image/gif: add writer implementation

R=r, nigeltao
CC=golang-dev
https://golang.org/cl/10896043

src/pkg/go/build/deps_test.go

@@ -202,7 +202,7 @@ var pkgDeps = map[string][]string{
 	"go/build":            {"L4", "OS", "GOPARSER"},
 	"html":                {"L4"},
 	"image/draw":          {"L4"},
-	"image/gif":           {"L4", "compress/lzw"},
+	"image/gif":           {"L4", "compress/lzw", "image/draw"},
 	"image/jpeg":          {"L4"},
 	"image/png":           {"L4", "compress/zlib"},
 	"index/suffixarray":   {"L4", "regexp"},

src/pkg/image/gif/reader.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package gif implements a GIF image decoder.
+// Package gif implements a GIF image decoder and encoder.
 //
 // The GIF specification is at http://www.w3.org/Graphics/GIF/spec-gif89a.txt.
 package gif

src/pkg/image/gif/writer.go

@@ -0,0 +1,329 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gif
+
+import (
+	"bufio"
+	"compress/lzw"
+	"errors"
+	"image"
+	"image/color"
+	"image/draw"
+	"io"
+)
+
+// Graphic control extension fields.
+const (
+	gcLabel     = 0xF9
+	gcBlockSize = 0x04
+)
+
+var log2Lookup = [8]int{2, 4, 8, 16, 32, 64, 128, 256}
+
+func log2(x int) int {
+	for i, v := range log2Lookup {
+		if x <= v {
+			return i
+		}
+	}
+	return -1
+}
+
+// Little-endian.
+func writeUint16(b []uint8, u uint16) {
+	b[0] = uint8(u)
+	b[1] = uint8(u >> 8)
+}
+
+// writer is a buffered writer.
+type writer interface {
+	Flush() error
+	io.Writer
+	io.ByteWriter
+}
+
+// encoder encodes an image to the GIF format.
+type encoder struct {
+	// w is the writer to write to. err is the first error encountered during
+	// writing. All attempted writes after the first error become no-ops.
+	w   writer
+	err error
+	// g is a reference to the data that is being encoded.
+	g *GIF
+	// bitsPerPixel is the number of bits required to represent each color
+	// in the image.
+	bitsPerPixel int
+	// buf is a scratch buffer. It must be at least 768 so we can write the color map.
+	buf [1024]byte
+}
+
+// blockWriter writes the block structure of GIF image data, which
+// comprises (n, (n bytes)) blocks, with 1 <= n <= 255. It is the
+// writer given to the LZW encoder, which is thus immune to the
+// blocking.
+type blockWriter struct {
+	e *encoder
+}
+
+func (b blockWriter) Write(data []byte) (int, error) {
+	if b.e.err != nil {
+		return 0, b.e.err
+	}
+	if len(data) == 0 {
+		return 0, nil
+	}
+	total := 0
+	for total < len(data) {
+		n := copy(b.e.buf[1:256], data[total:])
+		total += n
+		b.e.buf[0] = uint8(n)
+
+		n, b.e.err = b.e.w.Write(b.e.buf[:n+1])
+		if b.e.err != nil {
+			return 0, b.e.err
+		}
+	}
+	return total, b.e.err
+}
+
+func (e *encoder) flush() {
+	if e.err != nil {
+		return
+	}
+	e.err = e.w.Flush()
+}
+
+func (e *encoder) write(p []byte) {
