byte, V210FrameSize(12, 1))
+	if err := g.GenerateFrame(frame, 1); err != nil {
+		t.Fatalf("GenerateFrame(1): %v", err)
+	}
+	if frame[0] != 0x22 {
+		t.Fatalf("frame 1 patch byte = %#x, want 0x22", frame[0])
+	}
+
+	if err := g.GenerateFrame(frame, 2); err != nil {
+		t.Fatalf("GenerateFrame(2): %v", err)
+	}
+	if frame[0] != 0x11 {
+		t.Fatalf("old patch byte = %#x, want restored base 0x11", frame[0])
+	}
+	if frame[16] != 0x33 {
+		t.Fatalf("frame 2 patch byte = %#x, want 0x33", frame[16])
+	}
+}
+
+func TestPackV210Block(t *testing.T) {
+	var pixels [6]YCbCr10
+	for i := range pixels {
+		pixels[i] = YCbCr10{
+			Y:  uint32(100 + i),
+			Cb: uint32(200 + i),
+			Cr: uint32(300 + i),
+		}
+	}
+	frame := make([]byte, 16)
+	packV210Block(frame, pixels)
+
+	for x := range pixels {
+		y, cb, cr := sampleV210(frame, 6, x, 0)
+		chromaX := x - x%2
+		if y != pixels[x].Y || cb != pixels[chromaX].Cb || cr != pixels[chromaX].Cr {
+			t.Fatalf("pixel %d: got %d/%d/%d, want %d/%d/%d",
+				x, y, cb, cr, pixels[x].Y, pixels[chromaX].Cb, pixels[chromaX].Cr)
+		}
+	}
+}
+
+func TestPackV210BlockMasksComponents(t *testing.T) {
+	pixel := YCbCr10{Y: 0xC01, Cb: 0xC02, Cr: 0xC03}
+	frame := make([]byte, 16)
+	packV210Block(frame, [6]YCbCr10{pixel, pixel, pixel, pixel, pixel, pixel})
+	y, cb, cr := sampleV210(frame, 6, 0, 0)
+	if y != 1 || cb != 2 || cr != 3 {
+		t.Fatalf("masked components = %d/%d/%d, want 1/2/3", y, cb, cr)
+	}
+}
+
+func TestCPUGeneratorErrors(t *testing.T) {
+	wantErr := errors.New("patch failed")
+	g, err := NewCPUGenerator(6, 1, fillFrame(0), func([]byte, int, int, int) error {
+		return wantErr
+	})
+	if err != nil {
+		t.Fatalf("NewCPUGenerator: %v", err)
+	}
+	if err := g.GenerateFrame(make([]byte, V210FrameSize(6, 1)-1), 0); err == nil || !strings.Contains(err.Error(), "too small") {
+		t.Fatalf("small destination error = %v", err)
+	}
+	if err := g.GenerateFrame(make([]byte, V210FrameSize(6, 1)), 4); !errors.Is(err, wantErr) {
+		t.Fatalf("patch error = %v, want wrapped %v", err, wantErr)
+	}
+}
+
+func TestCPUGeneratorUsesPaddedV210Rows(t *testing.T) {
+	const width, height = 100, 2
+	g, err := NewCPUPatternGenerator(width, height, "gray-ramp")
+	if err != nil {
+		t.Fatalf("NewCPUPatternGenerator: %v", err)
+	}
+	frame := make([]byte, V210FrameSize(width, height))
+	if err := g.GenerateFrame(frame, 0); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	for y := 0; y < height; y++ {
+		gotY, gotCb, gotCr := sampleV210(frame, width, 0, y)
+		if gotY != 64 || gotCb != 512 || gotCr != 512 {
+			t.Fatalf("row %d first pixel = %d/%d/%d, want 64/512/512", y, gotY, gotCb, gotCr)
+		}
+		padding := frame[y*V210LineSize(width)+V210ActiveLineSize(width) : (y+1)*V210LineSize(width)]
+		for i, b := range padding {
+			if b != 0 {
+				t.Fatalf("row %d padding byte %d = %#x, want 0", y, i, b)
+			}
+		}
+	}
+}

internal/generator/text.go

@@ -33,9 +33,19 @@ func LoadFace(path string, size float64) (font.Face, error) {
 	if err != nil {
 		return nil, fmt.Errorf("text: %w", err)
 	}
+	face, err := NewFace(data, size)
+	if err != nil {
+		return nil, fmt.Errorf("text: load %s: %w", path, err)
+	}
+	return face, nil
+}
+
+// NewFace parses TTF/OTF data and builds a render-ready face at the given
+// pixel size (DPI 72, full hinting for crisp video text).
+func NewFace(data []byte, size float64) (font.Face, error) {
 	f, err := opentype.Parse(data)
 	if err != nil {
-		return nil, fmt.Errorf("text: parse %s: %w", path, err)
+		return nil, fmt.Errorf("parse font: %w", err)
 	}
 	face, err := opentype.NewFace(f, &opentype.FaceOptions{
 		Size:    size,
@@ -43,7 +53,7 @@ func LoadFace(path string, size float64) (font.Face, error) {
 		Hinting: font.HintingFull,
 	})
 	if err != nil {
-		return nil, fmt.Errorf("text: face %s: %w", path, err)
+		return nil, fmt.Errorf("create font face: %w", err)
 	}
 	return face, nil
 }
@@ -179,15 +189,16 @@ func (o *TextOverlay) pack() {
 // ApplyV210 stamps the pre-packed text tile over a packed v210 frame.
 // The tile occupies whole 16-byte blocks, so this is a row-wise copy.
 func (o *TextOverlay) ApplyV210(dest []byte) error {
-	lastPixel := (o.y+o.h-1)*o.frameW + o.x + o.w - 1
+	frameStride := V210LineSize(o.frameW)
-	if need := (lastPixel/6 + 1) * 16; len(dest) < need {
+	tileStride := (o.w / 6) * 16
+	need := (o.y+o.h-1)*frameStride + o.x/6*16 + tileStride
+	if len(dest) < need {
 		return fmt.Errorf("text: dest %d bytes too small, need %d", len(dest), need)
 	}
-	tileStride := (o.w / 6) * 16
 	for row := 0; row < o.h; row++ {
-		frameBlock := ((o.y+row)*o.frameW + o.x) / 6
+		frameOffset := (o.y+row)*frameStride + o.x/6*16
 		src := o.blocks[row*tileStride : (row+1)*tileStride]
-		copy(dest[frameBlock*16:frameBlock*16+tileStride], src)
+		copy(dest[frameOffset:frameOffset+tileStride], src)
 	}
 	return nil
 }

internal/generator/text_test.go

@@ -1,17 +1,17 @@
 package generator
 
 import (
-	"path/filepath"
 	"strings"
 	"testing"
 
+	"mxl-pattern-generator/assets"
+
 	"golang.org/x/image/font"
 )
 
 func testFace(t *testing.T, size float64) font.Face {
 	t.Helper()
-	face, err := LoadFace(filepath.Join("..", "..", "assets", "fonts",
+	face, err := NewFace(assets.JetBrainsMono, size)
-		"JetBrainsMonoNLNerdFontMono-Regular.ttf"), size)
 	if err != nil {
 		t.Fatalf("LoadFace: %v", err)
 	}
@@ -163,7 +163,7 @@ func TestTextOverlayApplyV210(t *testing.T) {
 		t.Fatalf("NewTextOverlay: %v", err)
 	}
 
