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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
 
package png
 
import (
        "bufio"
        "compress/zlib"
        "hash/crc32"
        "image"
        "image/color"
        "io"
        "strconv"
)
 
type encoder struct {
        w      io.Writer
        m      image.Image
        cb     int
        err    error
        header [8]byte
        footer [4]byte
        tmp    [3 * 256]byte
}
 
// Big-endian.
func writeUint32(b []uint8, u uint32) {
        b[0] = uint8(u >> 24)
        b[1] = uint8(u >> 16)
        b[2] = uint8(u >> 8)
        b[3] = uint8(u >> 0)
}
 
type opaquer interface {
        Opaque() bool
}
 
// Returns whether or not the image is fully opaque.
func opaque(m image.Image) bool {
        if o, ok := m.(opaquer); ok {
                return o.Opaque()
        }
        b := m.Bounds()
        for y := b.Min.Y; y < b.Max.Y; y++ {
                for x := b.Min.X; x < b.Max.X; x++ {
                        _, _, _, a := m.At(x, y).RGBA()
                        if a != 0xffff {
                                return false
                        }
                }
        }
        return true
}
 
// The absolute value of a byte interpreted as a signed int8.
func abs8(d uint8) int {
        if d < 128 {
                return int(d)
        }
        return 256 - int(d)
}
 
func (e *encoder) writeChunk(b []byte, name string) {
        if e.err != nil {
                return
        }
        n := uint32(len(b))
        if int(n) != len(b) {
                e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))
                return
        }
        writeUint32(e.header[0:4], n)
        e.header[4] = name[0]
        e.header[5] = name[1]
        e.header[6] = name[2]
        e.header[7] = name[3]
        crc := crc32.NewIEEE()
        crc.Write(e.header[4:8])
        crc.Write(b)
        writeUint32(e.footer[0:4], crc.Sum32())
 
        _, e.err = e.w.Write(e.header[0:8])
        if e.err != nil {
                return
        }
        _, e.err = e.w.Write(b)
        if e.err != nil {
                return
        }
        _, e.err = e.w.Write(e.footer[0:4])
}
 
func (e *encoder) writeIHDR() {
        b := e.m.Bounds()
        writeUint32(e.tmp[0:4], uint32(b.Dx()))
        writeUint32(e.tmp[4:8], uint32(b.Dy()))
        // Set bit depth and color type.
        switch e.cb {
        case cbG8:
                e.tmp[8] = 8
                e.tmp[9] = ctGrayscale
        case cbTC8:
                e.tmp[8] = 8
                e.tmp[9] = ctTrueColor
        case cbP8:
                e.tmp[8] = 8
                e.tmp[9] = ctPaletted
        case cbTCA8:
                e.tmp[8] = 8
                e.tmp[9] = ctTrueColorAlpha
        case cbG16:
                e.tmp[8] = 16
                e.tmp[9] = ctGrayscale
        case cbTC16:
                e.tmp[8] = 16
                e.tmp[9] = ctTrueColor
        case cbTCA16:
                e.tmp[8] = 16
                e.tmp[9] = ctTrueColorAlpha
        }
        e.tmp[10] = 0 // default compression method
        e.tmp[11] = 0 // default filter method
        e.tmp[12] = 0 // non-interlaced
        e.writeChunk(e.tmp[0:13], "IHDR")
}
 
func (e *encoder) writePLTE(p color.Palette) {
        if len(p) < 1 || len(p) > 256 {
                e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))
                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)
        }
        e.writeChunk(e.tmp[0:3*len(p)], "PLTE")
}
 
func (e *encoder) maybeWritetRNS(p color.Palette) {
        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.
//
// This method should only be called from writeIDATs (via writeImage).
// No other code should treat an encoder as an io.Writer.
func (e *encoder) Write(b []byte) (int, error) {
        e.writeChunk(b, "IDAT")
        if e.err != nil {
                return 0, e.err
        }
        return len(b), nil
}
 
