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image/gif: encode disposal, bg index and Config.
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The previous CL implemented decoding, but not encoding.

Also return the global color map (if present) for DecodeConfig.

Change-Id: I3b99c93720246010c9fe0924dc40a67875dfc852
Reviewed-on: https://go-review.googlesource.com/9389
Reviewed-by: Rob Pike <[email protected]>
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@nigeltao
nigeltao committed Apr 28, 2015
1 parent 0c62c93 commit baf3814
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Showing 3 changed files with 222 additions and 73 deletions.
87 changes: 38 additions & 49 deletions src/image/gif/reader.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -32,15 +32,9 @@ type reader interface {
// Masks etc.
const (
// Fields.
fColorMapFollows = 1 << 7

// Screen Descriptor flags.
sdGlobalColorTable = 1 << 7

// Image fields.
ifLocalColorTable = 1 << 7
ifInterlace = 1 << 6
ifPixelSizeMask = 7
fColorTable = 1 << 7
fInterlace = 1 << 6
fColorTableBitsMask = 7

// Graphic control flags.
gcTransparentColorSet = 1 << 0
Expand Down Expand Up @@ -77,15 +71,11 @@ type decoder struct {
vers string
width int
height int
headerFields byte
backgroundIndex byte
loopCount int
delayTime int
backgroundIndex byte
disposalMethod byte

// Unused from header.
aspect byte

// From image descriptor.
imageFields byte

Expand All @@ -94,7 +84,6 @@ type decoder struct {
hasTransparentIndex bool

// Computed.
pixelSize uint
globalColorMap color.Palette

// Used when decoding.
Expand Down Expand Up @@ -134,7 +123,7 @@ func (b *blockReader) Read(p []byte) (int, error) {
b.err = io.EOF
return 0, b.err
}
b.slice = b.tmp[0:blockLen]
b.slice = b.tmp[:blockLen]
if _, b.err = io.ReadFull(b.r, b.slice); b.err != nil {
return 0, b.err
}
Expand All @@ -161,12 +150,6 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
return nil
}

if d.headerFields&fColorMapFollows != 0 {
if d.globalColorMap, err = d.readColorMap(); err != nil {
return err
}
}

for {
c, err := d.r.ReadByte()
if err != nil {
Expand All @@ -183,9 +166,9 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
if err != nil {
return err
}
useLocalColorMap := d.imageFields&fColorMapFollows != 0
useLocalColorMap := d.imageFields&fColorTable != 0
if useLocalColorMap {
m.Palette, err = d.readColorMap()
m.Palette, err = d.readColorMap(d.imageFields)
if err != nil {
return err
}
Expand Down Expand Up @@ -241,7 +224,7 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
}

// Undo the interlacing if necessary.
if d.imageFields&ifInterlace != 0 {
if d.imageFields&fInterlace != 0 {
uninterlace(m)
}

Expand All @@ -267,40 +250,35 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
}

func (d *decoder) readHeaderAndScreenDescriptor() error {
_, err := io.ReadFull(d.r, d.tmp[0:13])
_, err := io.ReadFull(d.r, d.tmp[:13])
if err != nil {
return err
}
d.vers = string(d.tmp[0:6])
d.vers = string(d.tmp[:6])
if d.vers != "GIF87a" && d.vers != "GIF89a" {
return fmt.Errorf("gif: can't recognize format %s", d.vers)
}
d.width = int(d.tmp[6]) + int(d.tmp[7])<<8
d.height = int(d.tmp[8]) + int(d.tmp[9])<<8
d.headerFields = d.tmp[10]
if d.headerFields&sdGlobalColorTable != 0 {
if fields := d.tmp[10]; fields&fColorTable != 0 {
d.backgroundIndex = d.tmp[11]
// readColorMap overwrites the contents of d.tmp, but that's OK.
if d.globalColorMap, err = d.readColorMap(fields); err != nil {
return err
}
}
d.aspect = d.tmp[12]
// d.tmp[12] is the Pixel Aspect Ratio, which is ignored.
d.loopCount = -1
d.pixelSize = uint(d.headerFields&7) + 1
return nil
}