-	frame := make([]byte, frameW*frameH*8/3)
+	frame := make([]byte, V210FrameSize(frameW, frameH))
 	for i := range frame {
 		frame[i] = 0x5A // marker: untouched regions must survive
 	}
@@ -192,7 +192,7 @@ func TestTextOverlayApplyV210(t *testing.T) {
 		{o.x + o.w, o.y + o.h}, // corner
 		{100, 1000},            // far away
 	} {
-		off := ((p[1]*frameW + p[0]) / 6) * 16
+		off := p[1]*V210LineSize(frameW) + p[0]/6*16
 		for i := 0; i < 16; i++ {
 			if frame[off+i] != 0x5A {
 				t.Fatalf("block at (%d,%d) modified outside the box", p[0], p[1])

internal/generator/v210.go

@@ -0,0 +1,37 @@
+package generator
+
+const v210RowAlignment = 128
+
+// V210ActiveLineSize returns the number of bytes containing pixel data in one
+// v210 row, including the final partial six-pixel block when needed.
+func V210ActiveLineSize(width int) int {
+	return (width + 5) / 6 * 16
+}
+
+// V210LineSize returns the MXL v210 row stride. MXL stores every row at a
+// 128-byte boundary, equivalent to rounding the width up to 48 pixels.
+func V210LineSize(width int) int {
+	return ((width + 47) / 48) * v210RowAlignment
+}
+
+// V210FrameSize returns the complete MXL payload size for a v210 frame.
+func V210FrameSize(width, height int) int {
+	return V210LineSize(width) * height
+}
+
+// AlphaLineSize returns the byte stride of one packed 10-bit alpha row. Each
+// little-endian 32-bit word contains three alpha samples and two unused bits.
+func AlphaLineSize(width int) int {
+	return ((width + 2) / 3) * 4
+}
+
+// AlphaFrameSize returns the size of the alpha plane in a v210a frame.
+func AlphaFrameSize(width, height int) int {
+	return AlphaLineSize(width) * height
+}
+
+// V210AFrameSize returns the total size of a v210a payload: the complete v210
+// fill plane followed by the complete packed 10-bit alpha plane.
+func V210AFrameSize(width, height int) int {
+	return V210FrameSize(width, height) + AlphaFrameSize(width, height)
+}

internal/generator/v210_test.go

@@ -1,16 +1,84 @@
 package generator
 
-import "encoding/binary"
+import (
+	"encoding/binary"
+	"fmt"
+	"testing"
+)
+
+func TestV210Sizes(t *testing.T) {
+	tests := []struct {
+		width      int
+		activeLine int
+		line       int
+	}{
+		{width: 1920, activeLine: 5120, line: 5120},
+		{width: 1280, activeLine: 3424, line: 3456},
+		{width: 100, activeLine: 272, line: 384},
+		{width: 54, activeLine: 144, line: 256},
+		{width: 48, activeLine: 128, line: 128},
+	}
+	for _, tc := range tests {
+		if got := V210ActiveLineSize(tc.width); got != tc.activeLine {
+			t.Errorf("V210ActiveLineSize(%d) = %d, want %d", tc.width, got, tc.activeLine)
+		}
+		if got := V210LineSize(tc.width); got != tc.line {
+			t.Errorf("V210LineSize(%d) = %d, want %d", tc.width, got, tc.line)
+		}
+		if got := V210FrameSize(tc.width, 2); got != tc.line*2 {
+			t.Errorf("V210FrameSize(%d, 2) = %d, want %d", tc.width, got, tc.line*2)
+		}
+	}
+}
+
+func TestAlphaSizes(t *testing.T) {
+	tests := []struct {
+		width     int
+		height    int
+		lineSize  int
+		frameSize int
+		v210aSize int
+	}{
+		{
+			width: 1920, height: 1080,
+			lineSize: 2560, frameSize: 2_764_800, v210aSize: 8_294_400,
+		},
+		{
+			width: 1280, height: 720,
+			lineSize: 1708, frameSize: 1_229_760, v210aSize: 3_718_080,
+		},
+		{
+			width: 100, height: 2,
+			lineSize: 136, frameSize: 272, v210aSize: 1040,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(fmt.Sprintf("%dx%d", tc.width, tc.height), func(t *testing.T) {
+			if got := AlphaLineSize(tc.width); got != tc.lineSize {
+				t.Errorf("AlphaLineSize(%d) = %d, want %d", tc.width, got, tc.lineSize)
+			}
+			if got := AlphaFrameSize(tc.width, tc.height); got != tc.frameSize {
+				t.Errorf("AlphaFrameSize(%d, %d) = %d, want %d", tc.width, tc.height, got, tc.frameSize)
+			}
+			if got := V210AFrameSize(tc.width, tc.height); got != tc.v210aSize {
+				t.Errorf("V210AFrameSize(%d, %d) = %d, want %d", tc.width, tc.height, got, tc.v210aSize)
+			}
+			if got := V210AFrameSize(tc.width, tc.height) - AlphaFrameSize(tc.width, tc.height); got != V210FrameSize(tc.width, tc.height) {
+				t.Errorf("alpha plane starts at byte %d, want %d", got, V210FrameSize(tc.width, tc.height))
+			}
+		})
+	}
+}
 
 func sampleV210(buf []byte, width, x, y int) (yc, cb, cr uint32) {
-	pixel := y*width + x
+	offset := y*V210LineSize(width) + x/6*16
-	offset := (pixel / 6) * 16
 	w0 := binary.LittleEndian.Uint32(buf[offset:])
 	w1 := binary.LittleEndian.Uint32(buf[offset+4:])
 	w2 := binary.LittleEndian.Uint32(buf[offset+8:])
 	w3 := binary.LittleEndian.Uint32(buf[offset+12:])
 
-	switch pixel % 6 {
+	switch x % 6 {
 	case 0:
 		return (w0 >> 10) & 0x3FF, w0 & 0x3FF, (w0 >> 20) & 0x3FF
 	case 1:

internal/generator/v210a.go

@@ -0,0 +1,73 @@
+package generator
+
+import "fmt"
+
+// V210AGenerator combines a v210 fill generator with a packed 10-bit alpha
+// plane. It owns the fill generator and closes it from Close.
+type V210AGenerator struct {
+	fill      FrameGenerator
+	width     int
+	height    int
+	fillSize  int
+	fillBase  []byte
+	alphaBase []byte
+}
+
+var _ FrameGenerator = (*V210AGenerator)(nil)
+
+func NewV210AGenerator(
+	fill FrameGenerator,
+	width, height uint,
+	fillDynamic bool,
+) (*V210AGenerator, error) {
+	if fill == nil {
+		return nil, fmt.Errorf("v210a: fill generator is nil")
+	}
+	if width == 0 || height == 0 {
+		return nil, fmt.Errorf("v210a: width and height must be greater than zero, got %dx%d", width, height)
+	}
+	if width%2 != 0 {
+		return nil, fmt.Errorf("v210a: width must be even for 4:2:2 video, got %d", width)
+	}
+
+	g := &V210AGenerator{
+		fill:      fill,
+		width:     int(width),
+		height:    int(height),
+		fillSize:  V210FrameSize(int(width), int(height)),
+		alphaBase: make([]byte, AlphaFrameSize(int(width), int(height))),
+	}
+	if err := fillAlphaPlane(g.alphaBase, g.width, g.height, alphaOpaque); err != nil {
+		return nil, fmt.Errorf("v210a: initialize alpha plane: %w", err)
+	}
+	if !fillDynamic {
+		g.fillBase = make([]byte, g.fillSize)
+		if err := fill.GenerateFrame(g.fillBase, 0); err != nil {
+			return nil, fmt.Errorf("v210a: initialize static fill: %w", err)
+		}
+	}
+	return g, nil
+}
+
+func (g *V210AGenerator) GenerateFrame(dst []byte, frameIndex int) error {
+	need := V210AFrameSize(g.width, g.height)
+	if len(dst) < need {
+		return fmt.Errorf("v210a: destination is too small: got %d bytes, need %d", len(dst), need)
+	}
+
+	if g.fillBase != nil {
+		copy(dst[:g.fillSize], g.fillBase)
+	} else if err := g.fill.GenerateFrame(dst[:g.fillSize], frameIndex); err != nil {
+		return fmt.Errorf("v210a: generate dynamic fill frame %d: %w", frameIndex, err)
+	}
+	alpha := dst[g.fillSize:need]
+	copy(alpha, g.alphaBase)
+	if err := patchAlphaMovingSquare(alpha, g.width, g.height, frameIndex); err != nil {
+		return fmt.Errorf("v210a: patch alpha frame %d: %w", frameIndex, err)
+	}
+	return nil
+}
+
+func (g *V210AGenerator) Close() error {
+	return g.fill.Close()
+}

