summaryrefslogtreecommitdiffstats
path: root/src/image/jpeg/writer.go
diff options
context:
space:
mode:
Diffstat (limited to 'src/image/jpeg/writer.go')
-rw-r--r--src/image/jpeg/writer.go641
1 files changed, 641 insertions, 0 deletions
diff --git a/src/image/jpeg/writer.go b/src/image/jpeg/writer.go
new file mode 100644
index 0000000..0027f78
--- /dev/null
+++ b/src/image/jpeg/writer.go
@@ -0,0 +1,641 @@
+// Copyright 2011 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 jpeg
+
+import (
+ "bufio"
+ "errors"
+ "image"
+ "image/color"
+ "io"
+)
+
+// min returns the minimum of two integers.
+func min(x, y int) int {
+ if x < y {
+ return x
+ }
+ return y
+}
+
+// div returns a/b rounded to the nearest integer, instead of rounded to zero.
+func div(a, b int32) int32 {
+ if a >= 0 {
+ return (a + (b >> 1)) / b
+ }
+ return -((-a + (b >> 1)) / b)
+}
+
+// bitCount counts the number of bits needed to hold an integer.
+var bitCount = [256]byte{
+ 0, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4,
+ 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+ 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8,
+}
+
+type quantIndex int
+
+const (
+ quantIndexLuminance quantIndex = iota
+ quantIndexChrominance
+ nQuantIndex
+)
+
+// unscaledQuant are the unscaled quantization tables in zig-zag order. Each
+// encoder copies and scales the tables according to its quality parameter.
+// The values are derived from section K.1 after converting from natural to
+// zig-zag order.
+var unscaledQuant = [nQuantIndex][blockSize]byte{
+ // Luminance.
+ {
+ 16, 11, 12, 14, 12, 10, 16, 14,
+ 13, 14, 18, 17, 16, 19, 24, 40,
+ 26, 24, 22, 22, 24, 49, 35, 37,
+ 29, 40, 58, 51, 61, 60, 57, 51,
+ 56, 55, 64, 72, 92, 78, 64, 68,
+ 87, 69, 55, 56, 80, 109, 81, 87,
+ 95, 98, 103, 104, 103, 62, 77, 113,
+ 121, 112, 100, 120, 92, 101, 103, 99,
+ },
+ // Chrominance.
+ {
+ 17, 18, 18, 24, 21, 24, 47, 26,
+ 26, 47, 99, 66, 56, 66, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ 99, 99, 99, 99, 99, 99, 99, 99,
+ },
+}
+
+type huffIndex int
+
+const (
+ huffIndexLuminanceDC huffIndex = iota
+ huffIndexLuminanceAC
+ huffIndexChrominanceDC
+ huffIndexChrominanceAC
+ nHuffIndex
+)
+
+// huffmanSpec specifies a Huffman encoding.
+type huffmanSpec struct {
+ // count[i] is the number of codes of length i bits.
+ count [16]byte
+ // value[i] is the decoded value of the i'th codeword.
+ value []byte
+}
+
+// theHuffmanSpec is the Huffman encoding specifications.
+// This encoder uses the same Huffman encoding for all images.
+var theHuffmanSpec = [nHuffIndex]huffmanSpec{
+ // Luminance DC.
+ {
+ [16]byte{0, 1, 5, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0},
+ []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
+ },
+ // Luminance AC.
+ {
+ [16]byte{0, 2, 1, 3, 3, 2, 4, 3, 5, 5, 4, 4, 0, 0, 1, 125},
+ []byte{
+ 0x01, 0x02, 0x03, 0x00, 0x04, 0x11, 0x05, 0x12,
+ 0x21, 0x31, 0x41, 0x06, 0x13, 0x51, 0x61, 0x07,
+ 0x22, 0x71, 0x14, 0x32, 0x81, 0x91, 0xa1, 0x08,
+ 0x23, 0x42, 0xb1, 0xc1, 0x15, 0x52, 0xd1, 0xf0,
+ 0x24, 0x33, 0x62, 0x72, 0x82, 0x09, 0x0a, 0x16,
+ 0x17, 0x18, 0x19, 0x1a, 0x25, 0x26, 0x27, 0x28,
+ 0x29, 0x2a, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39,
+ 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48, 0x49,
+ 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58, 0x59,
+ 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69,
+ 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79,
+ 0x7a, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
+ 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97, 0x98,
+ 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5, 0xa6, 0xa7,
+ 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6,
+ 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3, 0xc4, 0xc5,
+ 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2, 0xd3, 0xd4,
+ 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda, 0xe1, 0xe2,
+ 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9, 0xea,
+ 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
+ 0xf9, 0xfa,
+ },
+ },
+ // Chrominance DC.