// Chooses the filter to use for encoding the current row, and applies it.
// The return value is the index of the filter and also of the row in cr that has had it applied.
func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {
        // We try all five filter types, and pick the one that minimizes the sum of absolute differences.
        // This is the same heuristic that libpng uses, although the filters are attempted in order of
        // estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than
        // in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).
        cdat0 := cr[0][1:]
        cdat1 := cr[1][1:]
        cdat2 := cr[2][1:]
        cdat3 := cr[3][1:]
        cdat4 := cr[4][1:]
        pdat := pr[1:]
        n := len(cdat0)
 
        // The up filter.
        sum := 0
        for i := 0; i < n; i++ {
                cdat2[i] = cdat0[i] - pdat[i]
                sum += abs8(cdat2[i])
        }
        best := sum
        filter := ftUp
 
        // The Paeth filter.
        sum = 0
        for i := 0; i < bpp; i++ {
                cdat4[i] = cdat0[i] - paeth(0, pdat[i], 0)
                sum += abs8(cdat4[i])
        }
        for i := bpp; i < n; i++ {
                cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])
                sum += abs8(cdat4[i])
                if sum >= best {
                        break
                }
        }
        if sum < best {
                best = sum
                filter = ftPaeth
        }
 
        // The none filter.
        sum = 0
        for i := 0; i < n; i++ {
                sum += abs8(cdat0[i])
                if sum >= best {
                        break
                }
        }
        if sum < best {
                best = sum
                filter = ftNone
        }
 
        // The sub filter.
        sum = 0
        for i := 0; i < bpp; i++ {
                cdat1[i] = cdat0[i]
                sum += abs8(cdat1[i])
        }
        for i := bpp; i < n; i++ {
                cdat1[i] = cdat0[i] - cdat0[i-bpp]
                sum += abs8(cdat1[i])
                if sum >= best {
                        break
                }
        }
        if sum < best {
                best = sum
                filter = ftSub
        }
 
        // The average filter.
        sum = 0
        for i := 0; i < bpp; i++ {
                cdat3[i] = cdat0[i] - pdat[i]/2
                sum += abs8(cdat3[i])
        }
        for i := bpp; i < n; i++ {
                cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)
                sum += abs8(cdat3[i])
                if sum >= best {
                        break
                }
        }
        if sum < best {
                best = sum
                filter = ftAverage
        }
 
        return filter
}
 
func writeImage(w io.Writer, m image.Image, cb int) error {
        zw, err := zlib.NewWriter(w)
        if err != nil {
                return err
        }
        defer zw.Close()
 
        bpp := 0 // Bytes per pixel.
 
        switch cb {
        case cbG8:
                bpp = 1
        case cbTC8:
                bpp = 3
        case cbP8:
                bpp = 1
        case cbTCA8:
                bpp = 4
        case cbTC16:
                bpp = 6
        case cbTCA16:
                bpp = 8
        case cbG16:
                bpp = 2
        }
        // cr[*] and pr are the bytes for the current and previous row.
        // cr[0] is unfiltered (or equivalently, filtered with the ftNone filter).
        // cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the
        // other PNG filter types. These buffers are allocated once and re-used for each row.
        // The +1 is for the per-row filter type, which is at cr[*][0].
        b := m.Bounds()
        var cr [nFilter][]uint8
        for i := range cr {
                cr[i] = make([]uint8, 1+bpp*b.Dx())
                cr[i][0] = uint8(i)
        }
        pr := make([]uint8, 1+bpp*b.Dx())
 
