[ARM] Fix typo in comment in arm_expand_prologue

[official-gcc.git] / libgo / go / image / jpeg / 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) 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)
                }
        }
}

// 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)
                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 {
                                                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
}