internal/generator/v210a_test.go

@@ -0,0 +1,171 @@
+package generator
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+type fakeFrameGenerator struct {
+	generateErr error
+	closeErr    error
+	closed      bool
+	calls       int
+}
+
+func (g *fakeFrameGenerator) GenerateFrame(dst []byte, frameIndex int) error {
+	g.calls++
+	if g.generateErr != nil {
+		return g.generateErr
+	}
+	for i := range dst {
+		dst[i] = byte(frameIndex)
+	}
+	return nil
+}
+
+func (g *fakeFrameGenerator) Close() error {
+	g.closed = true
+	return g.closeErr
+}
+
+func TestV210AGeneratorLayoutAndAlpha(t *testing.T) {
+	const width, height = 304, 200
+	fill := &fakeFrameGenerator{}
+	g, err := NewV210AGenerator(fill, width, height, true)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	dst := make([]byte, V210AFrameSize(width, height))
+	if err := g.GenerateFrame(dst, 7); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	fillSize := V210FrameSize(width, height)
+	for i, b := range dst[:fillSize] {
+		if b != 7 {
+			t.Fatalf("fill byte %d = %#x, want 0x07", i, b)
+		}
+	}
+	alpha := dst[fillSize:]
+	if got := sampleAlpha(alpha, width, width/2, height/2); got != alphaTransparent {
+		t.Errorf("square center = %d, want transparent %d", got, alphaTransparent)
+	}
+	if got := sampleAlpha(alpha, width, 10, height/2); got != alphaOpaque {
+		t.Errorf("outside square = %d, want opaque %d", got, alphaOpaque)
+	}
+}
+
+func TestV210AGeneratorRestoresAlphaBase(t *testing.T) {
+	const width, height = 304, 200
+	g, err := NewV210AGenerator(&fakeFrameGenerator{}, width, height, true)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	dst := make([]byte, V210AFrameSize(width, height))
+	if err := g.GenerateFrame(dst, 0); err != nil {
+		t.Fatalf("GenerateFrame(0): %v", err)
+	}
+	alpha := dst[V210FrameSize(width, height):]
+	if got := sampleAlpha(alpha, width, width/2, height/2); got != alphaTransparent {
+		t.Fatalf("frame 0 center = %d, want transparent %d", got, alphaTransparent)
+	}
+
+	if err := g.GenerateFrame(dst, 79); err != nil {
+		t.Fatalf("GenerateFrame(79): %v", err)
+	}
+	if got := sampleAlpha(alpha, width, width/2, height/2); got != alphaOpaque {
+		t.Errorf("old square position = %d, want restored opaque %d", got, alphaOpaque)
+	}
+	if got := sampleAlpha(alpha, width, 250, height/2); got != alphaTransparent {
+		t.Errorf("new square position = %d, want transparent %d", got, alphaTransparent)
+	}
+}
+
+func TestV210AGeneratorCachesStaticFill(t *testing.T) {
+	const width, height = 100, 20
+	fill := &fakeFrameGenerator{}
+	g, err := NewV210AGenerator(fill, width, height, false)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	if fill.calls != 1 {
+		t.Fatalf("fill calls after construction = %d, want 1", fill.calls)
+	}
+
+	dst := make([]byte, V210AFrameSize(width, height))
+	for _, frameIndex := range []int{7, 19} {
+		if err := g.GenerateFrame(dst, frameIndex); err != nil {
+			t.Fatalf("GenerateFrame(%d): %v", frameIndex, err)
+		}
+	}
+	if fill.calls != 1 {
+		t.Errorf("fill calls after two frames = %d, want 1", fill.calls)
+	}
+	for i, b := range dst[:V210FrameSize(width, height)] {
+		if b != 0 {
+			t.Fatalf("cached fill byte %d = %#x, want frame-zero value 0", i, b)
+		}
+	}
+}
+
+func TestV210AGeneratorRegeneratesDynamicFill(t *testing.T) {
+	const width, height = 100, 20
+	fill := &fakeFrameGenerator{}
+	g, err := NewV210AGenerator(fill, width, height, true)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	if fill.calls != 0 {
+		t.Fatalf("fill calls after construction = %d, want 0", fill.calls)
+	}
+
+	dst := make([]byte, V210AFrameSize(width, height))
+	for _, frameIndex := range []int{7, 19} {
+		if err := g.GenerateFrame(dst, frameIndex); err != nil {
+			t.Fatalf("GenerateFrame(%d): %v", frameIndex, err)
+		}
+	}
+	if fill.calls != 2 {
+		t.Errorf("fill calls after two frames = %d, want 2", fill.calls)
+	}
+	if got := dst[0]; got != 19 {
+		t.Errorf("dynamic fill byte = %#x, want frame-index value 0x13", got)
+	}
+}
+
+func TestV210AGeneratorErrors(t *testing.T) {
+	if _, err := NewV210AGenerator(nil, 1920, 1080, false); err == nil || !strings.Contains(err.Error(), "nil") {
+		t.Fatalf("nil fill error = %v", err)
+	}
+
+	fillErr := errors.New("fill failed")
+	if _, err := NewV210AGenerator(&fakeFrameGenerator{generateErr: fillErr}, 100, 20, false); !errors.Is(err, fillErr) {
+		t.Fatalf("static fill initialization error = %v, want wrapped %v", err, fillErr)
+	}
+	g, err := NewV210AGenerator(&fakeFrameGenerator{generateErr: fillErr}, 100, 20, true)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	if err := g.GenerateFrame(make([]byte, V210AFrameSize(100, 20)-1), 0); err == nil || !strings.Contains(err.Error(), "too small") {
+		t.Fatalf("small destination error = %v", err)
+	}
+	if err := g.GenerateFrame(make([]byte, V210AFrameSize(100, 20)), 3); !errors.Is(err, fillErr) {
+		t.Fatalf("fill error = %v, want wrapped %v", err, fillErr)
+	}
+}
+
+func TestV210AGeneratorClosesFill(t *testing.T) {
+	closeErr := errors.New("close failed")
+	fill := &fakeFrameGenerator{closeErr: closeErr}
+	g, err := NewV210AGenerator(fill, 100, 20, false)
+	if err != nil {
+		t.Fatalf("NewV210AGenerator: %v", err)
+	}
+	if err := g.Close(); !errors.Is(err, closeErr) {
+		t.Fatalf("Close = %v, want %v", err, closeErr)
+	}
+	if !fill.closed {
+		t.Fatal("wrapped fill generator was not closed")
+	}
+}

internal/generator/wgpu.go

@@ -6,7 +6,6 @@ import (
 	"context"
 	"encoding/binary"
 	"fmt"
-	"os"
 	"time"
 
