png: grayscale support.

R=nigeltao_golang, r
CC=golang-dev
https://golang.org/cl/1897049

src/pkg/image/png/reader.go

@@ -124,6 +124,8 @@ func (d *decoder) parseIHDR(r io.Reader, crc hash.Hash32, length uint32) os.Erro
 	}
 	d.colorType = d.tmp[9]
 	switch d.colorType {
+	case ctGrayscale:
+		d.image = image.NewGray(int(w), int(h))
 	case ctTrueColor:
 		d.image = image.NewRGBA(int(w), int(h))
 	case ctPaletted:
@@ -154,7 +156,7 @@ func (d *decoder) parsePLTE(r io.Reader, crc hash.Hash32, length uint32) os.Erro
 			palette[i] = image.RGBAColor{d.tmp[3*i+0], d.tmp[3*i+1], d.tmp[3*i+2], 0xff}
 		}
 		d.image.(*image.Paletted).Palette = image.PalettedColorModel(palette)
-	case ctTrueColor, ctTrueColorAlpha:
+	case ctGrayscale, ctTrueColor, ctTrueColorAlpha:
 		// As per the PNG spec, a PLTE chunk is optional (and for practical purposes,
 		// ignorable) for the ctTrueColor and ctTrueColorAlpha color types (section 4.1.2).
 		return nil
@@ -174,8 +176,10 @@ func (d *decoder) parsetRNS(r io.Reader, crc hash.Hash32, length uint32) os.Erro
 	}
 	crc.Write(d.tmp[0:n])
 	switch d.colorType {
+	case ctGrayscale:
+		return UnsupportedError("grayscale transparency")
 	case ctTrueColor:
-		return UnsupportedError("TrueColor transparency")
+		return UnsupportedError("truecolor transparency")
 	case ctPaletted:
 		p := d.image.(*image.Paletted).Palette
 		if n > len(p) {
@@ -214,11 +218,15 @@ func (d *decoder) idatReader(idat io.Reader) os.Error {
 	bpp := 0 // Bytes per pixel.
 	maxPalette := uint8(0)
 	var (
+		gray     *image.Gray
 		rgba     *image.RGBA
-		nrgba    *image.NRGBA
 		paletted *image.Paletted
+		nrgba    *image.NRGBA
 	)
 	switch d.colorType {
+	case ctGrayscale:
+		bpp = 1
+		gray = d.image.(*image.Gray)
 	case ctTrueColor:
 		bpp = 3
 		rgba = d.image.(*image.RGBA)
@@ -276,6 +284,10 @@ func (d *decoder) idatReader(idat io.Reader) os.Error {
 
 		// Convert from bytes to colors.
 		switch d.colorType {
+		case ctGrayscale:
+			for x := 0; x < d.width; x++ {
+				gray.Set(x, y, image.GrayColor{cdat[x]})
+			}
 		case ctTrueColor:
 			for x := 0; x < d.width; x++ {
 				rgba.Set(x, y, image.RGBAColor{cdat[3*x+0], cdat[3*x+1], cdat[3*x+2], 0xff})

src/pkg/image/png/reader_test.go

@@ -19,7 +19,7 @@ var filenames = []string{
 	//"basn0g01",	// bit depth is not 8
 	//"basn0g02",	// bit depth is not 8
 	//"basn0g04",	// bit depth is not 8
-	//"basn0g08",	// grayscale color model
+	"basn0g08",
 	//"basn0g16",	// bit depth is not 8
 	"basn2c08",
 	//"basn2c16",	// bit depth is not 8
@@ -56,6 +56,8 @@ func sng(w io.WriteCloser, filename string, png image.Image) {
 	var paletted *image.Paletted
 	cpm, _ := cm.(image.PalettedColorModel)
 	switch {
+	case cm == image.GrayColorModel:
+		io.WriteString(w, "    using grayscale;\n")
 	case cm == image.RGBAColorModel:
 		io.WriteString(w, "    using color;\n")
 	case cm == image.NRGBAColorModel:
@@ -89,6 +91,11 @@ func sng(w io.WriteCloser, filename string, png image.Image) {
 	io.WriteString(w, "IMAGE {\n    pixels hex\n")
 	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
 		switch {
+		case cm == image.GrayColorModel:
+			for x := bounds.Min.X; x < bounds.Max.X; x++ {
+				gray := png.At(x, y).(image.GrayColor)
+				fmt.Fprintf(w, "%02x", gray.Y)
+			}
 		case cm == image.RGBAColorModel:
 			for x := bounds.Min.X; x < bounds.Max.X; x++ {
 				rgba := png.At(x, y).(image.RGBAColor)

src/pkg/image/png/writer.go

@@ -243,6 +243,8 @@ func writeImage(w io.Writer, m image.Image, ct uint8) os.Error {
 	bpp := 0 // Bytes per pixel.
 	var paletted *image.Paletted
 	switch ct {
+	case ctGrayscale:
+		bpp = 1
 	case ctTrueColor:
 		bpp = 3
 	case ctPaletted:
@@ -267,6 +269,11 @@ func writeImage(w io.Writer, m image.Image, ct uint8) os.Error {
 	for y := b.Min.Y; y < b.Max.Y; y++ {
 		// Convert from colors to bytes.
 		switch ct {
+		case ctGrayscale:
+			for x := b.Min.X; x < b.Max.X; x++ {
+				c := image.GrayColorModel.Convert(m.At(x, y)).(image.GrayColor)
+				cr[0][x+1] = c.Y
+			}
 		case ctTrueColor:
 			for x := b.Min.X; x < b.Max.X; x++ {
 				// We have previously verified that the alpha value is fully opaque.
@@ -338,12 +345,19 @@ func Encode(w io.Writer, m image.Image) os.Error {
 	var e encoder
 	e.w = w
 	e.m = m
-	e.colorType = uint8(ctTrueColorAlpha)
+	e.colorType = ctTrueColorAlpha
 	pal, _ := m.(*image.Paletted)
 	if pal != nil {
 		e.colorType = ctPaletted
-	} else if opaque(m) {
-		e.colorType = ctTrueColor
+	} else {
+		switch m.ColorModel() {
+		case image.GrayColorModel:
+			e.colorType = ctGrayscale
+		default:
+			if opaque(m) {
+				e.colorType = ctTrueColor
+			}
+		}
 	}
 
 	_, e.err = io.WriteString(w, pngHeader)