alpha patterns

internal/video/generator.go

@@ -17,7 +17,31 @@ func newFrameGenerator(cfg Config, pattern pattern) (generator.FrameGenerator, B
 	newCPU := func() (generator.FrameGenerator, error) {
 		return generator.NewCPUPatternGenerator(cfg.Width(), cfg.Height(), cfg.Pattern)
 	}
-	return selectFrameGenerator(cfg.Backend, newGPU, newCPU)
+	fill, backend, err := selectFrameGenerator(cfg.Backend, newGPU, newCPU)
+	if err != nil {
+		return nil, "", err
+	}
+	gen, err := wrapAlphaGenerator(cfg, fill)
+	if err != nil {
+		return nil, "", err
+	}
+	return gen, backend, nil
+}
+
+func wrapAlphaGenerator(cfg Config, fill generator.FrameGenerator) (generator.FrameGenerator, error) {
+	if !cfg.HasAlpha() {
+		return fill, nil
+	}
+
+	gen, err := generator.NewV210AGenerator(fill, cfg.Width(), cfg.Height())
+	if err != nil {
+		closeErr := fill.Close()
+		return nil, errors.Join(
+			fmt.Errorf("initialize v210a generator: %w", err),
+			closeErr,
+		)
+	}
+	return gen, nil
 }
 
 func selectFrameGenerator(