 	"github.com/gogpu/gputypes"
@@ -23,23 +22,26 @@ const wgpuWorkgroupSize = 64
 // buffer, the GPU DMAs it into a persistent host-visible buffer, and the
 // mapped contents are copied straight into the destination grain.
 type WGPUGenerator struct {
-	instance  *wgpu.Instance
+	instance         *wgpu.Instance
-	adapter   *wgpu.Adapter
+	adapter          *wgpu.Adapter
-	device    *wgpu.Device
+	device           *wgpu.Device
-	queue     *wgpu.Queue
+	queue            *wgpu.Queue
-	shader    *wgpu.ShaderModule
+	shader           *wgpu.ShaderModule
-	bgl       *wgpu.BindGroupLayout
+	bgl              *wgpu.BindGroupLayout
-	bg        *wgpu.BindGroup
+	bg               *wgpu.BindGroup
-	pl        *wgpu.PipelineLayout
+	pl               *wgpu.PipelineLayout
-	pipeline  *wgpu.ComputePipeline
+	pipeline         *wgpu.ComputePipeline
-	out       *wgpu.Buffer
+	out              *wgpu.Buffer
-	host      *wgpu.Buffer
+	host             *wgpu.Buffer
-	uniform   *wgpu.Buffer
+	uniform          *wgpu.Buffer
-	params    []byte
+	params           []byte
-	width     int
+	width            int
-	height    int
+	height           int
-	blocks    int
+	blocks           int
-	frameSize uint64
+	activeLineSize   int
+	lineSize         int
+	compactFrameSize uint64
+	frameSize        uint64
 }
 
 var _ FrameGenerator = (*WGPUGenerator)(nil)
@@ -47,17 +49,20 @@ var _ FrameGenerator = (*WGPUGenerator)(nil)
 // WGPUOption customizes NewWGPUGenerator.
 type WGPUOption func(*WGPUGenerator)
 
-func NewWGPUGenerator(width, height uint, kernelPath string, opts ...WGPUOption) (*WGPUGenerator, error) {
+func NewWGPUGenerator(width, height uint, wgsl string, opts ...WGPUOption) (*WGPUGenerator, error) {
 	g := &WGPUGenerator{
-		width:  int(width),
+		width:          int(width),
-		height: int(height),
+		height:         int(height),
-		blocks: int(width*height) / 6,
+		blocks:         ((int(width) + 5) / 6) * int(height),
-		params: make([]byte, 16),
+		activeLineSize: V210ActiveLineSize(int(width)),
+		lineSize:       V210LineSize(int(width)),
+		params:         make([]byte, 16),
 	}
 	for _, opt := range opts {
 		opt(g)
 	}
-	g.frameSize = uint64(g.blocks) * 16
+	g.compactFrameSize = uint64(g.activeLineSize * g.height)
+	g.frameSize = uint64(g.lineSize * g.height)
 	binary.LittleEndian.PutUint32(g.params[0:], uint32(width))
 	binary.LittleEndian.PutUint32(g.params[4:], uint32(height))
 
@@ -75,26 +80,21 @@ func NewWGPUGenerator(width, height uint, kernelPath string, opts ...WGPUOption)
 	}
 	g.queue = g.device.Queue()
 
-	wgsl, err := os.ReadFile(kernelPath)
-	if err != nil {
-		g.Close()
-		return nil, fmt.Errorf("wgpu: read kernel: %w", err)
-	}
 	if g.shader, err = g.device.CreateShaderModule(&wgpu.ShaderModuleDescriptor{
-		Label: "v210-shader", WGSL: string(wgsl),
+		Label: "v210-shader", WGSL: wgsl,
 	}); err != nil {
 		g.Close()
 		return nil, fmt.Errorf("wgpu: shader: %w", err)
 	}
 	if g.out, err = g.device.CreateBuffer(&wgpu.BufferDescriptor{
-		Label: "v210-out", Size: g.frameSize,
+		Label: "v210-out", Size: g.compactFrameSize,
 		Usage: wgpu.BufferUsageStorage | wgpu.BufferUsageCopySrc,
 	}); err != nil {
 		g.Close()
 		return nil, fmt.Errorf("wgpu: out buffer: %w", err)
 	}
 	if g.host, err = g.device.CreateBuffer(&wgpu.BufferDescriptor{
-		Label: "v210-host", Size: g.frameSize,
+		Label: "v210-host", Size: g.compactFrameSize,
 		Usage: wgpu.BufferUsageCopyDst | wgpu.BufferUsageMapRead,
 	}); err != nil {
 		g.Close()
@@ -124,7 +124,7 @@ func NewWGPUGenerator(width, height uint, kernelPath string, opts ...WGPUOption)
 	if g.bg, err = g.device.CreateBindGroup(&wgpu.BindGroupDescriptor{
 		Label: "v210-bg", Layout: g.bgl,
 		Entries: []wgpu.BindGroupEntry{
-			{Binding: 0, Buffer: g.out, Size: g.frameSize},
+			{Binding: 0, Buffer: g.out, Size: g.compactFrameSize},
 			{Binding: 1, Buffer: g.uniform, Size: uint64(len(g.params))},
 		},
 	}); err != nil {
@@ -170,7 +170,7 @@ func (g *WGPUGenerator) GenerateFrame(dest []byte, frameIndex int) error {
 	if err := pass.End(); err != nil {
 		return fmt.Errorf("wgpu: end pass: %w", err)
 	}
-	encoder.CopyBufferToBuffer(g.out, 0, g.host, 0, g.frameSize)
+	encoder.CopyBufferToBuffer(g.out, 0, g.host, 0, g.compactFrameSize)
 	cmd, err := encoder.Finish()
 	if err != nil {
 		return fmt.Errorf("wgpu: finish: %w", err)
@@ -181,15 +181,25 @@ func (g *WGPUGenerator) GenerateFrame(dest []byte, frameIndex int) error {
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
-	if err := g.host.Map(ctx, wgpu.MapModeRead, 0, g.frameSize); err != nil {
+	if err := g.host.Map(ctx, wgpu.MapModeRead, 0, g.compactFrameSize); err != nil {
 		return fmt.Errorf("wgpu: map: %w", err)
 	}
-	rng, err := g.host.MappedRange(0, g.frameSize)
+	rng, err := g.host.MappedRange(0, g.compactFrameSize)
 	if err != nil {
 		_ = g.host.Unmap()
 		return fmt.Errorf("wgpu: mapped range: %w", err)
 	}
-	copy(dest, rng.Bytes())
+	mapped := rng.Bytes()
+	if g.activeLineSize == g.lineSize {
+		copy(dest[:g.frameSize], mapped)
+	} else {
+		for y := 0; y < g.height; y++ {
+			src := mapped[y*g.activeLineSize : (y+1)*g.activeLineSize]
+			dst := dest[y*g.lineSize : (y+1)*g.lineSize]
+			copy(dst, src)
+			clear(dst[g.activeLineSize:])
+		}
+	}
 	return g.host.Unmap()
 }
 

internal/generator/wgpu_move_test.go

@@ -1,13 +1,18 @@
 package generator
 
 import (
-	"path/filepath"
 	"testing"
+
+	"mxl-pattern-generator/kernels"
 )
 
 func TestWGPUMoveSquare(t *testing.T) {
 	const width, height = 1920, 1080
-	g, err := NewWGPUGenerator(width, height, filepath.Join("..", "..", "kernels", "dynamic", "smpteBars.wgsl"))
+	shader, err := kernels.Read("dynamic/smpteBars.wgsl")
+	if err != nil {
+		t.Fatalf("read shader: %v", err)
+	}
+	g, err := NewWGPUGenerator(width, height, string(shader))
 	if err != nil {
 		t.Fatalf("init: %v", err)
 	}

internal/generator/wgpu_test.go

@@ -1,13 +1,18 @@
 package generator
 
 import (
-	"path/filepath"
 	"testing"
+
+	"mxl-pattern-generator/kernels"
 )
 
