image/png: support encoding of paletted images with alpha channel.

Encoder now writes tRNS chunk for non-opaque paletted images.
CL includes new test images (basn3a08-trns.[ps]ng).

R=nigeltao, rsc, r
CC=golang-dev
https://golang.org/cl/4432078

src/pkg/image/png/reader_test.go

@@ -28,6 +28,7 @@ var filenames = []string{
 	"basn3p02",
 	"basn3p04",
 	"basn3p08",
+	"basn3p08-trns",
 	"basn4a08",
 	"basn4a16",
 	"basn6a08",
@@ -98,17 +99,30 @@ func sng(w io.WriteCloser, filename string, png image.Image) {
 	// (the PNG spec section 11.3 says "Ancillary chunks may be ignored by a decoder").
 	io.WriteString(w, "gAMA {1.0000}\n")
 
-	// Write the PLTE (if applicable).
+	// Write the PLTE and tRNS (if applicable).
 	if cpm != nil {
+		lastAlpha := -1
 		io.WriteString(w, "PLTE {\n")
-		for i := 0; i < len(cpm); i++ {
-			r, g, b, _ := cpm[i].RGBA()
+		for i, c := range cpm {
+			r, g, b, a := c.RGBA()
+			if a != 0xffff {
+				lastAlpha = i
+			}
 			r >>= 8
 			g >>= 8
 			b >>= 8
 			fmt.Fprintf(w, "    (%3d,%3d,%3d)     # rgb = (0x%02x,0x%02x,0x%02x)\n", r, g, b, r, g, b)
 		}
 		io.WriteString(w, "}\n")
+		if lastAlpha != -1 {
+			io.WriteString(w, "tRNS {\n")
+			for i := 0; i <= lastAlpha; i++ {
+				_, _, _, a := cpm[i].RGBA()
+				a >>= 8
+				fmt.Fprintf(w, " %d", a)
+			}
+			io.WriteString(w, "}\n")
+		}
 	}
 
 	// Write the IMAGE.

src/pkg/image/png/testdata/pngsuite/README

@@ -10,6 +10,9 @@ The files basn0g01-30.png, basn0g02-29.png and basn0g04-31.png are in fact
 not part of pngsuite but were created from files in pngsuite. Their non-power-
 of-two sizes makes them useful for testing bit-depths smaller than a byte.
 
+basn3a08.png was generated from basn6a08.png using the pngnq tool, which
+converted it to the 8-bit paletted image with alpha values in tRNS chunk.
+
 The *.sng files in this directory were generated from the *.png files
 by the sng command-line tool and some hand editing. The files
 basn0g0{1,2,4}.sng were actually generated by first converting the PNG

src/pkg/image/png/writer.go

@@ -130,12 +130,8 @@ func (e *encoder) writePLTE(p image.PalettedColorModel) {
 		e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
 		return
 	}
-	for i := 0; i < len(p); i++ {
-		r, g, b, a := p[i].RGBA()
-		if a != 0xffff {
-			e.err = UnsupportedError("non-opaque palette color")
-			return
-		}
+	for i, c := range p {
+		r, g, b, _ := c.RGBA()
 		e.tmp[3*i+0] = uint8(r >> 8)
 		e.tmp[3*i+1] = uint8(g >> 8)
 		e.tmp[3*i+2] = uint8(b >> 8)
@@ -143,6 +139,21 @@ func (e *encoder) writePLTE(p image.PalettedColorModel) {
 	e.writeChunk(e.tmp[0:3*len(p)], "PLTE")
 }
 
+func (e *encoder) maybeWritetRNS(p image.PalettedColorModel) {
+	last := -1
+	for i, c := range p {
+		_, _, _, a := c.RGBA()
+		if a != 0xffff {
+			last = i
+		}
+		e.tmp[i] = uint8(a >> 8)
+	}
+	if last == -1 {
+		return
+	}
+	e.writeChunk(e.tmp[:last+1], "tRNS")
+}
+
 // An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
 // including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
 // should be relatively infrequent, since writeIDATs uses a bufio.Writer.
@@ -447,6 +458,7 @@ func Encode(w io.Writer, m image.Image) os.Error {
 	e.writeIHDR()
 	if pal != nil {
 		e.writePLTE(pal.Palette)
+		e.maybeWritetRNS(pal.Palette)
 	}
 	e.writeIDATs()
 	e.writeIEND()