        for y := b.Min.Y; y < b.Max.Y; y++ {
                // Convert from colors to bytes.
                i := 1
                switch cb {
                case cbG8:
                        for x := b.Min.X; x < b.Max.X; x++ {
                                c := color.GrayModel.Convert(m.At(x, y)).(color.Gray)
                                cr[0][i] = c.Y
                                i++
                        }
                case cbTC8:
                        // We have previously verified that the alpha value is fully opaque.
                        cr0 := cr[0]
                        if rgba, _ := m.(*image.RGBA); rgba != nil {
                                j0 := (y - b.Min.Y) * rgba.Stride
                                j1 := j0 + b.Dx()*4
                                for j := j0; j < j1; j += 4 {
                                        cr0[i+0] = rgba.Pix[j+0]
                                        cr0[i+1] = rgba.Pix[j+1]
                                        cr0[i+2] = rgba.Pix[j+2]
                                        i += 3
                                }
                        } else {
                                for x := b.Min.X; x < b.Max.X; x++ {
                                        r, g, b, _ := m.At(x, y).RGBA()
                                        cr0[i+0] = uint8(r >> 8)
                                        cr0[i+1] = uint8(g >> 8)
                                        cr0[i+2] = uint8(b >> 8)
                                        i += 3
                                }
                        }
                case cbP8:
                        if p, _ := m.(*image.Paletted); p != nil {
                                offset := (y - b.Min.Y) * p.Stride
                                copy(cr[0][1:], p.Pix[offset:offset+b.Dx()])
                        } else {
                                pi := m.(image.PalettedImage)
                                for x := b.Min.X; x < b.Max.X; x++ {
                                        cr[0][i] = pi.ColorIndexAt(x, y)
                                        i += 1
                                }
                        }
                case cbTCA8:
                        // Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
                        for x := b.Min.X; x < b.Max.X; x++ {
                                c := color.NRGBAModel.Convert(m.At(x, y)).(color.NRGBA)
                                cr[0][i+0] = c.R
                                cr[0][i+1] = c.G
                                cr[0][i+2] = c.B
                                cr[0][i+3] = c.A
                                i += 4
                        }
                case cbG16:
                        for x := b.Min.X; x < b.Max.X; x++ {
                                c := color.Gray16Model.Convert(m.At(x, y)).(color.Gray16)
                                cr[0][i+0] = uint8(c.Y >> 8)
                                cr[0][i+1] = uint8(c.Y)
                                i += 2
                        }
                case cbTC16:
                        // We have previously verified that the alpha value is fully opaque.
                        for x := b.Min.X; x < b.Max.X; x++ {
                                r, g, b, _ := m.At(x, y).RGBA()
                                cr[0][i+0] = uint8(r >> 8)
                                cr[0][i+1] = uint8(r)
                                cr[0][i+2] = uint8(g >> 8)
                                cr[0][i+3] = uint8(g)
                                cr[0][i+4] = uint8(b >> 8)
                                cr[0][i+5] = uint8(b)
                                i += 6
                        }
                case cbTCA16:
                        // Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
                        for x := b.Min.X; x < b.Max.X; x++ {
                                c := color.NRGBA64Model.Convert(m.At(x, y)).(color.NRGBA64)
                                cr[0][i+0] = uint8(c.R >> 8)
                                cr[0][i+1] = uint8(c.R)
                                cr[0][i+2] = uint8(c.G >> 8)
                                cr[0][i+3] = uint8(c.G)
                                cr[0][i+4] = uint8(c.B >> 8)
                                cr[0][i+5] = uint8(c.B)
                                cr[0][i+6] = uint8(c.A >> 8)
                                cr[0][i+7] = uint8(c.A)
                                i += 8
                        }
                }
 
                // Apply the filter.
                f := filter(&cr, pr, bpp)
 
                // Write the compressed bytes.
                _, err = zw.Write(cr[f])
                if err != nil {
                        return err
                }
 
                // The current row for y is the previous row for y+1.
                pr, cr[0] = cr[0], pr
        }
        return nil
}
 
// Write the actual image data to one or more IDAT chunks.
func (e *encoder) writeIDATs() {
        if e.err != nil {
                return
        }
        var bw *bufio.Writer
        bw = bufio.NewWriterSize(e, 1<<15)
        e.err = writeImage(bw, e.m, e.cb)
        if e.err != nil {
                return
        }
        e.err = bw.Flush()
}
 
func (e *encoder) writeIEND() { e.writeChunk(e.tmp[0:0], "IEND") }
 
// Encode writes the Image m to w in PNG format. Any Image may be encoded, but
// images that are not image.NRGBA might be encoded lossily.
func Encode(w io.Writer, m image.Image) error {
        // Obviously, negative widths and heights are invalid. Furthermore, the PNG
        // spec section 11.2.2 says that zero is invalid. Excessively large images are
        // also rejected.
        mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())
        if mw <= 0 || mh <= 0 || mw >= 1<<32 || mh >= 1<<32 {
                return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mw, 10))
        }
 
