alpha patterns

internal/generator/alpha_test.go

@@ -0,0 +1,161 @@
+package generator
+
+import (
+	"encoding/binary"
+	"strings"
+	"testing"
+)
+
+func TestPackAlphaBlock(t *testing.T) {
+	var dst [4]byte
+	packAlphaBlock(dst[:], [3]uint32{64, 512, 940})
+
+	word := binary.LittleEndian.Uint32(dst[:])
+
+	if got := word & 0x3ff; got != 64 {
+		t.Errorf("sample 0 = %d, want 64", got)
+	}
+	if got := (word >> 10) & 0x3ff; got != 512 {
+		t.Errorf("sample 1 = %d, want 512", got)
+	}
+	if got := (word >> 20) & 0x3ff; got != 940 {
+		t.Errorf("sample 2 = %d, want 940", got)
+	}
+	if got := word >> 30; got != 0 {
+		t.Errorf("unused bits = %d, want 0", got)
+	}
+}
+
+func TestPackAlphaBlockMasksSamples(t *testing.T) {
+	var dst [4]byte
+	packAlphaBlock(dst[:], [3]uint32{0x401, 0x802, 0xc03})
+
+	word := binary.LittleEndian.Uint32(dst[:])
+
+	if got := word & 0x3ff; got != 1 {
+		t.Errorf("sample 0 = %d, want 1", got)
+	}
+	if got := (word >> 10) & 0x3ff; got != 2 {
+		t.Errorf("sample 1 = %d, want 2", got)
+	}
+	if got := (word >> 20) & 0x3ff; got != 3 {
+		t.Errorf("sample 2 = %d, want 3", got)
+	}
+}
+
+func TestFillAlphaPlaneCompleteBlocks(t *testing.T) {
+	const width, height = 6, 2
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			if got := sampleAlpha(dst, width, x, y); got != alphaOpaque {
+				t.Errorf("sample (%d,%d) = %d, want %d", x, y, got, alphaOpaque)
+			}
+		}
+	}
+}
+
+func TestFillAlphaPlaneZerosPartialBlockPadding(t *testing.T) {
+	const width, height = 4, 2
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaTransparent); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+
+	stride := AlphaLineSize(width)
+	for y := 0; y < height; y++ {
+		for x := 0; x < width; x++ {
+			if got := sampleAlpha(dst, width, x, y); got != alphaTransparent {
+				t.Errorf("sample (%d,%d) = %d, want %d", x, y, got, alphaTransparent)
+			}
+		}
+		lastWord := binary.LittleEndian.Uint32(dst[y*stride+4:])
+		if got := (lastWord >> 10) & 0x3ff; got != 0 {
+			t.Errorf("row %d padding sample 1 = %d, want 0", y, got)
+		}
+		if got := (lastWord >> 20) & 0x3ff; got != 0 {
+			t.Errorf("row %d padding sample 2 = %d, want 0", y, got)
+		}
+		if got := lastWord >> 30; got != 0 {
+			t.Errorf("row %d unused bits = %d, want 0", y, got)
+		}
+	}
+}
+
+func TestFillAlphaPlaneRejectsSmallDestination(t *testing.T) {
+	const width, height = 6, 2
+	dst := make([]byte, AlphaFrameSize(width, height)-1)
+	err := fillAlphaPlane(dst, width, height, alphaOpaque)
+	if err == nil || !strings.Contains(err.Error(), "destination is too small") {
+		t.Fatalf("error = %v, want destination size error", err)
+	}
+}
+
+func TestPatchAlphaMovingSquare(t *testing.T) {
+	const width, height = 304, 200
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+	if err := patchAlphaMovingSquare(dst, width, height, 0); err != nil {
+		t.Fatalf("patchAlphaMovingSquare: %v", err)
+	}
+
+	if got := sampleAlpha(dst, width, width/2, height/2); got != alphaTransparent {
+		t.Errorf("square center = %d, want transparent %d", got, alphaTransparent)
+	}
+	if got := sampleAlpha(dst, width, 10, height/2); got != alphaOpaque {
+		t.Errorf("outside square = %d, want opaque %d", got, alphaOpaque)
+	}
+
+	// At frame zero the square begins at x=77. Its first three-sample word
+	// therefore contains two opaque samples followed by one transparent sample.
+	for x, want := range []uint32{alphaOpaque, alphaOpaque, alphaTransparent} {
+		if got := sampleAlpha(dst, width, 75+x, height/2); got != want {
+			t.Errorf("boundary sample x=%d = %d, want %d", 75+x, got, want)
+		}
+	}
+}
+
+func TestPatchAlphaMovingSquarePreservesPartialBlockPadding(t *testing.T) {
+	const width, height = 100, 200
+	dst := make([]byte, AlphaFrameSize(width, height))
+	if err := fillAlphaPlane(dst, width, height, alphaOpaque); err != nil {
+		t.Fatalf("fillAlphaPlane: %v", err)
+	}
+	if err := patchAlphaMovingSquare(dst, width, height, 0); err != nil {
+		t.Fatalf("patchAlphaMovingSquare: %v", err)
+	}
+
+	lastWordOffset := height/2*AlphaLineSize(width) + (width/3)*4
+	lastWord := binary.LittleEndian.Uint32(dst[lastWordOffset:])
+	if got := lastWord & 0x3ff; got != alphaTransparent {
+		t.Errorf("last visible sample = %d, want %d", got, alphaTransparent)
+	}
+	if got := lastWord >> 10; got != 0 {
+		t.Errorf("partial-block padding bits = %#x, want 0", got)
+	}
+}
+
+func TestPatchAlphaMovingSquareRejectsSmallDestination(t *testing.T) {
+	const width, height = 100, 200
+	err := patchAlphaMovingSquare(
+		make([]byte, AlphaFrameSize(width, height)-1),
+		width,
+		height,
+		0,
+	)
+	if err == nil || !strings.Contains(err.Error(), "destination is too small") {
+		t.Fatalf("error = %v, want destination size error", err)
+	}
+}
+
+func sampleAlpha(buf []byte, width, x, y int) uint32 {
+	offset := y*AlphaLineSize(width) + x/3*4
+	word := binary.LittleEndian.Uint32(buf[offset:])
+	return (word >> uint(x%3*10)) & 0x3ff
+}