image/gif: don't encode local color tables if they're the same as the

global color table.

Change-Id: Ia38f75708ed5e5b430680a1eecafb4fc8047269c
Reviewed-on: https://go-review.googlesource.com/9467Reviewed-by: Rob Pike <r@golang.org>

src/image/gif/writer.go

@@ -6,6 +6,7 @@ package gif
 
 import (
 	"bufio"
+	"bytes"
 	"compress/lzw"
 	"errors"
 	"image"
@@ -53,8 +54,12 @@ type encoder struct {
 	err error
 	// g is a reference to the data that is being encoded.
 	g GIF
-	// buf is a scratch buffer. It must be at least 768 so we can write the color map.
-	buf [1024]byte
+	// globalCT is the size in bytes of the global color table.
+	globalCT int
+	// buf is a scratch buffer. It must be at least 256 for the blockWriter.
+	buf              [256]byte
+	globalColorTable [3 * 256]byte
+	localColorTable  [3 * 256]byte
 }
 
 // blockWriter writes the block structure of GIF image data, which
@@ -127,7 +132,8 @@ func (e *encoder) writeHeader() {
 		e.buf[1] = e.g.BackgroundIndex
 		e.buf[2] = 0x00 // Pixel Aspect Ratio.
 		e.write(e.buf[:3])
-		e.writeColorTable(p, paddedSize)
+		e.globalCT = encodeColorTable(e.globalColorTable[:], p, paddedSize)
+		e.write(e.globalColorTable[:e.globalCT])
 	} else {
 		// All frames have a local color table, so a global color table
 		// is not needed.
@@ -155,25 +161,22 @@ func (e *encoder) writeHeader() {
 	}
 }
 
-func (e *encoder) writeColorTable(p color.Palette, size int) {
-	if e.err != nil {
-		return
-	}
-
-	for i := 0; i < log2Lookup[size]; i++ {
+func encodeColorTable(dst []byte, p color.Palette, size int) int {
+	n := log2Lookup[size]
+	for i := 0; i < n; i++ {
 		if i < len(p) {
 			r, g, b, _ := p[i].RGBA()
-			e.buf[3*i+0] = uint8(r >> 8)
-			e.buf[3*i+1] = uint8(g >> 8)
-			e.buf[3*i+2] = uint8(b >> 8)
+			dst[3*i+0] = uint8(r >> 8)
+			dst[3*i+1] = uint8(g >> 8)
+			dst[3*i+2] = uint8(b >> 8)
 		} else {
 			// Pad with black.
-			e.buf[3*i+0] = 0x00
-			e.buf[3*i+1] = 0x00
-			e.buf[3*i+2] = 0x00
+			dst[3*i+0] = 0x00
+			dst[3*i+1] = 0x00
+			dst[3*i+2] = 0x00
 		}
 	}
-	e.write(e.buf[:3*log2Lookup[size]])
+	return 3 * n
 }
 
 func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte) {
@@ -232,11 +235,15 @@ func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte)
 	e.write(e.buf[:9])
 
 	paddedSize := log2(len(pm.Palette)) // Size of Local Color Table: 2^(1+n).
-	// Interlacing is not supported.
-	e.writeByte(0x80 | uint8(paddedSize))
-
-	// Local Color Table.
-	e.writeColorTable(pm.Palette, paddedSize)
+	ct := encodeColorTable(e.localColorTable[:], pm.Palette, paddedSize)
+	if ct != e.globalCT || !bytes.Equal(e.globalColorTable[:ct], e.localColorTable[:ct]) {
+		// Use a local color table.
+		e.writeByte(fColorTable | uint8(paddedSize))
+		e.write(e.localColorTable[:ct])
+	} else {
+		// Use the global color table.
+		e.writeByte(0)
+	}
 
 	litWidth := paddedSize + 1
 	if litWidth < 2 {

src/image/gif/writer_test.go

@@ -357,7 +357,60 @@ func TestEncodeImplicitConfigSize(t *testing.T) {
 	}
 }
 
-// TODO: add test for when a frame has the same color map (palette) as the global one.
+func TestEncodePalettes(t *testing.T) {
+	const w, h = 5, 5
+	pals := []color.Palette{{
+		color.RGBA{0x00, 0x00, 0x00, 0xff},
+		color.RGBA{0x01, 0x00, 0x00, 0xff},
+		color.RGBA{0x02, 0x00, 0x00, 0xff},
+	}, {
+		color.RGBA{0x00, 0x00, 0x00, 0xff},
+		color.RGBA{0x00, 0x01, 0x00, 0xff},
+	}, {
+		color.RGBA{0x00, 0x00, 0x03, 0xff},
+		color.RGBA{0x00, 0x00, 0x02, 0xff},
+		color.RGBA{0x00, 0x00, 0x01, 0xff},
+		color.RGBA{0x00, 0x00, 0x00, 0xff},
+	}, {
+		color.RGBA{0x10, 0x07, 0xf0, 0xff},
+		color.RGBA{0x20, 0x07, 0xf0, 0xff},
+		color.RGBA{0x30, 0x07, 0xf0, 0xff},
+		color.RGBA{0x40, 0x07, 0xf0, 0xff},
+		color.RGBA{0x50, 0x07, 0xf0, 0xff},
+	}}
+	g0 := &GIF{
+		Image: []*image.Paletted{
+			image.NewPaletted(image.Rect(0, 0, w, h), pals[0]),
+			image.NewPaletted(image.Rect(0, 0, w, h), pals[1]),
+			image.NewPaletted(image.Rect(0, 0, w, h), pals[2]),
+			image.NewPaletted(image.Rect(0, 0, w, h), pals[3]),
+		},
+		Delay:    make([]int, len(pals)),
+		Disposal: make([]byte, len(pals)),
+		Config: image.Config{
+			ColorModel: pals[2],
+			Width:      w,
+			Height:     h,
+		},
+	}
+
+	var buf bytes.Buffer
+	if err := EncodeAll(&buf, g0); err != nil {
+		t.Fatalf("EncodeAll: %v", err)
+	}
+	g1, err := DecodeAll(&buf)
+	if err != nil {
+		t.Fatalf("DecodeAll: %v", err)
+	}
+	if len(g0.Image) != len(g1.Image) {
+		t.Fatalf("image lengths differ: %d and %d", len(g0.Image), len(g1.Image))
+	}
+	for i, m := range g1.Image {
+		if got, want := m.Palette, pals[i]; !palettesEqual(got, want) {
+			t.Errorf("frame %d:\ngot  %v\nwant %v", i, got, want)
+		}
+	}
+}
 
 func BenchmarkEncode(b *testing.B) {
 	b.StopTimer()