        var e encoder
        e.w = w
        e.m = m
 
        var pal color.Palette
        // cbP8 encoding needs PalettedImage's ColorIndexAt method.
        if _, ok := m.(image.PalettedImage); ok {
                pal, _ = m.ColorModel().(color.Palette)
        }
        if pal != nil {
                e.cb = cbP8
        } else {
                switch m.ColorModel() {
                case color.GrayModel:
                        e.cb = cbG8
                case color.Gray16Model:
                        e.cb = cbG16
                case color.RGBAModel, color.NRGBAModel, color.AlphaModel:
                        if opaque(m) {
                                e.cb = cbTC8
                        } else {
                                e.cb = cbTCA8
                        }
                default:
                        if opaque(m) {
                                e.cb = cbTC16
                        } else {
                                e.cb = cbTCA16
                        }
                }
        }
 
        _, e.err = io.WriteString(w, pngHeader)
        e.writeIHDR()
        if pal != nil {
                e.writePLTE(pal)
                e.maybeWritetRNS(pal)
        }
        e.writeIDATs()
        e.writeIEND()
        return e.err
}

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Subversion Repositories openrisc

[/] [openrisc/] [trunk/] [gnu-dev/] [or1k-gcc/] [libgo/] [go/] [image/] [png/] [writer.go] - Blame information for rev 747

