src/image/jpeg/writer.go - go.git - Git at Google

 // 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"
 )

 // 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 of the spec, 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+1 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. It is also the
 // same Huffman encoding used by section K.3 of the spec.
 //
 // The DC tables have 12 decoded values, called categories.
 //
 // The AC tables have 162 decoded values: bytes that pack a 4-bit Run and a
 // 4-bit Size. There are 16 valid Runs and 10 valid Sizes, plus two special R|S
 // cases: 0|0 (meaning EOB) and F|0 (meaning ZRL).
 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
 }
	// 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"
	)

	// 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 of the spec, 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+1 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. It is also the
	// same Huffman encoding used by section K.3 of the spec.
	//
	// The DC tables have 12 decoded values, called categories.
	//
	// The AC tables have 162 decoded values: bytes that pack a 4-bit Run and a
	// 4-bit Size. There are 16 valid Runs and 10 valid Sizes, plus two special R\|S
	// cases: 0\|0 (meaning EOB) and F\|0 (meaning ZRL).
	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.X4
	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 := 16y + 2x
	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
	}