 func TestWGPUSMPTEPattern(t *testing.T) {
 	const width, height = 1920, 1080
-	g, err := NewWGPUGenerator(width, height, filepath.Join("..", "..", "kernels", "static", "smpteBars.wgsl"))
+	shader, err := kernels.Read("static/smpteBars.wgsl")
+	if err != nil {
+		t.Fatalf("read shader: %v", err)
+	}
+	g, err := NewWGPUGenerator(width, height, string(shader))
 	if err != nil {
 		t.Fatalf("init: %v", err)
 	}
@@ -49,3 +54,37 @@ func TestWGPUSMPTEPattern(t *testing.T) {
 		})
 	}
 }
+
+func TestWGPUGeneratorUsesPaddedV210Rows(t *testing.T) {
+	const width, height = 100, 2
+	shader, err := kernels.Read("static/ebu75.wgsl")
+	if err != nil {
+		t.Fatalf("read shader: %v", err)
+	}
+	g, err := NewWGPUGenerator(width, height, string(shader))
+	if err != nil {
+		t.Fatalf("init: %v", err)
+	}
+	defer g.Close()
+
+	buf := make([]byte, V210FrameSize(width, height))
+	for i := range buf {
+		buf[i] = 0xff
+	}
+	if err := g.GenerateFrame(buf, 0); err != nil {
+		t.Fatalf("GenerateFrame: %v", err)
+	}
+
+	for y := 0; y < height; y++ {
+		gotY, gotCb, gotCr := sampleV210(buf, width, 0, y)
+		if gotY != 721 || gotCb != 512 || gotCr != 512 {
+			t.Fatalf("row %d first pixel = %d/%d/%d, want 721/512/512", y, gotY, gotCb, gotCr)
+		}
+		padding := buf[y*V210LineSize(width)+V210ActiveLineSize(width) : (y+1)*V210LineSize(width)]
+		for i, b := range padding {
+			if b != 0 {
+				t.Fatalf("row %d padding byte %d = %#x, want 0", y, i, b)
+			}
+		}
+	}
+}

internal/video/config.go

@@ -6,10 +6,19 @@ import (
 	"github.com/qvest-digital/go-mxl/mxl"
 )
 
+type Backend string
+
+const (
+	BackendAuto Backend = "auto"
+	BackendGPU  Backend = "gpu"
+	BackendCPU  Backend = "cpu"
+)
+
 type Config struct {
 	Definition flowdef.Video
 	Pattern    string
 	Overlay    OverlayConfig
+	Backend    Backend
 }
 
 type OverlayConfig struct {
@@ -31,6 +40,10 @@ func (c Config) Height() uint {
 	return c.Definition.FrameHeight
 }
 
+func (c Config) HasAlpha() bool {
+	return c.Definition.MediaType == flowdef.MediaTypeV210A
+}
+
 func (c Config) Rate() mxl.Rational {
 	return mxl.Rational{
 		Num: int64(c.Definition.GrainRate.Numerator),

internal/video/generator.go

@@ -0,0 +1,89 @@
+package video
+
+import (
+	"errors"
+	"fmt"
+	"log"
+
+	"mxl-pattern-generator/internal/generator"
+)
+
+type generatorFactory func() (generator.FrameGenerator, error)
+
+func newFrameGenerator(cfg Config, pattern pattern) (generator.FrameGenerator, Backend, error) {
+	newGPU := func() (generator.FrameGenerator, error) {
+		return generator.NewWGPUGenerator(cfg.Width(), cfg.Height(), pattern.kernelSource)
+	}
+	newCPU := func() (generator.FrameGenerator, error) {
+		return generator.NewCPUPatternGenerator(cfg.Width(), cfg.Height(), cfg.Pattern)
+	}
+	fill, backend, err := selectFrameGenerator(cfg.Backend, newGPU, newCPU)
+	if err != nil {
+		return nil, "", err
+	}
+	gen, err := wrapAlphaGenerator(cfg, fill, pattern.dynamic)
+	if err != nil {
+		return nil, "", err
+	}
+	return gen, backend, nil
+}
+
+func wrapAlphaGenerator(
+	cfg Config,
+	fill generator.FrameGenerator,
+	fillDynamic bool,
+) (generator.FrameGenerator, error) {
+	if !cfg.HasAlpha() {
+		return fill, nil
+	}
+
+	gen, err := generator.NewV210AGenerator(fill, cfg.Width(), cfg.Height(), fillDynamic)
+	if err != nil {
+		closeErr := fill.Close()
+		return nil, errors.Join(
+			fmt.Errorf("initialize v210a generator: %w", err),
+			closeErr,
+		)
+	}
+	return gen, nil
+}
+
+func selectFrameGenerator(
+	backend Backend,
+	newGPU, newCPU generatorFactory,
+) (generator.FrameGenerator, Backend, error) {
+	switch backend {
+	case BackendGPU:
+		gen, err := newGPU()
+		if err != nil {
+			return nil, "", fmt.Errorf("initialize GPU video generator: %w", err)
+		}
+		return gen, BackendGPU, nil
+
+	case BackendCPU:
+		gen, err := newCPU()
+		if err != nil {
+			return nil, "", fmt.Errorf("initialize CPU video generator: %w", err)
+		}
+		return gen, BackendCPU, nil
+
+	case "", BackendAuto:
+		gpuGen, gpuErr := newGPU()
+		if gpuErr == nil {
+			return gpuGen, BackendGPU, nil
+		}
+		log.Printf("GPU video generator unavailable, falling back to CPU: %v", gpuErr)
+
+		cpuGen, cpuErr := newCPU()
+		if cpuErr == nil {
+			return cpuGen, BackendCPU, nil
+		}
+		return nil, "", errors.Join(
+			fmt.Errorf("initialize GPU video generator: %w", gpuErr),
+			fmt.Errorf("initialize CPU video generator: %w", cpuErr),
+		)
+
+	default:
+		return nil, "", fmt.Errorf("unsupported video backend %q", backend)
+	}
+}