Line No.	Rev	Author	Line
1	747	jeremybenn	`// Copyright 2009 The Go Authors. All rights reserved.`
2			`// Use of this source code is governed by a BSD-style`
3			`// license that can be found in the LICENSE file.`
4
5			`package png`
6
7			`import (`
8			`"bufio"`
9			`"compress/zlib"`
10			`"hash/crc32"`
11			`"image"`
12			`"image/color"`
13			`"io"`
14			`"strconv"`
15			`)`
16
17			`type encoder struct {`
18			`w io.Writer`
19			`m image.Image`
20			`cb int`
21			`err error`
22			`header [8]byte`
23			`footer [4]byte`
24			`tmp [3 * 256]byte`
25			`}`
26
27			`// Big-endian.`
28			`func writeUint32(b []uint8, u uint32) {`
29			`b[0] = uint8(u >> 24)`
30			`b[1] = uint8(u >> 16)`
31			`b[2] = uint8(u >> 8)`
32			`b[3] = uint8(u >> 0)`
33			`}`
34
35			`type opaquer interface {`
36			`Opaque() bool`
37			`}`
38
39			`// Returns whether or not the image is fully opaque.`
40			`func opaque(m image.Image) bool {`
41			`if o, ok := m.(opaquer); ok {`
42			`return o.Opaque()`
43			`}`
44			`b := m.Bounds()`
45			`for y := b.Min.Y; y < b.Max.Y; y++ {`
46			`for x := b.Min.X; x < b.Max.X; x++ {`
47			`_, _, _, a := m.At(x, y).RGBA()`
48			`if a != 0xffff {`
49			`return false`
50			`}`
51			`}`
52			`}`
53			`return true`
54			`}`
55
56			`// The absolute value of a byte interpreted as a signed int8.`
57			`func abs8(d uint8) int {`
58			`if d < 128 {`
59			`return int(d)`
60			`}`
61			`return 256 - int(d)`
62			`}`
63
64			`func (e *encoder) writeChunk(b []byte, name string) {`
65			`if e.err != nil {`
66			`return`
67			`}`
68			`n := uint32(len(b))`
69			`if int(n) != len(b) {`
70			`e.err = UnsupportedError(name + " chunk is too large: " + strconv.Itoa(len(b)))`
71			`return`
72			`}`
73			`writeUint32(e.header[0:4], n)`
74			`e.header[4] = name[0]`
75			`e.header[5] = name[1]`
76			`e.header[6] = name[2]`
77			`e.header[7] = name[3]`
78			`crc := crc32.NewIEEE()`
79			`crc.Write(e.header[4:8])`
80			`crc.Write(b)`
81			`writeUint32(e.footer[0:4], crc.Sum32())`
82
83			`_, e.err = e.w.Write(e.header[0:8])`
84			`if e.err != nil {`
85			`return`
86			`}`
87			`_, e.err = e.w.Write(b)`
88			`if e.err != nil {`
89			`return`
90			`}`
91			`_, e.err = e.w.Write(e.footer[0:4])`
92			`}`
93
94			`func (e *encoder) writeIHDR() {`
95			`b := e.m.Bounds()`
96			`writeUint32(e.tmp[0:4], uint32(b.Dx()))`
97			`writeUint32(e.tmp[4:8], uint32(b.Dy()))`
98			`// Set bit depth and color type.`
99			`switch e.cb {`
100			`case cbG8:`
101			`e.tmp[8] = 8`
102			`e.tmp[9] = ctGrayscale`
103			`case cbTC8:`
104			`e.tmp[8] = 8`
105			`e.tmp[9] = ctTrueColor`
106			`case cbP8:`
107			`e.tmp[8] = 8`
108			`e.tmp[9] = ctPaletted`
109			`case cbTCA8:`
110			`e.tmp[8] = 8`
111			`e.tmp[9] = ctTrueColorAlpha`
112			`case cbG16:`
113			`e.tmp[8] = 16`
114			`e.tmp[9] = ctGrayscale`
115			`case cbTC16:`
116			`e.tmp[8] = 16`
117			`e.tmp[9] = ctTrueColor`
118			`case cbTCA16:`
119			`e.tmp[8] = 16`
120			`e.tmp[9] = ctTrueColorAlpha`
121			`}`
122			`e.tmp[10] = 0 // default compression method`
123			`e.tmp[11] = 0 // default filter method`
124			`e.tmp[12] = 0 // non-interlaced`
125			`e.writeChunk(e.tmp[0:13], "IHDR")`
126			`}`
127
128			`func (e *encoder) writePLTE(p color.Palette) {`
129			`if len(p) < 1 \|\| len(p) > 256 {`
130			`e.err = FormatError("bad palette length: " + strconv.Itoa(len(p)))`
131			`return`
132			`}`
