image/gif: expose disposal, bg index and Config

The background index in the global palette (located in the image.Config)
is necessary for interpreting GIF frames properly

Frame disposal information is necessary for interpreting GIF frames in
the context of a sequence (or animation)

Removes decoder.flags as it can be a local variable

Change-Id: I6790a7febf6ba0859175c834c807bc6413e6b194
Reviewed-on: https://go-review.googlesource.com/4620
Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/gif/reader.go

@@ -34,6 +34,9 @@ const (
 	// Fields.
 	fColorMapFollows = 1 << 7
 
+	// Screen Descriptor flags.
+	sdGlobalColorTable = 1 << 7
+
 	// Image fields.
 	ifLocalColorTable = 1 << 7
 	ifInterlace       = 1 << 6
@@ -41,6 +44,14 @@ const (
 
 	// Graphic control flags.
 	gcTransparentColorSet = 1 << 0
+	gcDisposalMethodMask  = 7 << 2
+)
+
+// Disposal Methods.
+const (
+	DisposalNone       = 0x01
+	DisposalBackground = 0x02
+	DisposalPrevious   = 0x03
 )
 
 // Section indicators.
@@ -66,11 +77,11 @@ type decoder struct {
 	vers            string
 	width           int
 	height          int
-	flags           byte
 	headerFields    byte
 	backgroundIndex byte
 	loopCount       int
 	delayTime       int
+	disposalMethod  byte
 
 	// Unused from header.
 	aspect byte
@@ -87,9 +98,10 @@ type decoder struct {
 	globalColorMap color.Palette
 
 	// Used when decoding.
-	delay []int
-	image []*image.Paletted
-	tmp   [1024]byte // must be at least 768 so we can read color map
+	delay    []int
+	disposal []byte
+	image    []*image.Paletted
+	tmp      [1024]byte // must be at least 768 so we can read color map
 }
 
 // blockReader parses the block structure of GIF image data, which
@@ -235,6 +247,7 @@ func (d *decoder) decode(r io.Reader, configOnly bool) error {
 
 			d.image = append(d.image, m)
 			d.delay = append(d.delay, d.delayTime)
+			d.disposal = append(d.disposal, d.disposalMethod)
 			// The GIF89a spec, Section 23 (Graphic Control Extension) says:
 			// "The scope of this extension is the first graphic rendering block
 			// to follow." We therefore reset the GCE fields to zero.
@@ -265,7 +278,9 @@ func (d *decoder) readHeaderAndScreenDescriptor() error {
 	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]
-	d.backgroundIndex = d.tmp[11]
+	if d.headerFields&sdGlobalColorTable != 0 {
+		d.backgroundIndex = d.tmp[11]
+	}
 	d.aspect = d.tmp[12]
 	d.loopCount = -1
 	d.pixelSize = uint(d.headerFields&7) + 1
@@ -346,9 +361,10 @@ func (d *decoder) readGraphicControl() error {
 	if _, err := io.ReadFull(d.r, d.tmp[0:6]); err != nil {
 		return fmt.Errorf("gif: can't read graphic control: %s", err)
 	}
-	d.flags = d.tmp[1]
+	flags := d.tmp[1]
+	d.disposalMethod = (flags & gcDisposalMethodMask) >> 2
 	d.delayTime = int(d.tmp[2]) | int(d.tmp[3])<<8
-	if d.flags&gcTransparentColorSet != 0 {
+	if flags&gcTransparentColorSet != 0 {
 		d.transparentIndex = d.tmp[4]
 		d.hasTransparentIndex = true
 	}
@@ -428,7 +444,11 @@ 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.
+	BackgroundIndex byte
 }
 
 // DecodeAll reads a GIF image from r and returns the sequential frames
@@ -442,6 +462,13 @@ func DecodeAll(r io.Reader) (*GIF, error) {
 		Image:     d.image,
 		LoopCount: d.loopCount,
 		Delay:     d.delay,
+		Disposal:  d.disposal,
+		Config: image.Config{
+			ColorModel: d.globalColorMap,
+			Width:      d.width,
+			Height:     d.height,
+		},
+		BackgroundIndex: d.backgroundIndex,
 	}
 	return gif, nil
 }