internal/video/generator_test.go

@@ -0,0 +1,178 @@
+package video
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"mxl-pattern-generator/internal/flowdef"
+	"mxl-pattern-generator/internal/generator"
+)
+
+type stubFrameGenerator struct{}
+
+func (*stubFrameGenerator) GenerateFrame([]byte, int) error { return nil }
+func (*stubFrameGenerator) Close() error                    { return nil }
+
+func TestSelectFrameGenerator(t *testing.T) {
+	gpuErr := errors.New("no GPU")
+	cpuErr := errors.New("no CPU pattern")
+	tests := []struct {
+		name          string
+		backend       Backend
+		gpuErr        error
+		cpuErr        error
+		wantBackend   Backend
+		wantGPUCalls  int
+		wantCPUCalls  int
+		wantErrSubstr []string
+	}{
+		{name: "explicit GPU", backend: BackendGPU, wantBackend: BackendGPU, wantGPUCalls: 1},
+		{name: "explicit CPU", backend: BackendCPU, wantBackend: BackendCPU, wantCPUCalls: 1},
+		{name: "auto prefers GPU", backend: BackendAuto, wantBackend: BackendGPU, wantGPUCalls: 1},
+		{name: "zero value is auto", wantBackend: BackendGPU, wantGPUCalls: 1},
+		{
+			name:         "auto falls back to CPU",
+			backend:      BackendAuto,
+			gpuErr:       gpuErr,
+			wantBackend:  BackendCPU,
+			wantGPUCalls: 1,
+			wantCPUCalls: 1,
+		},
+		{
+			name:          "auto reports both failures",
+			backend:       BackendAuto,
+			gpuErr:        gpuErr,
+			cpuErr:        cpuErr,
+			wantGPUCalls:  1,
+			wantCPUCalls:  1,
+			wantErrSubstr: []string{"GPU video generator", "CPU video generator"},
+		},
+		{
+			name:          "explicit GPU does not fall back",
+			backend:       BackendGPU,
+			gpuErr:        gpuErr,
+			wantGPUCalls:  1,
+			wantErrSubstr: []string{"GPU video generator"},
+		},
+		{
+			name:          "explicit CPU does not try GPU",
+			backend:       BackendCPU,
+			cpuErr:        cpuErr,
+			wantCPUCalls:  1,
+			wantErrSubstr: []string{"CPU video generator"},
+		},
+		{
+			name:          "invalid backend",
+			backend:       Backend("invalid"),
+			wantErrSubstr: []string{"unsupported video backend"},
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			gpuCalls, cpuCalls := 0, 0
+			factory := func(calls *int, err error) generatorFactory {
+				return func() (generator.FrameGenerator, error) {
+					*calls++
+					if err != nil {
+						return nil, err
+					}
+					return &stubFrameGenerator{}, nil
+				}
+			}
+
+			gen, backend, err := selectFrameGenerator(
+				tc.backend,
+				factory(&gpuCalls, tc.gpuErr),
+				factory(&cpuCalls, tc.cpuErr),
+			)
+			if gpuCalls != tc.wantGPUCalls || cpuCalls != tc.wantCPUCalls {
+				t.Fatalf("factory calls GPU/CPU = %d/%d, want %d/%d",
+					gpuCalls, cpuCalls, tc.wantGPUCalls, tc.wantCPUCalls)
+			}
+			if len(tc.wantErrSubstr) == 0 {
+				if err != nil {
+					t.Fatalf("selectFrameGenerator: %v", err)
+				}
+				if gen == nil || backend != tc.wantBackend {
+					t.Fatalf("generator/backend = %v/%q, want non-nil/%q", gen, backend, tc.wantBackend)
+				}
+				return
+			}
+			if err == nil {
+				t.Fatal("selectFrameGenerator returned nil error")
+			}
+			for _, substring := range tc.wantErrSubstr {
+				if !strings.Contains(err.Error(), substring) {
+					t.Errorf("error = %q, want substring %q", err, substring)
+				}
+			}
+		})
+	}
+}
+
+func TestWrapAlphaGenerator(t *testing.T) {
+	const id = "5fbec3b1-1b0f-417d-9059-8b94a47197ed"
+	rate := flowdef.Rational{Numerator: 25, Denominator: 1}
+	tests := []struct {
+		name      string
+		alpha     bool
+		wantAlpha bool
+	}{
+		{name: "v210"},
+		{name: "v210a", alpha: true, wantAlpha: true},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			var (
+				definition flowdef.Video
+				err        error
+			)
+			if tc.alpha {
+				definition, err = flowdef.NewV210AVideo(id, 100, 200, rate)
+			} else {
+				definition, err = flowdef.NewV210Video(id, 100, 200, rate)
+			}
+			if err != nil {
+				t.Fatalf("create flow definition: %v", err)
+			}
+			cfg := Config{
+				Definition: definition,
+				Pattern:    "gray-ramp",
+				Backend:    BackendCPU,
+			}
+			fill := &stubFrameGenerator{}
+
+			gen, err := wrapAlphaGenerator(cfg, fill, false)
+			if err != nil {
+				t.Fatalf("wrapAlphaGenerator: %v", err)
+			}
+			defer gen.Close()
+			_, gotAlpha := gen.(*generator.V210AGenerator)
+			if gotAlpha != tc.wantAlpha {
+				t.Errorf("v210a wrapper present = %v, want %v", gotAlpha, tc.wantAlpha)
+			}
+			if !tc.wantAlpha && gen != fill {
+				t.Error("v210 generator was unexpectedly replaced")
+			}
+		})
+	}
+}
+
+func TestConfigHasAlpha(t *testing.T) {
+	for _, tc := range []struct {
+		mediaType string
+		want      bool
+	}{
+		{mediaType: flowdef.MediaTypeV210},
+		{mediaType: flowdef.MediaTypeV210A, want: true},
+		{mediaType: "video/unknown"},
+	} {
+		cfg := Config{Definition: flowdef.Video{Common: flowdef.Common{MediaType: tc.mediaType}}}
+		if got := cfg.HasAlpha(); got != tc.want {
+			t.Errorf("HasAlpha() for %q = %v, want %v", tc.mediaType, got, tc.want)
+		}
+	}
+}

internal/video/pattern.go

@@ -1,6 +1,11 @@
 package video
 
-import "sort"
+import (
+	"fmt"
+	"sort"
+
+	"mxl-pattern-generator/kernels"
+)
 
 type PatternInfo struct {
 	Name        string
@@ -9,52 +14,66 @@ type PatternInfo struct {
 
 type pattern struct {
 	PatternInfo
-	kernelPath string
+	kernelSource string
+	dynamic      bool
 }
 
 var patterns = map[string]pattern{
 	"ebu75": {
-		PatternInfo: PatternInfo{Name: "ebu75", Description: "EBU 75% Color Bar Signal"},
+		PatternInfo:  PatternInfo{Name: "ebu75", Description: "EBU 75% Color Bar Signal"},
-		kernelPath:  "kernels/static/ebu75.wgsl",
+		kernelSource: mustReadKernel("static/ebu75.wgsl"),
 	},
 	"ebu75-move": {
-		PatternInfo: PatternInfo{Name: "ebu75-move", Description: "EBU 75% Color Bar Signal with moving square"},
+		PatternInfo:  PatternInfo{Name: "ebu75-move", Description: "EBU 75% Color Bar Signal with moving square"},
-		kernelPath:  "kernels/dynamic/ebu75.wgsl",
+		kernelSource: mustReadKernel("dynamic/ebu75.wgsl"),
+		dynamic:      true,
 	},
 	"ebu100": {
-		PatternInfo: PatternInfo{Name: "ebu100", Description: "EBU 100% Color Bar Signal"},
+		PatternInfo:  PatternInfo{Name: "ebu100", Description: "EBU 100% Color Bar Signal"},
-		kernelPath:  "kernels/static/ebu100.wgsl",
+		kernelSource: mustReadKernel("static/ebu100.wgsl"),
 	},
 	"ebu100-move": {
-		PatternInfo: PatternInfo{Name: "ebu100-move", Description: "EBU 100% Color Bar Signal with moving square"},
+		PatternInfo:  PatternInfo{Name: "ebu100-move", Description: "EBU 100% Color Bar Signal with moving square"},
-		kernelPath:  "kernels/dynamic/ebu100.wgsl",
+		kernelSource: mustReadKernel("dynamic/ebu100.wgsl"),
+		dynamic:      true,
 	},
 	"smpte": {
-		PatternInfo: PatternInfo{Name: "smpte", Description: "SMPTE RP-219 Color Bar Signal"},
+		PatternInfo:  PatternInfo{Name: "smpte", Description: "SMPTE RP-219 Color Bar Signal"},
-		kernelPath:  "kernels/static/smpteBars.wgsl",
+		kernelSource: mustReadKernel("static/smpteBars.wgsl"),
 	},
 	"smpte-move": {
-		PatternInfo: PatternInfo{Name: "smpte-move", Description: "SMPTE RP-219 Color Bar Signal with moving square"},
+		PatternInfo:  PatternInfo{Name: "smpte-move", Description: "SMPTE RP-219 Color Bar Signal with moving square"},
-		kernelPath:  "kernels/dynamic/smpteBars.wgsl",
+		kernelSource: mustReadKernel("dynamic/smpteBars.wgsl"),
+		dynamic:      true,
 	},
 	"gray-bars": {
-		PatternInfo: PatternInfo{Name: "gray-bars", Description: "13-step grayscale bars (Y 64..940)"},
+		PatternInfo:  PatternInfo{Name: "gray-bars", Description: "13-step grayscale bars (Y 64..940)"},
-		kernelPath:  "kernels/static/yBars.wgsl",
+		kernelSource: mustReadKernel("static/yBars.wgsl"),
 	},
 	"gray-bars-move": {
-		PatternInfo: PatternInfo{Name: "gray-bars-move", Description: "13-step grayscale bars (Y 64..940) with moving square"},
+		PatternInfo:  PatternInfo{Name: "gray-bars-move", Description: "13-step grayscale bars (Y 64..940) with moving square"},
-		kernelPath:  "kernels/dynamic/yBars.wgsl",
+		kernelSource: mustReadKernel("dynamic/yBars.wgsl"),
+		dynamic:      true,
 	},
 	"gray-ramp": {
-		PatternInfo: PatternInfo{Name: "gray-ramp", Description: "Y gradient (black -> 100% white)"},
+		PatternInfo:  PatternInfo{Name: "gray-ramp", Description: "Y gradient (black -> 100% white)"},
-		kernelPath:  "kernels/static/yRamp.wgsl",
+		kernelSource: mustReadKernel("static/yRamp.wgsl"),
 	},
 	"gray-ramp-move": {
-		PatternInfo: PatternInfo{Name: "gray-ramp-move", Description: "Y gradient with moving square"},
+		PatternInfo:  PatternInfo{Name: "gray-ramp-move", Description: "Y gradient with moving square"},
-		kernelPath:  "kernels/dynamic/yRamp.wgsl",
+		kernelSource: mustReadKernel("dynamic/yRamp.wgsl"),
+		dynamic:      true,
 	},
 }
 