133			`for i, c := range p {`
134			`r, g, b, _ := c.RGBA()`
135			`e.tmp[3*i+0] = uint8(r >> 8)`
136			`e.tmp[3*i+1] = uint8(g >> 8)`
137			`e.tmp[3*i+2] = uint8(b >> 8)`
138			`}`
139			`e.writeChunk(e.tmp[0:3*len(p)], "PLTE")`
140			`}`
141
142			`func (e *encoder) maybeWritetRNS(p color.Palette) {`
143			`last := -1`
144			`for i, c := range p {`
145			`_, _, _, a := c.RGBA()`
146			`if a != 0xffff {`
147			`last = i`
148			`}`
149			`e.tmp[i] = uint8(a >> 8)`
150			`}`
151			`if last == -1 {`
152			`return`
153			`}`
154			`e.writeChunk(e.tmp[:last+1], "tRNS")`
155			`}`
156
157			`// An encoder is an io.Writer that satisfies writes by writing PNG IDAT chunks,`
158			`// including an 8-byte header and 4-byte CRC checksum per Write call. Such calls`
159			`// should be relatively infrequent, since writeIDATs uses a bufio.Writer.`
160			`//`
161			`// This method should only be called from writeIDATs (via writeImage).`
162			`// No other code should treat an encoder as an io.Writer.`
163			`func (e *encoder) Write(b []byte) (int, error) {`
164			`e.writeChunk(b, "IDAT")`
165			`if e.err != nil {`
166			`return 0, e.err`
167			`}`
168			`return len(b), nil`
169			`}`
170
171			`// Chooses the filter to use for encoding the current row, and applies it.`
172			`// The return value is the index of the filter and also of the row in cr that has had it applied.`
173			`func filter(cr *[nFilter][]byte, pr []byte, bpp int) int {`
174			`// We try all five filter types, and pick the one that minimizes the sum of absolute differences.`
175			`// This is the same heuristic that libpng uses, although the filters are attempted in order of`
176			`// estimated most likely to be minimal (ftUp, ftPaeth, ftNone, ftSub, ftAverage), rather than`
177			`// in their enumeration order (ftNone, ftSub, ftUp, ftAverage, ftPaeth).`
178			`cdat0 := cr[0][1:]`
179			`cdat1 := cr[1][1:]`
180			`cdat2 := cr[2][1:]`
181			`cdat3 := cr[3][1:]`
182			`cdat4 := cr[4][1:]`
183			`pdat := pr[1:]`
184			`n := len(cdat0)`
185
186			`// The up filter.`
187			`sum := 0`
188			`for i := 0; i < n; i++ {`
189			`cdat2[i] = cdat0[i] - pdat[i]`
190			`sum += abs8(cdat2[i])`
191			`}`
192			`best := sum`
193			`filter := ftUp`
194
195			`// The Paeth filter.`
196			`sum = 0`
197			`for i := 0; i < bpp; i++ {`
198			`cdat4[i] = cdat0[i] - paeth(0, pdat[i], 0)`
199			`sum += abs8(cdat4[i])`
200			`}`
201			`for i := bpp; i < n; i++ {`
202			`cdat4[i] = cdat0[i] - paeth(cdat0[i-bpp], pdat[i], pdat[i-bpp])`
203			`sum += abs8(cdat4[i])`
204			`if sum >= best {`
205			`break`
206			`}`
207			`}`
208			`if sum < best {`
209			`best = sum`
210			`filter = ftPaeth`
211			`}`
212
213			`// The none filter.`
214			`sum = 0`
215			`for i := 0; i < n; i++ {`
216			`sum += abs8(cdat0[i])`
217			`if sum >= best {`
218			`break`
219			`}`
220			`}`
221			`if sum < best {`
222			`best = sum`
223			`filter = ftNone`
224			`}`
225
226			`// The sub filter.`
227			`sum = 0`
228			`for i := 0; i < bpp; i++ {`
229			`cdat1[i] = cdat0[i]`
230			`sum += abs8(cdat1[i])`
231			`}`
232			`for i := bpp; i < n; i++ {`
233			`cdat1[i] = cdat0[i] - cdat0[i-bpp]`
234			`sum += abs8(cdat1[i])`
235			`if sum >= best {`
236			`break`
237			`}`
238			`}`
239			`if sum < best {`
240			`best = sum`
241			`filter = ftSub`
242			`}`
243
244			`// The average filter.`
245			`sum = 0`