+ {
+ [16]byte{0, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0},
+ []byte{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11},
+ },
+ // Chrominance AC.
+ {
+ [16]byte{0, 2, 1, 2, 4, 4, 3, 4, 7, 5, 4, 4, 0, 1, 2, 119},
+ []byte{
+ 0x00, 0x01, 0x02, 0x03, 0x11, 0x04, 0x05, 0x21,
+ 0x31, 0x06, 0x12, 0x41, 0x51, 0x07, 0x61, 0x71,
+ 0x13, 0x22, 0x32, 0x81, 0x08, 0x14, 0x42, 0x91,
+ 0xa1, 0xb1, 0xc1, 0x09, 0x23, 0x33, 0x52, 0xf0,
+ 0x15, 0x62, 0x72, 0xd1, 0x0a, 0x16, 0x24, 0x34,
+ 0xe1, 0x25, 0xf1, 0x17, 0x18, 0x19, 0x1a, 0x26,
+ 0x27, 0x28, 0x29, 0x2a, 0x35, 0x36, 0x37, 0x38,
+ 0x39, 0x3a, 0x43, 0x44, 0x45, 0x46, 0x47, 0x48,
+ 0x49, 0x4a, 0x53, 0x54, 0x55, 0x56, 0x57, 0x58,
+ 0x59, 0x5a, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
+ 0x69, 0x6a, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78,
+ 0x79, 0x7a, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87,
+ 0x88, 0x89, 0x8a, 0x92, 0x93, 0x94, 0x95, 0x96,
+ 0x97, 0x98, 0x99, 0x9a, 0xa2, 0xa3, 0xa4, 0xa5,
+ 0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xb2, 0xb3, 0xb4,
+ 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xc2, 0xc3,
+ 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xd2,
+ 0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9, 0xda,
+ 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7, 0xe8, 0xe9,
+ 0xea, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8,
+ 0xf9, 0xfa,
+ },
+ },
+}
+
+// huffmanLUT is a compiled look-up table representation of a huffmanSpec.
+// Each value maps to a uint32 of which the 8 most significant bits hold the
+// codeword size in bits and the 24 least significant bits hold the codeword.
+// The maximum codeword size is 16 bits.
+type huffmanLUT []uint32
+
+func (h *huffmanLUT) init(s huffmanSpec) {
+ maxValue := 0
+ for _, v := range s.value {
+ if int(v) > maxValue {
+ maxValue = int(v)
+ }
+ }
+ *h = make([]uint32, maxValue+1)
+ code, k := uint32(0), 0
+ for i := 0; i < len(s.count); i++ {
+ nBits := uint32(i+1) << 24
+ for j := uint8(0); j < s.count[i]; j++ {
+ (*h)[s.value[k]] = nBits | code
+ code++
+ k++
+ }
+ code <<= 1
+ }
+}
+
+// theHuffmanLUT are compiled representations of theHuffmanSpec.
+var theHuffmanLUT [4]huffmanLUT
+
+func init() {
+ for i, s := range theHuffmanSpec {
+ theHuffmanLUT[i].init(s)
+ }
+}
+
+// writer is a buffered writer.
+type writer interface {
+ Flush() error
+ io.Writer
+ io.ByteWriter
+}
+
+// encoder encodes an image to the JPEG format.
+type encoder struct {
+ // w is the writer to write to. err is the first error encountered during
+ // writing. All attempted writes after the first error become no-ops.