func (d *decoder) readColorMap() (color.Palette, error) {
if d.pixelSize > 8 {
return nil, fmt.Errorf("gif: can't handle %d bits per pixel", d.pixelSize)
}
numColors := 1 << d.pixelSize
if d.imageFields&ifLocalColorTable != 0 {
numColors = 1 << ((d.imageFields & ifPixelSizeMask) + 1)
}
numValues := 3 * numColors
_, err := io.ReadFull(d.r, d.tmp[0:numValues])
func (d *decoder) readColorMap(fields byte) (color.Palette, error) {
n := 1 << (1 + uint(fields&fColorTableBitsMask))
_, err := io.ReadFull(d.r, d.tmp[:3*n])
if err != nil {
return nil, fmt.Errorf("gif: short read on color map: %s", err)
}
colorMap := make(color.Palette, numColors)
colorMap := make(color.Palette, n)
j := 0
for i := range colorMap {
colorMap[i] = color.RGBA{d.tmp[j+0], d.tmp[j+1], d.tmp[j+2], 0xFF}
Expand Down Expand Up @@ -333,7 +311,7 @@ func (d *decoder) readExtension() error {
return fmt.Errorf("gif: unknown extension 0x%.2x", extension)
}
if size > 0 {
if _, err := io.ReadFull(d.r, d.tmp[0:size]); err != nil {
if _, err := io.ReadFull(d.r, d.tmp[:size]); err != nil {
return err
}
}
Expand All @@ -358,7 +336,7 @@ func (d *decoder) readExtension() error {
}

func (d *decoder) readGraphicControl() error {
if _, err := io.ReadFull(d.r, d.tmp[0:6]); err != nil {
if _, err := io.ReadFull(d.r, d.tmp[:6]); err != nil {
return fmt.Errorf("gif: can't read graphic control: %s", err)
}
flags := d.tmp[1]
Expand All @@ -372,7 +350,7 @@ func (d *decoder) readGraphicControl() error {
}

func (d *decoder) newImageFromDescriptor() (*image.Paletted, error) {
if _, err := io.ReadFull(d.r, d.tmp[0:9]); err != nil {
if _, err := io.ReadFull(d.r, d.tmp[:9]); err != nil {
return nil, fmt.Errorf("gif: can't read image descriptor: %s", err)
}
left := int(d.tmp[0]) + int(d.tmp[1])<<8
Expand All @@ -396,7 +374,7 @@ func (d *decoder) readBlock() (int, error) {
if n == 0 || err != nil {
return 0, err
}
return io.ReadFull(d.r, d.tmp[0:n])
return io.ReadFull(d.r, d.tmp[:n])
}

// interlaceScan defines the ordering for a pass of the interlace algorithm.
Expand Down Expand Up @@ -444,10 +422,21 @@ func Decode(r io.Reader) (image.Image, error) {
type GIF struct {
Image []*image.Paletted // The successive images.
Delay []int // The successive delay times, one per frame, in 100ths of a second.
Disposal []byte // The successive disposal methods, one per frame.
LoopCount int // The loop count.
Config image.Config
// The background index in the Global Color Map.
// Disposal is the successive disposal methods, one per frame. For
// backwards compatibility, a nil Disposal is valid to pass to EncodeAll,
// and implies that each frame's disposal method is 0 (no disposal
// specified).
Disposal []byte
// Config is the global color map (palette), width and height. A nil or
// empty-color.Palette Config.ColorModel means that each frame has its own
// color map and there is no global color map. For backwards compatibility,
// a zero-valued Config is valid to pass to EncodeAll, and implies that the
// overall GIF's width and height equals the first frame's width and
// height.
Config image.Config
// BackgroundIndex is the background index in the global color map, for use
// with the DisposalBackground disposal method.
BackgroundIndex byte
}

