1 files changed, 333 insertions, 0 deletions
diff --git a/src/image/gif/writer.go b/src/image/gif/writer.go
new file mode 100644
index 000000000..49abde704
--- /dev/null
+++ b/src/image/gif/writer.go
@@ -0,0 +1,333 @@
+// 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/color/palette"
+	"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
+	// 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
+	}
+
+	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])
+
+	// All frames have a local color table, so a global color table
+	// is not needed.
+	e.buf[0] = 0x00
+	e.buf[1] = 0x00 // Background Color Index.
+	e.buf[2] = 0x00 // Pixel Aspect Ratio.
+	e.write(e.buf[:3])
+
+	// 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 := paddedSize + 1
+	if litWidth < 2 {
+		litWidth = 2
+	}
+	e.writeByte(uint8(litWidth)) // LZW Minimum Code Size.
+
+	lzww := lzw.NewWriter(blockWriter{e: e}, lzw.LSB, litWidth)
+	if dx := b.Dx(); dx == pm.Stride {
+		_, e.err = lzww.Write(pm.Pix)
+		if e.err != nil {
+			lzww.Close()
+			return
+		}
+	} else {
+		for i, y := 0, b.Min.Y; y < b.Max.Y; i, y = i+pm.Stride, y+1 {
+			_, e.err = lzww.Write(pm.Pix[i : i+dx])
+			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.
+	// palette.Plan9 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, palette.Plan9[: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},
+	})
+}