+ w writer
+ err error
+ // buf is a scratch buffer.
+ buf [16]byte
+ // bits and nBits are accumulated bits to write to w.
+ bits, nBits uint32
+ // quant is the scaled quantization tables, in zig-zag order.
+ quant [nQuantIndex][blockSize]byte
+}
+
+func (e *encoder) flush() {
+ if e.err != nil {
+ return
+ }
+ e.err = e.w.Flush()
+}
+
+func (e *encoder) write(p []byte) {
+ if e.err != nil {
+ return
+ }
+ _, e.err = e.w.Write(p)
+}
+
+func (e *encoder) writeByte(b byte) {
+ if e.err != nil {
+ return
+ }
+ e.err = e.w.WriteByte(b)
+}
+
+// emit emits the least significant nBits bits of bits to the bit-stream.
+// The precondition is bits < 1<<nBits && nBits <= 16.
+func (e *encoder) emit(bits, nBits uint32) {
+ nBits += e.nBits
+ bits <<= 32 - nBits
+ bits |= e.bits
+ for nBits >= 8 {
+ b := uint8(bits >> 24)
+ e.writeByte(b)
+ if b == 0xff {
+ e.writeByte(0x00)
+ }
+ bits <<= 8
+ nBits -= 8
+ }
+ e.bits, e.nBits = bits, nBits
+}
+
+// emitHuff emits the given value with the given Huffman encoder.
+func (e *encoder) emitHuff(h huffIndex, value int32) {
+ x := theHuffmanLUT[h][value]
+ e.emit(x&(1<<24-1), x>>24)
+}
+
+// emitHuffRLE emits a run of runLength copies of value encoded with the given
+// Huffman encoder.
+func (e *encoder) emitHuffRLE(h huffIndex, runLength, value int32) {
+ a, b := value, value
+ if a < 0 {
+ a, b = -value, value-1
+ }
+ var nBits uint32
+ if a < 0x100 {
+ nBits = uint32(bitCount[a])
+ } else {
+ nBits = 8 + uint32(bitCount[a>>8])
+ }
+ e.emitHuff(h, runLength<<4|int32(nBits))
+ if nBits > 0 {
+ e.emit(uint32(b)&(1<<nBits-1), nBits)
+ }
+}
+
+// writeMarkerHeader writes the header for a marker with the given length.
+func (e *encoder) writeMarkerHeader(marker uint8, markerlen int) {
+ e.buf[0] = 0xff
+ e.buf[1] = marker
+ e.buf[2] = uint8(markerlen >> 8)
+ e.buf[3] = uint8(markerlen & 0xff)
+ e.write(e.buf[:4])
+}
+
+// writeDQT writes the Define Quantization Table marker.
+func (e *encoder) writeDQT() {
+ const markerlen = 2 + int(nQuantIndex)*(1+blockSize)
+ e.writeMarkerHeader(dqtMarker, markerlen)
+ for i := range e.quant {
+ e.writeByte(uint8(i))
+ e.write(e.quant[i][:])
+ }
+}
+
+// writeSOF0 writes the Start Of Frame (Baseline Sequential) marker.
+func (e *encoder) writeSOF0(size image.Point, nComponent int) {
+ markerlen := 8 + 3*nComponent
+ e.writeMarkerHeader(sof0Marker, markerlen)
+ e.buf[0] = 8 // 8-bit color.
+ e.buf[1] = uint8(size.Y >> 8)
+ e.buf[2] = uint8(size.Y & 0xff)
+ e.buf[3] = uint8(size.X >> 8)
+ e.buf[4] = uint8(size.X & 0xff)
+ e.buf[5] = uint8(nComponent)
+ if nComponent == 1 {
+ e.buf[6] = 1
+ // No subsampling for grayscale image.
+ e.buf[7] = 0x11
+ e.buf[8] = 0x00
+ } else {
+ for i := 0; i < nComponent; i++ {
+ e.buf[3*i+6] = uint8(i + 1)
+ // We use 4:2:0 chroma subsampling.
