alpha patterns

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
+}