MXL Pattern Generator Architecture Review

Reviewed on 2026-09-16 against commit f37f480.

Summary

The current stack is a good fit for a test-pattern generator:

Go is well suited to CLI handling, MXL lifecycle management, timing, and synthetic audio generation.
go-mxl provides the correct data model: discrete grains for video and continuous samples for audio.
wgpu/WGSL is a reasonable portable GPU abstraction for producing v210 video.
CPU-rendered text is appropriate because text changes infrequently and font rendering does not need to be implemented in a shader.

The project is still prototype-quality in its orchestration and configuration layers. Before adding substantial audio functionality, flow-definition handling, lifecycle management, and the failing tests should be corrected.

Recommended audio architecture

Video and audio should run in separate goroutines. Each goroutine should own its own MXL writer and timing loop because video and audio have different units and rates:

run
  |-- shared context, signal handling, and error propagation
  |-- video goroutine
  |     OpenGrain -> render -> overlay -> Commit
  `-- audio goroutine
        OpenSamples -> synthesize -> Commit

This is also consistent with go-mxl: a writer is not safe for concurrent use, so a writer should belong to exactly one goroutine.

The goroutines should not synchronize by sending a notification for every video frame. That would make audio timing depend on GPU latency and Go scheduler jitter. Instead, both loops should use the MXL clock independently:

videoIndex := mxl.CurrentIndex(videoRate)
audioIndex := mxl.CurrentIndex(audioRate)

After committing data, each loop advances its own index and uses mxl.NsUntilIndex for pacing. Both loops should share a context.Context; an error in either flow should cancel the other flow and be returned to the caller.

For audio, start with CPU generation and approximately 10 ms batches (480 samples at 48 kHz). A sine wave is inexpensive to calculate, so GPU audio generation would add complexity without a useful performance benefit.

Audio phase should be derived from the absolute sample index:

phase := 2 * math.Pi * frequency *
    float64(firstSample+i) / sampleRate

This preserves continuity across batches and after a timing resynchronization.

Video and audio should expose different interfaces because a video frame and a range of audio samples are fundamentally different units:

type VideoGenerator interface {
    GenerateFrame(dst []byte, frameIndex uint64) error
    Close() error
}

type AudioGenerator interface {
    GenerateSamples(channels [][]byte, firstSample, sampleCount uint64) error
    Close() error
}

For audio/float32, the audio package may use []float32 internally and keep byte encoding at the MXL boundary.

Issue status

#	Issue	Status	Current observation
1	The validated `--domain` value was ignored in favor of `/dev/shm/mxl`.	Fixed	Logging, `mxl.NewInstance`, and reuse messages now use `args.domain`.
2	An external video flow definition does not populate runtime width, height, FPS, or UUID.	Fixed	External definitions are parsed and validated as `flowdef.Video`; `video.Config` supplies their dimensions, rate, and ID to the generator and writer.
3	`checkArgs` received `appArgs` by value, so generated UUIDs were discarded.	Fixed	Argument validation mutates the actual configuration, and the hard-coded video UUID has been removed.
4	The default pattern was `bars`, which did not exist.	Fixed	The default is now `ebu75`, which exists in the pattern registry.
5	`NewFlowDefJSON(TYPE_AUDIO, ...)` produces a video/v210 definition.	Fixed	The generic discriminator-based builder was removed. Video and audio have separate schema types, and video uses the typed `NewV210Video` constructor. An audio constructor will be added with audio generation.
6	The wgpu path was described as zero-copy although it performs GPU readback and a CPU copy.	Deferred — fix after audio	Every frame is copied from GPU storage to a mapped host buffer and then copied into the MXL payload. The path is synchronous and serial. This may require substantial benchmarking and architectural work, so audio implementation takes priority. Update the documentation now, but defer optimization or redesign until audio is complete.
7	The test suite had three failures.	Open	`TestNewTextOverlay`, `TestWGPUMoveSquare`, and `TestWGPUGenerator` still fail.
8	The Makefile clean target uses `fm -f` instead of `rm -f`.	Fixed	The clean target now uses `rm -f`.

Additional implementation priorities

Fix the existing tests or update incorrect expectations after confirming the intended color values and overlay positioning.
Introduce a run(...) error orchestration function using shared cancellation and error propagation instead of calling log.Fatalf throughout the media loop.
Add a typed audio flow-definition constructor, CPU audio generator, and continuous-flow writer loop using OpenSamples, ChannelFragments, and Commit.
After audio is complete, measure end-to-end frame time and missed deadlines at 1080p50/60 and UHD. The current wgpu path may be adequate, but it is neither zero-copy nor asynchronous. Treat GPU readback optimization as a separate, potentially large task.

Suggested package layout

cmd/mxl-pattern/
    main.go

internal/config/
    config.go
    flow.go

internal/video/
    generator.go
    wgpu.go
    overlay.go
    runner.go

internal/audio/
    generator.go
    sine.go
    runner.go

internal/app/
    run.go

The exact directory names are less important than keeping CLI parsing, typed configuration, media generation, and MXL writing as separate responsibilities.

Verification

The following command was used:

GOCACHE=/tmp/go-mxl-gen-cache go test ./...

Package compilation succeeds, but the generator package fails these tests:

TestNewTextOverlay: expected a centered text box, but its center was reported as 45 instead of approximately 960.
TestWGPUMoveSquare: tick 79 produced 590/512/512 instead of expected 893/176/543.
TestWGPUGenerator: pixel (0,0) produced Y=414 instead of expected Y=721.

Conclusion

Keep the chosen Go + go-mxl + wgpu stack. Implement audio on the CPU in its own goroutine and give video and audio separate writers, indices, and pacing loops. Coordinate them through a shared context and the common MXL timebase, not through per-frame messages. Typed external-video configuration is now implemented; the next architectural work is extracting the video runner and adding typed audio construction and generation. The three rendering-test failures remain a separate correctness task.

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