+ e.buf[3*i+7] = "\x22\x11\x11"[i]
+ e.buf[3*i+8] = "\x00\x01\x01"[i]
+ }
+ }
+ e.write(e.buf[:3*(nComponent-1)+9])
+}
+
+// writeDHT writes the Define Huffman Table marker.
+func (e *encoder) writeDHT(nComponent int) {
+ markerlen := 2
+ specs := theHuffmanSpec[:]
+ if nComponent == 1 {
+ // Drop the Chrominance tables.
+ specs = specs[:2]
+ }
+ for _, s := range specs {
+ markerlen += 1 + 16 + len(s.value)
+ }
+ e.writeMarkerHeader(dhtMarker, markerlen)
+ for i, s := range specs {
+ e.writeByte("\x00\x10\x01\x11"[i])
+ e.write(s.count[:])
+ e.write(s.value)
+ }
+}
+
+// writeBlock writes a block of pixel data using the given quantization table,
+// returning the post-quantized DC value of the DCT-transformed block. b is in
+// natural (not zig-zag) order.
+func (e *encoder) writeBlock(b *block, q quantIndex, prevDC int32) int32 {
+ fdct(b)
+ // Emit the DC delta.
+ dc := div(b[0], 8*int32(e.quant[q][0]))
+ e.emitHuffRLE(huffIndex(2*q+0), 0, dc-prevDC)
+ // Emit the AC components.
+ h, runLength := huffIndex(2*q+1), int32(0)
+ for zig := 1; zig < blockSize; zig++ {
+ ac := div(b[unzig[zig]], 8*int32(e.quant[q][zig]))
+ if ac == 0 {
+ runLength++
+ } else {
+ for runLength > 15 {
+ e.emitHuff(h, 0xf0)
+ runLength -= 16
+ }
+ e.emitHuffRLE(h, runLength, ac)
+ runLength = 0
+ }
+ }
+ if runLength > 0 {
+ e.emitHuff(h, 0x00)
+ }
+ return dc
+}
+
+// toYCbCr converts the 8x8 region of m whose top-left corner is p to its
+// YCbCr values.
+func toYCbCr(m image.Image, p image.Point, yBlock, cbBlock, crBlock *block) {
+ b := m.Bounds()
+ xmax := b.Max.X - 1
+ ymax := b.Max.Y - 1
+ for j := 0; j < 8; j++ {
+ for i := 0; i < 8; i++ {
+ r, g, b, _ := m.At(min(p.X+i, xmax), min(p.Y+j, ymax)).RGBA()
+ yy, cb, cr := color.RGBToYCbCr(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+ yBlock[8*j+i] = int32(yy)
+ cbBlock[8*j+i] = int32(cb)
+ crBlock[8*j+i] = int32(cr)
+ }
+ }
+}
+
+// grayToY stores the 8x8 region of m whose top-left corner is p in yBlock.
+func grayToY(m *image.Gray, p image.Point, yBlock *block) {
+ b := m.Bounds()
+ xmax := b.Max.X - 1
+ ymax := b.Max.Y - 1
+ pix := m.Pix
+ for j := 0; j < 8; j++ {
+ for i := 0; i < 8; i++ {
+ idx := m.PixOffset(min(p.X+i, xmax), min(p.Y+j, ymax))
+ yBlock[8*j+i] = int32(pix[idx])
+ }
+ }
+}
+
+// rgbaToYCbCr is a specialized version of toYCbCr for image.RGBA images.
+func rgbaToYCbCr(m *image.RGBA, p image.Point, yBlock, cbBlock, crBlock *block) {
+ b := m.Bounds()
+ xmax := b.Max.X - 1
+ ymax := b.Max.Y - 1
+ for j := 0; j < 8; j++ {
+ sj := p.Y + j
+ if sj > ymax {
+ sj = ymax
+ }
+ offset := (sj-b.Min.Y)*m.Stride - b.Min.X*4
+ for i := 0; i < 8; i++ {
+ sx := p.X + i
+ if sx > xmax {
+ sx = xmax
+ }
+ pix := m.Pix[offset+sx*4:]
+ yy, cb, cr := color.RGBToYCbCr(pix[0], pix[1], pix[2])
+ yBlock[8*j+i] = int32(yy)
+ cbBlock[8*j+i] = int32(cb)
+ crBlock[8*j+i] = int32(cr)
+ }
+ }
+}
+
+// yCbCrToYCbCr is a specialized version of toYCbCr for image.YCbCr images.