+func mustReadKernel(name string) string {
+	source, err := kernels.Read(name)
+	if err != nil {
+		panic(fmt.Sprintf("read embedded video kernel %q: %v", name, err))
+	}
+	return string(source)
+}
+
 func HasPattern(name string) bool {
 	_, ok := patterns[name]
 	return ok

internal/video/pattern_test.go

@@ -0,0 +1,54 @@
+package video
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestPatterns(t *testing.T) {
+	got := Patterns()
+	if len(got) == 0 {
+		t.Fatal("Patterns returned no video patterns")
+	}
+
+	seen := make(map[string]struct{}, len(got))
+	for i, pattern := range got {
+		if pattern.Name == "" {
+			t.Fatalf("pattern %d has an empty name", i)
+		}
+		if pattern.Description == "" {
+			t.Fatalf("pattern %q has an empty description", pattern.Name)
+		}
+		if !HasPattern(pattern.Name) {
+			t.Fatalf("Patterns returned %q, but HasPattern rejected it", pattern.Name)
+		}
+		if _, exists := seen[pattern.Name]; exists {
+			t.Fatalf("duplicate pattern name %q", pattern.Name)
+		}
+		seen[pattern.Name] = struct{}{}
+		if i > 0 && got[i-1].Name >= pattern.Name {
+			t.Fatalf("patterns are not sorted: %q appears before %q", got[i-1].Name, pattern.Name)
+		}
+	}
+}
+
+func TestHasPatternRejectsUnknownName(t *testing.T) {
+	if HasPattern("not-a-pattern") {
+		t.Fatal("HasPattern accepted an unknown pattern")
+	}
+}
+
+func TestPatternRegistryKeysMatchNames(t *testing.T) {
+	for name, pattern := range patterns {
+		if pattern.Name != name {
+			t.Errorf("pattern map key %q does not match pattern name %q", name, pattern.Name)
+		}
+		if pattern.kernelSource == "" {
+			t.Errorf("pattern %q has empty kernel source", name)
+		}
+		wantDynamic := strings.HasSuffix(name, "-move")
+		if pattern.dynamic != wantDynamic {
+			t.Errorf("pattern %q dynamic = %t, want %t", name, pattern.dynamic, wantDynamic)
+		}
+	}
+}

internal/video/runner.go

@@ -6,6 +6,8 @@ import (
 	"errors"
 	"fmt"
 	"log"
+
+	"mxl-pattern-generator/assets"
 	"mxl-pattern-generator/internal/generator"
 
 	"github.com/qvest-digital/go-mxl/mxl"
@@ -17,11 +19,11 @@ func Run(ctx context.Context, inst *mxl.Instance, cfg Config) (runErr error) {
 		return fmt.Errorf("unknown video pattern %q", cfg.Pattern)
 	}
 
-	// TODO: fall back to a CPU generator if GPU initialization fails.
+	gen, backend, err := newFrameGenerator(cfg, pattern)
-	gen, err := generator.NewWGPUGenerator(cfg.Width(), cfg.Height(), pattern.kernelPath)
 	if err != nil {
-		return fmt.Errorf("initialize wgpu video generator: %w", err)
+		return err
 	}
+	log.Printf("video generator backend: %s", backend)
 	defer func() {
 		if err := gen.Close(); err != nil {
 			runErr = errors.Join(runErr, fmt.Errorf("close video generator: %w", err))
@@ -33,6 +35,21 @@ func Run(ctx context.Context, inst *mxl.Instance, cfg Config) (runErr error) {
 		return err
 	}
 
+	var staticFrame []byte
+	if !pattern.dynamic && !cfg.HasAlpha() {
+		frameSize := generator.V210FrameSize(int(cfg.Width()), int(cfg.Height()))
+		staticFrame = make([]byte, frameSize)
+
+		if err := gen.GenerateFrame(staticFrame, 0); err != nil {
+			return fmt.Errorf("generate static frame: %w", err)
+		}
+		if overlay != nil {
+			if err := overlay.ApplyV210(staticFrame); err != nil {
+				return fmt.Errorf("apply text overlay to static frame: %w", err)
+			}
+		}
+	}
+
 	flowJSON, err := json.Marshal(cfg.Definition)
 	if err != nil {
 		return fmt.Errorf("marshal video flow definition: %w", err)
@@ -68,13 +85,25 @@ func Run(ctx context.Context, inst *mxl.Instance, cfg Config) (runErr error) {
 		if err != nil {
 			return fmt.Errorf("open video grain %d: %w", idx, err)
 		}
-		if err := gen.GenerateFrame(grain.Payload, int(tick)); err != nil {
+		if staticFrame != nil {
-			return cancelGrain(grain, fmt.Errorf("generate frame for grain %d: %w", idx, err))
+			if len(grain.Payload) != len(staticFrame) {
-		}
+				return cancelGrain(grain, fmt.Errorf(
-		if overlay != nil {
+					"video grain payload size %d, expected %d",
-			if err := overlay.ApplyV210(grain.Payload); err != nil {
+					len(grain.Payload),
-				return cancelGrain(grain, fmt.Errorf("apply text overlay to grain %d: %w", idx, err))
+					len(staticFrame),
+				))
 			}
+			copy(grain.Payload, staticFrame)
+		} else {
+			if err := gen.GenerateFrame(grain.Payload, int(tick)); err != nil {
+				return cancelGrain(grain, fmt.Errorf("generate frame for grain %d: %w", idx, err))
+			}
+			if overlay != nil {
+				if err := overlay.ApplyV210(grain.Payload); err != nil {
+					return cancelGrain(grain, fmt.Errorf("apply text overlay to grain %d: %w", idx, err))
+				}
+			}
+			tick++
 		}
 		if err := grain.Commit(grain.TotalSlices, 0); err != nil {
 			return fmt.Errorf("commit video grain %d: %w", idx, err)
@@ -82,7 +111,6 @@ func Run(ctx context.Context, inst *mxl.Instance, cfg Config) (runErr error) {
 
 		grainsWritten++
 		idx++
-		tick++
 		if grainsWritten%100 == 0 {
 			log.Printf("video grains written=%d, next index=%d", grainsWritten, idx)
 		}
@@ -101,13 +129,8 @@ func buildTextOverlay(cfg Config) (overlay *generator.TextOverlay, resultErr err
 	if cfg.Overlay.Text == "" {
 		return nil, nil
 	}
-	if cfg.Overlay.X < 0 || cfg.Overlay.X > int(cfg.Width()) ||
-		cfg.Overlay.Y < 0 || cfg.Overlay.Y > int(cfg.Height()) {
-		return nil, fmt.Errorf("text overlay position (%d, %d) is outside the %dx%d video frame",
-			cfg.Overlay.X, cfg.Overlay.Y, cfg.Width(), cfg.Height())
-	}
 
