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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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func writeUint32(b []uint8, u uint32) {
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type opaquer interface {
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// Returns whether or not the image is fully opaque.
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func opaque(m image.Image) bool {
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if o, ok := m.(opaquer); ok {
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for y := b.Min.Y; y < b.Max.Y; y++ {
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for x := b.Min.X; x < b.Max.X; x++ {
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_, _, _, a := m.At(x, y).RGBA()
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// The absolute value of a byte interpreted as a signed int8.
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func abs8(d uint8) int {
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func (e *encoder) writeChunk(b []byte, name string) {
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e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))
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writeUint32(e.header[0:4], n)
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crc := crc32.NewIEEE()
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crc.Write(e.header[4:8])
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writeUint32(e.footer[0:4], crc.Sum32())
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_, e.err = e.w.Write(e.header[0:8])
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_, e.err = e.w.Write(b)
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_, e.err = e.w.Write(e.footer[0:4])
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func (e *encoder) writeIHDR() {
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writeUint32(e.tmp[0:4], uint32(b.Dx()))
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writeUint32(e.tmp[4:8], uint32(b.Dy()))
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// Set bit depth and color type.
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e.tmp[9] = ctGrayscale
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e.tmp[9] = ctTrueColor
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e.tmp[9] = ctPaletted
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e.tmp[9] = ctTrueColorAlpha
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e.tmp[9] = ctGrayscale
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e.tmp[9] = ctTrueColor
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e.tmp[9] = ctTrueColorAlpha
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e.tmp[10] = 0 // default compression method
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e.tmp[11] = 0 // default filter method
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e.tmp[12] = 0 // non-interlaced
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e.writeChunk(e.tmp[0:13], "IHDR")
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func (e *encoder) writePLTE(p image.PalettedColorModel) {
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if len(p) < 1 || len(p) > 256 {
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e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
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for i := 0; i < len(p); i++ {
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r, g, b, a := p[i].RGBA()
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e.err = UnsupportedError("non-opaque palette color")
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e.tmp[3*i+0] = uint8(r >> 8)
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e.tmp[3*i+1] = uint8(g >> 8)
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e.tmp[3*i+2] = uint8(b >> 8)
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e.writeChunk(e.tmp[0:3*len(p)], "PLTE")
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// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,
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// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls
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// should be relatively infrequent, since writeIDATs uses a bufio.Writer.
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// This method should only be called from writeIDATs (via writeImage).
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// No other code should treat an encoder as an io.Writer.
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// Note that, because the zlib Reader may involve an io.Pipe, e.Write calls may
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// occur on a separate go-routine than the e.writeIDATs call, and care should be
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// taken that e's state (such as its tmp buffer) is not modified concurrently.
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func (e *encoder) Write(b []byte) (int, os.Error) {
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e.writeChunk(b, "IDAT")
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// Chooses the filter to use for encoding the current row, and applies it.
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// The return value is the index of the filter and also of the row in cr that has had it applied.
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func filter(cr [][]byte, pr []byte, bpp int) int {
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// We try all five filter types, and pick the one that minimizes the sum of absolute differences.
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// This is the same heuristic that libpng uses, although the filters are attempted in order of
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// estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than
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// in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).
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for i := 0; i < n; i++ {
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cdat2[i] = cdat0[i] - pdat[i]
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sum += abs8(cdat2[i])
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for i := 0; i < bpp; i++ {
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cdat4[i] = cdat0[i] - paeth(0, pdat[i], 0)
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sum += abs8(cdat4[i])
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for i := bpp; i < n; i++ {
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cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])
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sum += abs8(cdat4[i])
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for i := 0; i < n; i++ {
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sum += abs8(cdat0[i])
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for i := 0; i < bpp; i++ {
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sum += abs8(cdat1[i])
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for i := bpp; i < n; i++ {
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cdat1[i] = cdat0[i] - cdat0[i-bpp]
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sum += abs8(cdat1[i])
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// The average filter.
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for i := 0; i < bpp; i++ {
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cdat3[i] = cdat0[i] - pdat[i]/2
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sum += abs8(cdat3[i])
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for i := bpp; i < n; i++ {
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cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)
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sum += abs8(cdat3[i])
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func writeImage(w io.Writer, m image.Image, cb int) os.Error {
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zw, err := zlib.NewWriter(w)
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bpp := 0 // Bytes per pixel.