246			`for i := 0; i < bpp; i++ {`
247			`cdat3[i] = cdat0[i] - pdat[i]/2`
248			`sum += abs8(cdat3[i])`
249			`}`
250			`for i := bpp; i < n; i++ {`
251			`cdat3[i] = cdat0[i] - uint8((int(cdat0[i-bpp])+int(pdat[i]))/2)`
252			`sum += abs8(cdat3[i])`
253			`if sum >= best {`
254			`break`
255			`}`
256			`}`
257			`if sum < best {`
258			`best = sum`
259			`filter = ftAverage`
260			`}`
261
262			`return filter`
263			`}`
264
265			`func writeImage(w io.Writer, m image.Image, cb int) error {`
266			`zw, err := zlib.NewWriter(w)`
267			`if err != nil {`
268			`return err`
269			`}`
270			`defer zw.Close()`
271
272			`bpp := 0 // Bytes per pixel.`
273
274			`switch cb {`
275			`case cbG8:`
276			`bpp = 1`
277			`case cbTC8:`
278			`bpp = 3`
279			`case cbP8:`
280			`bpp = 1`
281			`case cbTCA8:`
282			`bpp = 4`
283			`case cbTC16:`
284			`bpp = 6`
285			`case cbTCA16:`
286			`bpp = 8`
287			`case cbG16:`
288			`bpp = 2`
289			`}`
290			`// cr[*] and pr are the bytes for the current and previous row.`
291			`// cr[0] is unfiltered (or equivalently, filtered with the ftNone filter).`
292			`// cr[ft], for non-zero filter types ft, are buffers for transforming cr[0] under the`
293			`// other PNG filter types. These buffers are allocated once and re-used for each row.`
294			`// The +1 is for the per-row filter type, which is at cr[*][0].`
295			`b := m.Bounds()`
296			`var cr [nFilter][]uint8`
297			`for i := range cr {`
298			`cr[i] = make([]uint8, 1+bpp*b.Dx())`
299			`cr[i][0] = uint8(i)`
300			`}`
301			`pr := make([]uint8, 1+bpp*b.Dx())`
302
303			`for y := b.Min.Y; y < b.Max.Y; y++ {`
304			`// Convert from colors to bytes.`
305			`i := 1`
306			`switch cb {`
307			`case cbG8:`
308			`for x := b.Min.X; x < b.Max.X; x++ {`
309			`c := color.GrayModel.Convert(m.At(x, y)).(color.Gray)`
310			`cr[0][i] = c.Y`
311			`i++`
312			`}`
313			`case cbTC8:`
314			`// We have previously verified that the alpha value is fully opaque.`
315			`cr0 := cr[0]`
316			`if rgba, _ := m.(*image.RGBA); rgba != nil {`
317			`j0 := (y - b.Min.Y) * rgba.Stride`
318			`j1 := j0 + b.Dx()*4`
319			`for j := j0; j < j1; j += 4 {`
320			`cr0[i+0] = rgba.Pix[j+0]`
321			`cr0[i+1] = rgba.Pix[j+1]`
322			`cr0[i+2] = rgba.Pix[j+2]`
323			`i += 3`
324			`}`
325			`} else {`
326			`for x := b.Min.X; x < b.Max.X; x++ {`
327			`r, g, b, _ := m.At(x, y).RGBA()`
328			`cr0[i+0] = uint8(r >> 8)`
329			`cr0[i+1] = uint8(g >> 8)`
330			`cr0[i+2] = uint8(b >> 8)`
331			`i += 3`
332			`}`
333			`}`
334			`case cbP8:`
335			`if p, _ := m.(*image.Paletted); p != nil {`
336			`offset := (y - b.Min.Y) * p.Stride`
337			`copy(cr[0][1:], p.Pix[offset:offset+b.Dx()])`
338			`} else {`
339			`pi := m.(image.PalettedImage)`
340			`for x := b.Min.X; x < b.Max.X; x++ {`
341			`cr[0][i] = pi.ColorIndexAt(x, y)`
342			`i += 1`
343			`}`
344			`}`
345			`case cbTCA8:`
346			`// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.`
347			`for x := b.Min.X; x < b.Max.X; x++ {`
348			`c := color.NRGBAModel.Convert(m.At(x, y)).(color.NRGBA)`
349			`cr[0][i+0] = c.R`
350			`cr[0][i+1] = c.G`
351			`cr[0][i+2] = c.B`
352			`cr[0][i+3] = c.A`
353			`i += 4`
354			`}`
355			`case cbG16:`
356			`for x := b.Min.X; x < b.Max.X; x++ {`
357			`c := color.Gray16Model.Convert(m.At(x, y)).(color.Gray16)`
358			`cr[0][i+0] = uint8(c.Y >> 8)`
359			`cr[0][i+1] = uint8(c.Y)`
360			`i += 2`