Expand Down
65 changes: 49 additions & 16 deletions src/image/gif/writer.go
View file
Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,7 @@ type encoder struct {
w writer
err error
// g is a reference to the data that is being encoded.
g *GIF
g GIF
// buf is a scratch buffer. It must be at least 768 so we can write the color map.
buf [1024]byte
}
Expand Down Expand Up @@ -116,18 +116,26 @@ func (e *encoder) writeHeader() {
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()))
writeUint16(e.buf[0:2], uint16(e.g.Config.Width))
writeUint16(e.buf[2:4], uint16(e.g.Config.Height))
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])
if p, ok := e.g.Config.ColorModel.(color.Palette); ok && len(p) > 0 {
paddedSize := log2(len(p)) // Size of Global Color Table: 2^(1+n).
e.buf[0] = fColorTable | uint8(paddedSize)
e.buf[1] = e.g.BackgroundIndex
e.buf[2] = 0x00 // Pixel Aspect Ratio.
e.write(e.buf[:3])
e.writeColorTable(p, paddedSize)
} else {
// 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 {
Expand Down Expand Up @@ -168,7 +176,7 @@ func (e *encoder) writeColorTable(p color.Palette, size int) {
e.write(e.buf[:3*log2Lookup[size]])
}

func (e *encoder) writeImageBlock(pm *image.Paletted, delay int) {
func (e *encoder) writeImageBlock(pm *image.Paletted, delay int, disposal byte) {
if e.err != nil {
return
}
Expand All @@ -192,14 +200,14 @@ func (e *encoder) writeImageBlock(pm *image.Paletted, delay int) {
}
}

if delay > 0 || transparentIndex != -1 {
if delay > 0 || disposal != 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
e.buf[3] = 0x01 | disposal<<2
} else {
e.buf[3] = 0x00
e.buf[3] = 0x00 | disposal<<2
}
writeUint16(e.buf[4:6], uint16(delay)) // Delay Time (1/100ths of a second)

Expand Down Expand Up @@ -281,7 +289,23 @@ func EncodeAll(w io.Writer, g *GIF) error {
g.LoopCount = 0
}

e := encoder{g: g}
e := encoder{g: *g}
// The GIF.Disposal, GIF.Config and GIF.BackgroundIndex fields were added
// in Go 1.5. Valid Go 1.4 code, such as when the Disposal field is omitted
// in a GIF struct literal, should still produce valid GIFs.
if e.g.Disposal != nil && len(e.g.Image) != len(e.g.Disposal) {
return errors.New("gif: mismatched image and disposal lengths")
}
if e.g.Config == (image.Config{}) {
b := g.Image[0].Bounds()
e.g.Config.Width = b.Dx()
e.g.Config.Height = b.Dy()
} else if e.g.Config.ColorModel != nil {
if _, ok := e.g.Config.ColorModel.(color.Palette); !ok {
return errors.New("gif: GIF color model must be a color.Palette")
}
}

if ww, ok := w.(writer); ok {
e.w = ww
} else {
Expand All @@ -290,7 +314,11 @@ func EncodeAll(w io.Writer, g *GIF) error {

e.writeHeader()
for i, pm := range g.Image {
e.writeImageBlock(pm, g.Delay[i])
disposal := uint8(0)
if g.Disposal != nil {
disposal = g.Disposal[i]
}
e.writeImageBlock(pm, g.Delay[i], disposal)
}
e.writeByte(sTrailer)
e.flush()
Expand Down Expand Up @@ -329,5 +357,10 @@ func Encode(w io.Writer, m image.Image, o *Options) error {
return EncodeAll(w, &GIF{
Image: []*image.Paletted{pm},
Delay: []int{0},
Config: image.Config{
ColorModel: pm.Palette,
Width: b.Dx(),
Height: b.Dy(),
},
})
}
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