+	if e.err != nil {
+		return
+	}
+	_, e.err = e.w.Write(p)
+}
+
+func (e *encoder) writeByte(b byte) {
+	if e.err != nil {
+		return
+	}
+	e.err = e.w.WriteByte(b)
+}
+
+func (e *encoder) writeHeader() {
+	if e.err != nil {
+		return
+	}
+	_, e.err = io.WriteString(e.w, "GIF89a")
+	if e.err != nil {
+		return
+	}
+
+	// TODO: This bases the global color table on the first image
+	// only.
+	pm := e.g.Image[0]
+	// Logical screen width and height.
+	writeUint16(e.buf[0:2], uint16(pm.Bounds().Dx()))
+	writeUint16(e.buf[2:4], uint16(pm.Bounds().Dy()))
+	e.write(e.buf[:4])
+
+	e.bitsPerPixel = log2(len(pm.Palette)) + 1
+	e.buf[0] = 0x80 | ((uint8(e.bitsPerPixel) - 1) << 4) | (uint8(e.bitsPerPixel) - 1)
+	e.buf[1] = 0x00 // Background Color Index.
+	e.buf[2] = 0x00 // Pixel Aspect Ratio.
+	e.write(e.buf[:3])
+
+	// Global Color Table.
+	e.writeColorTable(pm.Palette, e.bitsPerPixel-1)
+
+	// Add animation info if necessary.
+	if len(e.g.Image) > 1 {
+		e.buf[0] = 0x21 // Extension Introducer.
+		e.buf[1] = 0xff // Application Label.
+		e.buf[2] = 0x0b // Block Size.
+		e.write(e.buf[:3])
+		_, e.err = io.WriteString(e.w, "NETSCAPE2.0") // Application Identifier.
+		if e.err != nil {
+			return
+		}
+		e.buf[0] = 0x03 // Block Size.
+		e.buf[1] = 0x01 // Sub-block Index.
+		writeUint16(e.buf[2:4], uint16(e.g.LoopCount))
+		e.buf[4] = 0x00 // Block Terminator.
+		e.write(e.buf[:5])
+	}
+}
+
+func (e *encoder) writeColorTable(p color.Palette, size int) {
+	if e.err != nil {
+		return
+	}
+
+	for i := 0; i < log2Lookup[size]; 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)
+		} else {
+			// Pad with black.
+			e.buf[3*i+0] = 0x00
+			e.buf[3*i+1] = 0x00
+			e.buf[3*i+2] = 0x00
+		}
+	}
+	e.write(e.buf[:3*log2Lookup[size]])
+}
+
+func (e *encoder) writeImageBlock(pm *image.Paletted, delay int) {
+	if e.err != nil {
+		return
+	}
+
+	if len(pm.Palette) == 0 {
+		e.err = errors.New("gif: cannot encode image block with empty palette")
+		return
+	}
+
+	b := pm.Bounds()
+	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 || b.Min.X < 0 || b.Min.X >= 1<<16 || b.Min.Y < 0 || b.Min.Y >= 1<<16 {
+		e.err = errors.New("gif: image block is too large to encode")
+		return
+	}
+
+	transparentIndex := -1
+	for i, c := range pm.Palette {
+		if _, _, _, a := c.RGBA(); a == 0 {
+			transparentIndex = i
+			break
+		}
+	}
+
+	if delay > 0 || transparentIndex != -1 {
+		e.buf[0] = sExtension  // Extension Introducer.
+		e.buf[1] = gcLabel     // Graphic Control Label.
+		e.buf[2] = gcBlockSize // Block Size.
+		if transparentIndex != -1 {
+			e.buf[3] = 0x01
+		} else {
+			e.buf[3] = 0x00
+		}
+		writeUint16(e.buf[4:6], uint16(delay)) // Delay Time (1/100ths of a second)
+
+		// Transparent color index.
+		if transparentIndex != -1 {
+			e.buf[6] = uint8(transparentIndex)
