Text + MXL logo pattern

internal/generator/logo.go

@@ -0,0 +1,76 @@
+// Logo plane construction for texture-sampling kernels (v210_dvd_logo.wgsl).
+package generator
+
+import (
+	"fmt"
+	"image"
+	"math"
+	"os"
+
+	// PNG decoder registration for LoadLogoPNG.
+	_ "image/png"
+)
+
+// BuildLogoPlane converts an image into the packed logo plane consumed by
+// v210_dvd_logo.wgsl as read-only storage binding 2:
+//
+//	logo[0] = width | height<<16
+//	logo[1+dy*w+dx] = opaque<<30 | Cr<<20 | Cb<<10 | Y
+//
+// Rec.709 full-range RGB -> 10-bit studio swing, the same transform behind
+// the color-bar tables in the kernels (verified against all 21 bar values).
+// Semi-transparent pixels (alpha >= 50%) count as opaque; the rest show the
+// kernel background through.
+func BuildLogoPlane(img image.Image) ([]uint32, error) {
+	b := img.Bounds()
+	w, h := b.Dx(), b.Dy()
+	if w <= 0 || h <= 0 || w > 0xFFFF || h > 0xFFFF {
+		return nil, fmt.Errorf("logo: bad dimensions %dx%d", w, h)
+	}
+	plane := make([]uint32, 1+w*h)
+	plane[0] = uint32(w) | uint32(h)<<16
+	for yy := 0; yy < h; yy++ {
+		for xx := 0; xx < w; xx++ {
+			r, g, bl, a := img.At(b.Min.X+xx, b.Min.Y+yy).RGBA()
+			if a < 1<<15 {
+				continue // transparent: shader falls back to background
+			}
+			// At().RGBA() is premultiplied; unpremultiply to full-range RGB.
+			rf := float64(r) / float64(a)
+			gf := float64(g) / float64(a)
+			bf := float64(bl) / float64(a)
+			luma := 0.2126*rf + 0.7152*gf + 0.0722*bf
+			yc := clamp10(64 + math.Round(876*luma))
+			cb := clamp10(512 + math.Round(448*(bf-luma)/(1-0.0722)))
+			cr := clamp10(512 + math.Round(448*(rf-luma)/(1-0.2126)))
+			plane[1+yy*w+xx] = 1<<30 | cr<<20 | cb<<10 | yc
+		}
+	}
+	return plane, nil
+}
+
+// clamp10 keeps a 10-bit code inside the legal 4..1019 range.
+func clamp10(v float64) uint32 {
+	switch {
+	case v < 4:
+		return 4
+	case v > 1019:
+		return 1019
+	default:
+		return uint32(v)
+	}
+}
+
+// LoadLogoPNG reads a PNG file and builds the logo plane.
+func LoadLogoPNG(path string) ([]uint32, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, fmt.Errorf("logo: %w", err)
+	}
+	defer f.Close()
+	img, _, err := image.Decode(f)
+	if err != nil {
+		return nil, fmt.Errorf("logo: decode %s: %w", path, err)
+	}
+	return BuildLogoPlane(img)
+}