+func yCbCrToYCbCr(m *image.YCbCr, p image.Point, yBlock, cbBlock, crBlock *block) {
+ b := m.Bounds()
+ xmax := b.Max.X - 1
+ ymax := b.Max.Y - 1
+ for j := 0; j < 8; j++ {
+ sy := p.Y + j
+ if sy > ymax {
+ sy = ymax
+ }
+ for i := 0; i < 8; i++ {
+ sx := p.X + i
+ if sx > xmax {
+ sx = xmax
+ }
+ yi := m.YOffset(sx, sy)
+ ci := m.COffset(sx, sy)
+ yBlock[8*j+i] = int32(m.Y[yi])
+ cbBlock[8*j+i] = int32(m.Cb[ci])
+ crBlock[8*j+i] = int32(m.Cr[ci])
+ }
+ }
+}
+
+// scale scales the 16x16 region represented by the 4 src blocks to the 8x8
+// dst block.
+func scale(dst *block, src *[4]block) {
+ for i := 0; i < 4; i++ {
+ dstOff := (i&2)<<4 | (i&1)<<2
+ for y := 0; y < 4; y++ {
+ for x := 0; x < 4; x++ {
+ j := 16*y + 2*x
+ sum := src[i][j] + src[i][j+1] + src[i][j+8] + src[i][j+9]
+ dst[8*y+x+dstOff] = (sum + 2) >> 2
+ }
+ }
+ }
+}
+
+// sosHeaderY is the SOS marker "\xff\xda" followed by 8 bytes:
+// - the marker length "\x00\x08",
+// - the number of components "\x01",
+// - component 1 uses DC table 0 and AC table 0 "\x01\x00",
+// - the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
+// sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
+// should be 0x00, 0x3f, 0x00<<4 | 0x00.
+var sosHeaderY = []byte{
+ 0xff, 0xda, 0x00, 0x08, 0x01, 0x01, 0x00, 0x00, 0x3f, 0x00,
+}
+
+// sosHeaderYCbCr is the SOS marker "\xff\xda" followed by 12 bytes:
+// - the marker length "\x00\x0c",
+// - the number of components "\x03",
+// - component 1 uses DC table 0 and AC table 0 "\x01\x00",
+// - component 2 uses DC table 1 and AC table 1 "\x02\x11",
+// - component 3 uses DC table 1 and AC table 1 "\x03\x11",
+// - the bytes "\x00\x3f\x00". Section B.2.3 of the spec says that for
+// sequential DCTs, those bytes (8-bit Ss, 8-bit Se, 4-bit Ah, 4-bit Al)
+// should be 0x00, 0x3f, 0x00<<4 | 0x00.
+var sosHeaderYCbCr = []byte{
+ 0xff, 0xda, 0x00, 0x0c, 0x03, 0x01, 0x00, 0x02,
+ 0x11, 0x03, 0x11, 0x00, 0x3f, 0x00,
+}
+
+// writeSOS writes the StartOfScan marker.
+func (e *encoder) writeSOS(m image.Image) {
+ switch m.(type) {
+ case *image.Gray:
+ e.write(sosHeaderY)
+ default:
+ e.write(sosHeaderYCbCr)
+ }
+ var (
+ // Scratch buffers to hold the YCbCr values.
+ // The blocks are in natural (not zig-zag) order.
+ b block
+ cb, cr [4]block
+ // DC components are delta-encoded.
+ prevDCY, prevDCCb, prevDCCr int32
+ )
+ bounds := m.Bounds()
+ switch m := m.(type) {
+ // TODO(wathiede): switch on m.ColorModel() instead of type.
