alpha patterns

internal/generator/v210a.go

@@ -0,0 +1,63 @@
+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
+	alphaBase []byte
+}
+
+var _ FrameGenerator = (*V210AGenerator)(nil)
+
+func NewV210AGenerator(
+	fill FrameGenerator,
+	width, height uint,
+) (*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)
+	}
+	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 err := g.fill.GenerateFrame(dst[:g.fillSize], frameIndex); err != nil {
+		return fmt.Errorf("v210a: generate 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()
+}