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Source file src/image/jpeg/writer.go

Documentation: image/jpeg

  // 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
  }
  

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