+		} else {
+			e.buf[6] = 0x00
+		}
+		e.buf[7] = 0x00 // Block Terminator.
+		e.write(e.buf[:8])
+	}
+	e.buf[0] = sImageDescriptor
+	writeUint16(e.buf[1:3], uint16(b.Min.X))
+	writeUint16(e.buf[3:5], uint16(b.Min.Y))
+	writeUint16(e.buf[5:7], uint16(b.Dx()))
+	writeUint16(e.buf[7:9], uint16(b.Dy()))
+	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)
+
+	litWidth := e.bitsPerPixel
+	if litWidth < 2 {
+		litWidth = 2
+	}
+	e.writeByte(uint8(litWidth)) // LZW Minimum Code Size.
+
+	lzww := lzw.NewWriter(blockWriter{e: e}, lzw.LSB, litWidth)
+	_, e.err = lzww.Write(pm.Pix)
+	if e.err != nil {
+		lzww.Close()
+		return
+	}
+	lzww.Close()
+	e.writeByte(0x00) // Block Terminator.
+}
+
+// Options are the encoding parameters.
+type Options struct {
+	// NumColors is the maximum number of colors used in the image.
+	// It ranges from 1 to 256.
+	NumColors int
+
+	// Quantizer is used to produce a palette with size NumColors.
+	// color.Plan9Palette is used in place of a nil Quantizer.
+	Quantizer draw.Quantizer
+
+	// Drawer is used to convert the source image to the desired palette.
+	// draw.FloydSteinberg is used in place of a nil Drawer.
+	Drawer draw.Drawer
+}
+
+// EncodeAll writes the images in g to w in GIF format with the
+// given loop count and delay between frames.
+func EncodeAll(w io.Writer, g *GIF) error {
+	if len(g.Image) == 0 {
+		return errors.New("gif: must provide at least one image")
+	}
+
+	if len(g.Image) != len(g.Delay) {
+		return errors.New("gif: mismatched image and delay lengths")
+	}
+	if g.LoopCount < 0 {
+		g.LoopCount = 0
+	}
+
+	e := encoder{g: g}
+	if ww, ok := w.(writer); ok {
+		e.w = ww
+	} else {
+		e.w = bufio.NewWriter(w)
+	}
+
+	e.writeHeader()
+	for i, pm := range g.Image {
+		e.writeImageBlock(pm, g.Delay[i])
+	}
+	e.writeByte(sTrailer)
+	e.flush()
+	return e.err
+}
+
+// Encode writes the Image m to w in GIF format.
+func Encode(w io.Writer, m image.Image, o *Options) error {
+	// Check for bounds and size restrictions.
+	b := m.Bounds()
+	if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
+		return errors.New("gif: image is too large to encode")
+	}
+
+	opts := Options{}
+	if o != nil {
+		opts = *o
+	}
+	if opts.NumColors < 1 || 256 < opts.NumColors {
+		opts.NumColors = 256
+	}
+	if opts.Drawer == nil {
+		opts.Drawer = draw.FloydSteinberg
+	}
+
+	pm, ok := m.(*image.Paletted)
+	if !ok || len(pm.Palette) > opts.NumColors {
+		// TODO: Pick a better sub-sample of the Plan 9 palette.
+		pm = image.NewPaletted(b, color.Plan9Palette[:opts.NumColors])
+		if opts.Quantizer != nil {
+			pm.Palette = opts.Quantizer.Quantize(make(color.Palette, 0, opts.NumColors), m)
+		}
+		opts.Drawer.Draw(pm, b, m, image.ZP)
+	}
+
+	return EncodeAll(w, &GIF{
+		Image: []*image.Paletted{pm},
+		Delay: []int{0},
+	})
+}

