writer.go

// 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
}

type Meta struct {
	App2 []byte
}

// 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, meta *Meta) 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])
	if meta != nil {
		// Write APP2 data if specified
		if meta.App2 != nil {
			e.writeMarkerHeader(app2Marker, 2+len(meta.App2))
			e.write(meta.App2)
		}
	}
	// 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
}