gif: fix local color map and coordinates

R=nigeltao
CC=golang-dev
https://golang.org/cl/4759051

src/pkg/image/gif/reader.go

@@ -28,7 +28,9 @@ const (
 	fColorMapFollows = 1 << 7
 
 	// Image fields.
-	ifInterlace = 1 << 6
+	ifLocalColorTable = 1 << 7
+	ifInterlace       = 1 << 6
+	ifPixelSizeMask   = 7
 
 	// Graphic control flags.
 	gcTransparentColorSet = 1 << 0
@@ -190,7 +192,9 @@ Loop:
 			}
 
 			// Undo the interlacing if necessary.
-			d.uninterlace(m)
+			if d.imageFields&ifInterlace != 0 {
+				uninterlace(m)
+			}
 
 			d.image = append(d.image, m)
 			d.delay = append(d.delay, d.delayTime)
@@ -236,6 +240,9 @@ func (d *decoder) readColorMap() (image.PalettedColorModel, os.Error) {
 		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])
 	if err != nil {
@@ -322,15 +329,15 @@ func (d *decoder) newImageFromDescriptor() (*image.Paletted, os.Error) {
 	if _, err := io.ReadFull(d.r, d.tmp[0:9]); err != nil {
 		return nil, fmt.Errorf("gif: can't read image descriptor: %s", err)
 	}
-	// TODO: This code (throughout) ignores the top and left values,
-	// and assumes (in interlacing, for example) that the images'
-	// widths and heights are all the same.
-	_ = int(d.tmp[0]) + int(d.tmp[1])<<8 // TODO: honor left value
-	_ = int(d.tmp[2]) + int(d.tmp[3])<<8 // TODO: honor top value
+	left := int(d.tmp[0]) + int(d.tmp[1])<<8
+	top := int(d.tmp[2]) + int(d.tmp[3])<<8
 	width := int(d.tmp[4]) + int(d.tmp[5])<<8
 	height := int(d.tmp[6]) + int(d.tmp[7])<<8
 	d.imageFields = d.tmp[8]
-	return image.NewPaletted(width, height, nil), nil
+	m := image.NewPaletted(width, height, nil)
+	// Overwrite the rectangle to take account of left and top.
+	m.Rect = image.Rect(left, top, left+width, top+height)
+	return m, nil
 }
 
 func (d *decoder) readBlock() (int, os.Error) {
@@ -354,13 +361,11 @@ var interlacing = []interlaceScan{
 	{2, 1}, // Group 4 : Every 2nd. row, starting with row 1.
 }
 
-func (d *decoder) uninterlace(m *image.Paletted) {
-	if d.imageFields&ifInterlace == 0 {
-		return
-	}
+// uninterlace rearranges the pixels in m to account for interlaced input.
+func uninterlace(m *image.Paletted) {
 	var nPix []uint8
-	dx := d.width
-	dy := d.height
+	dx := m.Bounds().Dx()
+	dy := m.Bounds().Dy()
 	nPix = make([]uint8, dx*dy)
 	offset := 0 // steps through the input by sequential scan lines.
 	for _, pass := range interlacing {