src/pkg/image/gif/writer_test.go

@@ -0,0 +1,204 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gif
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	_ "image/png"
+	"io/ioutil"
+	"math/rand"
+	"os"
+	"testing"
+)
+
+func readImg(filename string) (image.Image, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	m, _, err := image.Decode(f)
+	return m, err
+}
+
+func readGIF(filename string) (*GIF, error) {
+	f, err := os.Open(filename)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	return DecodeAll(f)
+}
+
+func delta(u0, u1 uint32) int64 {
+	d := int64(u0) - int64(u1)
+	if d < 0 {
+		return -d
+	}
+	return d
+}
+
+// averageDelta returns the average delta in RGB space. The two images must
+// have the same bounds.
+func averageDelta(m0, m1 image.Image) int64 {
+	b := m0.Bounds()
+	var sum, n int64
+	for y := b.Min.Y; y < b.Max.Y; y++ {
+		for x := b.Min.X; x < b.Max.X; x++ {
+			c0 := m0.At(x, y)
+			c1 := m1.At(x, y)
+			r0, g0, b0, _ := c0.RGBA()
+			r1, g1, b1, _ := c1.RGBA()
+			sum += delta(r0, r1)
+			sum += delta(g0, g1)
+			sum += delta(b0, b1)
+			n += 3
+		}
+	}
+	return sum / n
+}
+
+var testCase = []struct {
+	filename  string
+	tolerance int64
+}{
+	{"../testdata/video-001.png", 1 << 12},
+	{"../testdata/video-001.gif", 0},
+	{"../testdata/video-001.interlaced.gif", 0},
+}
+
+func TestWriter(t *testing.T) {
+	for _, tc := range testCase {
+		m0, err := readImg(tc.filename)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		var buf bytes.Buffer
+		err = Encode(&buf, m0, nil)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		m1, err := Decode(&buf)
+		if err != nil {
+			t.Error(tc.filename, err)
+			continue
+		}
+		if m0.Bounds() != m1.Bounds() {
+			t.Errorf("%s, bounds differ: %v and %v", tc.filename, m0.Bounds(), m1.Bounds())
+			continue
+		}
+		// Compare the average delta to the tolerance level.
+		avgDelta := averageDelta(m0, m1)
+		if avgDelta > tc.tolerance {
+			t.Errorf("%s: average delta is too high. expected: %d, got %d", tc.filename, tc.tolerance, avgDelta)
+			continue
+		}
+	}
+}
+
+var frames = []string{
+	"../testdata/video-001.gif",
+	"../testdata/video-005.gray.gif",
+}
+
+func TestEncodeAll(t *testing.T) {
+	g0 := &GIF{
+		Image:     make([]*image.Paletted, len(frames)),
+		Delay:     make([]int, len(frames)),
+		LoopCount: 5,
+	}
+	for i, f := range frames {
+		m, err := readGIF(f)
+		if err != nil {
+			t.Error(f, err)
+		}
+		g0.Image[i] = m.Image[0]
+	}
+	var buf bytes.Buffer
+	if err := EncodeAll(&buf, g0); err != nil {
+		t.Fatal("EncodeAll:", err)
+	}
+	g1, err := DecodeAll(&buf)
+	if err != nil {
+		t.Fatal("DecodeAll:", err)
+	}
+	if g0.LoopCount != g1.LoopCount {
+		t.Errorf("loop counts differ: %d and %d", g0.LoopCount, g1.LoopCount)
+	}
+	for i := range g0.Image {
+		m0, m1 := g0.Image[i], g1.Image[i]
+		if m0.Bounds() != m1.Bounds() {
+			t.Errorf("%s, bounds differ: %v and %v", frames[i], m0.Bounds(), m1.Bounds())
+		}
+		d0, d1 := g0.Delay[i], g1.Delay[i]
+		if d0 != d1 {
+			t.Errorf("%s: delay values differ: %d and %d", frames[i], d0, d1)
+		}
+	}
+
+	g1.Delay = make([]int, 1)
+	if err := EncodeAll(ioutil.Discard, g1); err == nil {
+		t.Error("expected error from mismatched delay and image slice lengths")
+	}
+	if err := EncodeAll(ioutil.Discard, &GIF{}); err == nil {
+		t.Error("expected error from providing empty gif")
+	}
+}
+
+func BenchmarkEncode(b *testing.B) {
+	b.StopTimer()
+
+	bo := image.Rect(0, 0, 640, 480)
+	rnd := rand.New(rand.NewSource(123))
+
+	// Restrict to a 256-color paletted image to avoid quantization path.
+	palette := make(color.Palette, 256)
+	for i := range palette {
+		palette[i] = color.RGBA{
+			uint8(rnd.Intn(256)),
+			uint8(rnd.Intn(256)),
+			uint8(rnd.Intn(256)),
+			255,
+		}
+	}
+	img := image.NewPaletted(image.Rect(0, 0, 640, 480), palette)
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			img.Set(x, y, palette[rnd.Intn(256)])
+		}
+	}
+
+	b.SetBytes(640 * 480 * 4)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(ioutil.Discard, img, nil)
+	}
+}
+
+func BenchmarkQuantizedEncode(b *testing.B) {
+	b.StopTimer()
+	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
+	bo := img.Bounds()
+	rnd := rand.New(rand.NewSource(123))
+	for y := bo.Min.Y; y < bo.Max.Y; y++ {
+		for x := bo.Min.X; x < bo.Max.X; x++ {
+			img.SetRGBA(x, y, color.RGBA{
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				uint8(rnd.Intn(256)),
+				255,
+			})
+		}
+	}
+	b.SetBytes(640 * 480 * 4)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		Encode(ioutil.Discard, img, nil)
+	}
+}