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var paletted *image.Paletted
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paletted = m.(*image.Paletted)
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// cr[*] and pr are the bytes for the current and previous row.
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// cr[0] is unfiltered (or equivalently, filtered with the ftNone filter).
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// cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the
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// other PNG filter types. These buffers are allocated once and re-used for each row.
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// The +1 is for the per-row filter type, which is at cr[*][0].
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var cr [nFilter][]uint8
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for i := 0; i < len(cr); i++ {
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cr[i] = make([]uint8, 1+bpp*b.Dx())
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pr := make([]uint8, 1+bpp*b.Dx())
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for y := b.Min.Y; y < b.Max.Y; y++ {
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// Convert from colors to bytes.
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for x := b.Min.X; x < b.Max.X; x++ {
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c := image.GrayColorModel.Convert(m.At(x, y)).(image.GrayColor)
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for x := b.Min.X; x < b.Max.X; x++ {
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// We have previously verified that the alpha value is fully opaque.
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r, g, b, _ := m.At(x, y).RGBA()
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cr[0][3*x+1] = uint8(r >> 8)
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cr[0][3*x+2] = uint8(g >> 8)
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cr[0][3*x+3] = uint8(b >> 8)
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rowOffset := y * paletted.Stride
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copy(cr[0][b.Min.X+1:], paletted.Pix[rowOffset+b.Min.X:rowOffset+b.Max.X])
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// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
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for x := b.Min.X; x < b.Max.X; x++ {
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c := image.NRGBAColorModel.Convert(m.At(x, y)).(image.NRGBAColor)
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for x := b.Min.X; x < b.Max.X; x++ {
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c := image.Gray16ColorModel.Convert(m.At(x, y)).(image.Gray16Color)
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cr[0][2*x+1] = uint8(c.Y >> 8)
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cr[0][2*x+2] = uint8(c.Y)
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for x := b.Min.X; x < b.Max.X; x++ {
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// We have previously verified that the alpha value is fully opaque.
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r, g, b, _ := m.At(x, y).RGBA()
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cr[0][6*x+1] = uint8(r >> 8)
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cr[0][6*x+2] = uint8(r)
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cr[0][6*x+3] = uint8(g >> 8)
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cr[0][6*x+4] = uint8(g)
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cr[0][6*x+5] = uint8(b >> 8)
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cr[0][6*x+6] = uint8(b)
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// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
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for x := b.Min.X; x < b.Max.X; x++ {
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c := image.NRGBA64ColorModel.Convert(m.At(x, y)).(image.NRGBA64Color)
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cr[0][8*x+1] = uint8(c.R >> 8)
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cr[0][8*x+2] = uint8(c.R)
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cr[0][8*x+3] = uint8(c.G >> 8)
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cr[0][8*x+4] = uint8(c.G)
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cr[0][8*x+5] = uint8(c.B >> 8)
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cr[0][8*x+6] = uint8(c.B)
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cr[0][8*x+7] = uint8(c.A >> 8)
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cr[0][8*x+8] = uint8(c.A)
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f := filter(cr[0:nFilter], pr, bpp)
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// Write the compressed bytes.
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_, err = zw.Write(cr[f])
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// The current row for y is the previous row for y+1.
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pr, cr[0] = cr[0], pr
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// Write the actual image data to one or more IDAT chunks.
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func (e *encoder) writeIDATs() {
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bw, e.err = bufio.NewWriterSize(e, 1<<15)
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e.err = writeImage(bw, e.m, e.cb)
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func (e *encoder) writeIEND() { e.writeChunk(e.tmp[0:0], "IEND") }
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// Encode writes the Image m to w in PNG format. Any Image may be encoded, but
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// images that are not image.NRGBA might be encoded lossily.
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func Encode(w io.Writer, m image.Image) os.Error {
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// Obviously, negative widths and heights are invalid. Furthermore, the PNG
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// spec section 11.2.2 says that zero is invalid. Excessively large images are
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mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())
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if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 {
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return FormatError("invalid image size: " + strconv.Itoa64(mw) + "x" + strconv.Itoa64(mw))
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pal, _ := m.(*image.Paletted)
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switch m.ColorModel() {
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case image.GrayColorModel:
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case image.Gray16ColorModel:
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case image.RGBAColorModel, image.NRGBAColorModel, image.AlphaColorModel:
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_, e.err = io.WriteString(w, pngHeader)
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e.writePLTE(pal.Palette)