361			`}`
362			`case cbTC16:`
363			`// We have previously verified that the alpha value is fully opaque.`
364			`for x := b.Min.X; x < b.Max.X; x++ {`
365			`r, g, b, _ := m.At(x, y).RGBA()`
366			`cr[0][i+0] = uint8(r >> 8)`
367			`cr[0][i+1] = uint8(r)`
368			`cr[0][i+2] = uint8(g >> 8)`
369			`cr[0][i+3] = uint8(g)`
370			`cr[0][i+4] = uint8(b >> 8)`
371			`cr[0][i+5] = uint8(b)`
372			`i += 6`
373			`}`
374			`case cbTCA16:`
375			`// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.`
376			`for x := b.Min.X; x < b.Max.X; x++ {`
377			`c := color.NRGBA64Model.Convert(m.At(x, y)).(color.NRGBA64)`
378			`cr[0][i+0] = uint8(c.R >> 8)`
379			`cr[0][i+1] = uint8(c.R)`
380			`cr[0][i+2] = uint8(c.G >> 8)`
381			`cr[0][i+3] = uint8(c.G)`
382			`cr[0][i+4] = uint8(c.B >> 8)`
383			`cr[0][i+5] = uint8(c.B)`
384			`cr[0][i+6] = uint8(c.A >> 8)`
385			`cr[0][i+7] = uint8(c.A)`
386			`i += 8`
387			`}`
388			`}`
389
390			`// Apply the filter.`
391			`f := filter(&cr, pr, bpp)`
392
393			`// Write the compressed bytes.`
394			`_, err = zw.Write(cr[f])`
395			`if err != nil {`
396			`return err`
397			`}`
398
399			`// The current row for y is the previous row for y+1.`
400			`pr, cr[0] = cr[0], pr`
401			`}`
402			`return nil`
403			`}`
404
405			`// Write the actual image data to one or more IDAT chunks.`
406			`func (e *encoder) writeIDATs() {`
407			`if e.err != nil {`
408			`return`
409			`}`
410			`var bw *bufio.Writer`
411			`bw = bufio.NewWriterSize(e, 1<<15)`
412			`e.err = writeImage(bw, e.m, e.cb)`
413			`if e.err != nil {`
414			`return`
415			`}`
416			`e.err = bw.Flush()`
417			`}`
418
419			`func (e *encoder) writeIEND() { e.writeChunk(e.tmp[0:0], "IEND") }`
420
421			`// Encode writes the Image m to w in PNG format. Any Image may be encoded, but`
422			`// images that are not image.NRGBA might be encoded lossily.`
423			`func Encode(w io.Writer, m image.Image) error {`
424			`// Obviously, negative widths and heights are invalid. Furthermore, the PNG`
425			`// spec section 11.2.2 says that zero is invalid. Excessively large images are`
426			`// also rejected.`
427			`mw, mh := int64(m.Bounds().Dx()), int64(m.Bounds().Dy())`
428			`if mw <= 0 \|\| mh <= 0 \|\| mw >= 1<<32 \|\| mh >= 1<<32 {`
429			`return FormatError("invalid image size: " + strconv.FormatInt(mw, 10) + "x" + strconv.FormatInt(mw, 10))`
430			`}`
431
432			`var e encoder`
433			`e.w = w`
434			`e.m = m`
435
436			`var pal color.Palette`
437			`// cbP8 encoding needs PalettedImage's ColorIndexAt method.`
438			`if _, ok := m.(image.PalettedImage); ok {`
439			`pal, _ = m.ColorModel().(color.Palette)`
440			`}`
441			`if pal != nil {`
442			`e.cb = cbP8`
443			`} else {`
444			`switch m.ColorModel() {`
445			`case color.GrayModel:`
446			`e.cb = cbG8`
447			`case color.Gray16Model:`
448			`e.cb = cbG16`
449			`case color.RGBAModel, color.NRGBAModel, color.AlphaModel:`
450			`if opaque(m) {`
451			`e.cb = cbTC8`
452			`} else {`
453			`e.cb = cbTCA8`
454			`}`
455			`default:`
456			`if opaque(m) {`
457			`e.cb = cbTC16`
458			`} else {`
459			`e.cb = cbTCA16`
460			`}`
461			`}`
462			`}`
463
464			`_, e.err = io.WriteString(w, pngHeader)`
465			`e.writeIHDR()`
466			`if pal != nil {`
467			`e.writePLTE(pal)`
468			`e.maybeWritetRNS(pal)`
469			`}`
470			`e.writeIDATs()`
471			`e.writeIEND()`
472			`return e.err`
473			`}`