-	face, err := generator.LoadFace("assets/fonts/JetBrainsMonoNLNerdFontMono-Regular.ttf", 48)
+	face, err := generator.NewFace(assets.JetBrainsMono, 48)
 	if err != nil {
 		return nil, fmt.Errorf("load text overlay font: %w", err)
 	}

internal/video/runner_test.go

@@ -0,0 +1,77 @@
+package video
+
+import (
+	"strings"
+	"testing"
+
+	"mxl-pattern-generator/internal/flowdef"
+)
+
+const testVideoID = "b3bb5be7-9fe9-4324-a5bb-4c70e1084449"
+
+func testConfig(t *testing.T, overlay OverlayConfig) Config {
+	t.Helper()
+	definition, err := flowdef.NewV210Video(
+		testVideoID,
+		1920,
+		1080,
+		flowdef.Rational{Numerator: 50, Denominator: 1},
+	)
+	if err != nil {
+		t.Fatalf("NewV210Video: %v", err)
+	}
+	return Config{Definition: definition, Pattern: "ebu75", Overlay: overlay}
+}
+
+func TestBuildTextOverlayDisabled(t *testing.T) {
+	overlay, err := buildTextOverlay(testConfig(t, OverlayConfig{}))
+	if err != nil {
+		t.Fatalf("buildTextOverlay: %v", err)
+	}
+	if overlay != nil {
+		t.Fatal("buildTextOverlay returned an overlay for empty text")
+	}
+}
+
+func TestBuildTextOverlayPositioning(t *testing.T) {
+	tests := []struct {
+		name    string
+		overlay OverlayConfig
+	}{
+		{name: "explicit", overlay: OverlayConfig{Text: "MXL", X: 120, Y: 48}},
+		{name: "preset", overlay: OverlayConfig{Text: "MXL", Position: "cc"}},
+		{name: "preset ignores explicit coordinates", overlay: OverlayConfig{Text: "MXL", X: -1, Y: -1, Position: "cc"}},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			overlay, err := buildTextOverlay(testConfig(t, tc.overlay))
+			if err != nil {
+				t.Fatalf("buildTextOverlay: %v", err)
+			}
+			if overlay == nil {
+				t.Fatal("buildTextOverlay returned nil")
+			}
+		})
+	}
+}
+
+func TestBuildTextOverlayRejectsInvalidPosition(t *testing.T) {
+	tests := []struct {
+		name       string
+		overlay    OverlayConfig
+		wantErrSub string
+	}{
+		{name: "negative", overlay: OverlayConfig{Text: "MXL", X: -6}, wantErrSub: "must not be negative"},
+		{name: "unaligned x", overlay: OverlayConfig{Text: "MXL", X: 7}, wantErrSub: "divisible by 6"},
+		{name: "right overflow", overlay: OverlayConfig{Text: "MXL", X: 1902}, wantErrSub: "does not fit"},
+		{name: "bottom overflow", overlay: OverlayConfig{Text: "MXL", Y: 1070}, wantErrSub: "does not fit"},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			_, err := buildTextOverlay(testConfig(t, tc.overlay))
+			if err == nil || !strings.Contains(err.Error(), tc.wantErrSub) {
+				t.Fatalf("error = %v, want substring %q", err, tc.wantErrSub)
+			}
+		})
+	}
+}

kernels/dynamic/ebu100.wgsl

@@ -18,13 +18,14 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let py = (block * 6u) / params.width;
+    let py = block / blocks_per_row;
 
     // Bar order: 100% white, yellow, cyan, green, magenta, red, blue, black
     let y_tab  = array<u32, 8>(940u, 877u, 754u, 691u, 313u, 250u, 127u, 64u);

kernels/dynamic/ebu75.wgsl

@@ -18,13 +18,14 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let py = (block * 6u) / params.width;
+    let py = block / blocks_per_row;
 
     // Bar order: 75% white, yellow, cyan, green, magenta, red, blue, black
     let y_tab  = array<u32, 8>(721u, 674u, 581u, 534u, 251u, 204u, 111u, 64u);

kernels/dynamic/smpteBars.wgsl

@@ -18,13 +18,14 @@ fn bar_index(px: u32, bars_75_width: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let y_px = (block * 6u) / params.width;
+    let y_px = block / blocks_per_row;
 
     // 75% Color Table: white, yellow, cyan, green, magenta, red, blue
     let y_tab  = array<u32, 7>(721u, 674u, 581u, 534u, 251u, 204u, 111u);

kernels/dynamic/yBars.wgsl

@@ -19,15 +19,16 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
     let y_tab  = array<u32, 13>(64u, 137u, 210u, 283u, 356u, 429u, 502u, 575u, 648u, 721u, 794u, 867u, 940u);
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let py = (block * 6u) / params.width;
+    let py = block / blocks_per_row;
     // Moving square: horizontal oscillation around screen center.
     // frame is a small tick; converting the huge raw grain index here
     // would destroy f32 precision and freeze the motion.

kernels/dynamic/yRamp.wgsl

@@ -18,13 +18,14 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let py = (block * 6u) / params.width;
+    let py = block / blocks_per_row;
 
     // Bar order: 75% white, yellow, cyan, green, magenta, red, blue
     let y_tab  = array<u32, 7>(721u, 674u, 581u, 534u, 251u, 204u, 111u);

kernels/embed.go

@@ -0,0 +1,12 @@
+package kernels
+
+import "embed"
+
+// files contains every built-in video pattern shader.
+//
+//go:embed static/*.wgsl dynamic/*.wgsl
+var files embed.FS
+
+func Read(name string) ([]byte, error) {
+	return files.ReadFile(name)
+}

kernels/static/ebu100.wgsl

@@ -18,12 +18,13 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
 
     // Bar order: 100% white, yellow, cyan, green, magenta, red, blue, black
     let y_tab  = array<u32, 8>(940u, 877u, 754u, 691u, 313u, 250u, 127u, 64u);

kernels/static/ebu75.wgsl

@@ -18,12 +18,13 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
 
     // Bar order: 75% white, yellow, cyan, green, magenta, red, blue, black
     let y_tab  = array<u32, 8>(721u, 674u, 581u, 534u, 251u, 204u, 111u, 64u);

kernels/static/smpteBars.wgsl

@@ -18,13 +18,14 @@ fn bar_index(px: u32, bars_75_width: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
-    let y_px = (block * 6u) / params.width;
+    let y_px = block / blocks_per_row;
 
     // 75% Color Table: white, yellow, cyan, green, magenta, red, blue
     let y_tab  = array<u32, 7>(721u, 674u, 581u, 534u, 251u, 204u, 111u);

kernels/static/yBars.wgsl

@@ -19,7 +19,8 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
@@ -27,7 +28,7 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let y_tab  = array<u32, 13>(64u, 137u, 210u, 283u, 356u, 429u, 502u, 575u, 648u, 721u, 794u, 867u, 940u);
 
     // 6 = pixels per v210 block (NOT the bar count)
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
     var y: array<u32, 6>;
     var cb: array<u32, 6>;
     var cr: array<u32, 6>;

kernels/static/yRamp.wgsl

@@ -18,7 +18,8 @@ fn bar_index(px: u32) -> u32 {
 @compute @workgroup_size(64)
 fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let block = gid.x;
-    let total = (params.width * params.height) / 6u;
+    let blocks_per_row = (params.width + 5u) / 6u;
+    let total = blocks_per_row * params.height;
     if (block >= total) {
         return;
     }
@@ -28,7 +29,7 @@ fn main(@builtin(global_invocation_id) gid: vec3<u32>) {
     let cb_tab = array<u32, 7>(512u, 176u, 589u, 253u, 771u, 435u, 848u);
     let cr_tab = array<u32, 7>(512u, 543u, 176u, 207u, 817u, 848u, 481u);
 
-    let x = (block * 6u) % params.width;
+    let x = (block % blocks_per_row) * 6u;
     var y: array<u32, 6>;
     var cb: array<u32, 6>;
     var cr: array<u32, 6>;