+ case *image.Gray:
+ for y := bounds.Min.Y; y < bounds.Max.Y; y += 8 {
+ for x := bounds.Min.X; x < bounds.Max.X; x += 8 {
+ p := image.Pt(x, y)
+ grayToY(m, p, &b)
+ prevDCY = e.writeBlock(&b, 0, prevDCY)
+ }
+ }
+ default:
+ rgba, _ := m.(*image.RGBA)
+ ycbcr, _ := m.(*image.YCbCr)
+ for y := bounds.Min.Y; y < bounds.Max.Y; y += 16 {
+ for x := bounds.Min.X; x < bounds.Max.X; x += 16 {
+ for i := 0; i < 4; i++ {
+ xOff := (i & 1) * 8
+ yOff := (i & 2) * 4
+ p := image.Pt(x+xOff, y+yOff)
+ if rgba != nil {
+ rgbaToYCbCr(rgba, p, &b, &cb[i], &cr[i])
+ } else if ycbcr != nil {
+ yCbCrToYCbCr(ycbcr, p, &b, &cb[i], &cr[i])
+ } else {
+ toYCbCr(m, p, &b, &cb[i], &cr[i])
+ }
+ prevDCY = e.writeBlock(&b, 0, prevDCY)
+ }
+ scale(&b, &cb)
+ prevDCCb = e.writeBlock(&b, 1, prevDCCb)
+ scale(&b, &cr)
+ prevDCCr = e.writeBlock(&b, 1, prevDCCr)
+ }
+ }
+ }
+ // Pad the last byte with 1's.
+ e.emit(0x7f, 7)
+}
+
+// DefaultQuality is the default quality encoding parameter.
+const DefaultQuality = 75
+
+// Options are the encoding parameters.
+// Quality ranges from 1 to 100 inclusive, higher is better.
+type Options struct {
+ Quality int
+}
+
+// Encode writes the Image m to w in JPEG 4:2:0 baseline format with the given
+// options. Default parameters are used if a nil *Options is passed.
+func Encode(w io.Writer, m image.Image, o *Options) error {
+ b := m.Bounds()
+ if b.Dx() >= 1<<16 || b.Dy() >= 1<<16 {
+ return errors.New("jpeg: image is too large to encode")
+ }
+ var e encoder
+ if ww, ok := w.(writer); ok {
+ e.w = ww
+ } else {
+ e.w = bufio.NewWriter(w)
+ }
+ // Clip quality to [1, 100].
+ quality := DefaultQuality
+ if o != nil {
+ quality = o.Quality
+ if quality < 1 {
+ quality = 1
+ } else if quality > 100 {
+ quality = 100
+ }
+ }
+ // Convert from a quality rating to a scaling factor.
+ var scale int
+ if quality < 50 {
+ scale = 5000 / quality
+ } else {
+ scale = 200 - quality*2
+ }
+ // Initialize the quantization tables.
+ for i := range e.quant {
+ for j := range e.quant[i] {
+ x := int(unscaledQuant[i][j])
+ x = (x*scale + 50) / 100
+ if x < 1 {
+ x = 1
+ } else if x > 255 {
+ x = 255
+ }
+ e.quant[i][j] = uint8(x)
+ }
+ }
+ // Compute number of components based on input image type.
+ nComponent := 3
+ switch m.(type) {
+ // TODO(wathiede): switch on m.ColorModel() instead of type.
+ case *image.Gray:
+ nComponent = 1
+ }
+ // Write the Start Of Image marker.
+ e.buf[0] = 0xff
+ e.buf[1] = 0xd8
+ e.write(e.buf[:2])
+ // Write the quantization tables.
+ e.writeDQT()
+ // Write the image dimensions.
+ e.writeSOF0(b.Size(), nComponent)
+ // Write the Huffman tables.
+ e.writeDHT(nComponent)
+ // Write the image data.
+ e.writeSOS(m)
+ // Write the End Of Image marker.
+ e.buf[0] = 0xff
+ e.buf[1] = 0xd9
+ e.write(e.buf[:2])
+ e.flush()
+ return e.err
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-11-06 10:11:38 +0000