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747 |
jeremybenn |
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package jpeg implements a JPEG image decoder and encoder.
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//
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// JPEG is defined in ITU-T T.81: http://www.w3.org/Graphics/JPEG/itu-t81.pdf.
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package jpeg
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import (
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"bufio"
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"image"
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"image/color"
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"io"
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)
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// TODO(nigeltao): fix up the doc comment style so that sentences start with
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// the name of the type or function that they annotate.
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// A FormatError reports that the input is not a valid JPEG.
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type FormatError string
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func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }
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// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
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type UnsupportedError string
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func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }
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// Component specification, specified in section B.2.2.
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type component struct {
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h int // Horizontal sampling factor.
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v int // Vertical sampling factor.
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c uint8 // Component identifier.
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tq uint8 // Quantization table destination selector.
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}
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type block [blockSize]int
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const (
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blockSize = 64 // A DCT block is 8x8.
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dcTable = 0
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acTable = 1
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maxTc = 1
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maxTh = 3
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maxTq = 3
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// A grayscale JPEG image has only a Y component.
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nGrayComponent = 1
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// A color JPEG image has Y, Cb and Cr components.
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nColorComponent = 3
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// We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and therefore the
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// number of luma samples per chroma sample is at most 2 in the horizontal
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// and 2 in the vertical direction.
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maxH = 2
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maxV = 2
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)
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const (
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soiMarker = 0xd8 // Start Of Image.
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eoiMarker = 0xd9 // End Of Image.
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sof0Marker = 0xc0 // Start Of Frame (Baseline).
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sof2Marker = 0xc2 // Start Of Frame (Progressive).
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dhtMarker = 0xc4 // Define Huffman Table.
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dqtMarker = 0xdb // Define Quantization Table.
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sosMarker = 0xda // Start Of Scan.
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driMarker = 0xdd // Define Restart Interval.
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rst0Marker = 0xd0 // ReSTart (0).
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rst7Marker = 0xd7 // ReSTart (7).
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app0Marker = 0xe0 // APPlication specific (0).
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app15Marker = 0xef // APPlication specific (15).
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comMarker = 0xfe // COMment.
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)
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// Maps from the zig-zag ordering to the natural ordering.
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var unzig = [blockSize]int{
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0, 1, 8, 16, 9, 2, 3, 10,
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17, 24, 32, 25, 18, 11, 4, 5,
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12, 19, 26, 33, 40, 48, 41, 34,
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27, 20, 13, 6, 7, 14, 21, 28,
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35, 42, 49, 56, 57, 50, 43, 36,
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29, 22, 15, 23, 30, 37, 44, 51,
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58, 59, 52, 45, 38, 31, 39, 46,
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53, 60, 61, 54, 47, 55, 62, 63,
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}
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// If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering.
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type Reader interface {
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io.Reader
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ReadByte() (c byte, err error)
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}
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type decoder struct {
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r Reader
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width, height int
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img1 *image.Gray
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img3 *image.YCbCr
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ri int // Restart Interval.
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nComp int
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comp [nColorComponent]component
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huff [maxTc + 1][maxTh + 1]huffman
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quant [maxTq + 1]block
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b bits
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tmp [1024]byte
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}
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// Reads and ignores the next n bytes.
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func (d *decoder) ignore(n int) error {
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for n > 0 {
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m := len(d.tmp)
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if m > n {
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m = n
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}
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_, err := io.ReadFull(d.r, d.tmp[0:m])
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if err != nil {
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return err
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}
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n -= m
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}
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return nil
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}
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// Specified in section B.2.2.
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func (d *decoder) processSOF(n int) error {
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switch n {
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case 6 + 3*nGrayComponent:
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d.nComp = nGrayComponent
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case 6 + 3*nColorComponent:
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d.nComp = nColorComponent
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default:
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return UnsupportedError("SOF has wrong length")
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}
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_, err := io.ReadFull(d.r, d.tmp[:n])
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if err != nil {
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return err
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}
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// We only support 8-bit precision.
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if d.tmp[0] != 8 {
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return UnsupportedError("precision")
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}
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d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
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d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
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if int(d.tmp[5]) != d.nComp {
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return UnsupportedError("SOF has wrong number of image components")
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}
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for i := 0; i < d.nComp; i++ {
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hv := d.tmp[7+3*i]
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d.comp[i].h = int(hv >> 4)
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d.comp[i].v = int(hv & 0x0f)
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d.comp[i].c = d.tmp[6+3*i]
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d.comp[i].tq = d.tmp[8+3*i]
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if d.nComp == nGrayComponent {
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continue
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}
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// For color images, we only support 4:4:4, 4:2:2 or 4:2:0 chroma
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// downsampling ratios. This implies that the (h, v) values for the Y
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// component are either (1, 1), (2, 1) or (2, 2), and the (h, v)
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// values for the Cr and Cb components must be (1, 1).
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if i == 0 {
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if hv != 0x11 && hv != 0x21 && hv != 0x22 {
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return UnsupportedError("luma downsample ratio")
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}
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} else if hv != 0x11 {
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return UnsupportedError("chroma downsample ratio")
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}
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}
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return nil
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}
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// Specified in section B.2.4.1.
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func (d *decoder) processDQT(n int) error {
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const qtLength = 1 + blockSize
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for ; n >= qtLength; n -= qtLength {
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_, err := io.ReadFull(d.r, d.tmp[0:qtLength])
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if err != nil {
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return err
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}
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pq := d.tmp[0] >> 4
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if pq != 0 {
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return UnsupportedError("bad Pq value")
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}
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tq := d.tmp[0] & 0x0f
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if tq > maxTq {
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return FormatError("bad Tq value")
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}
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for i := range d.quant[tq] {
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d.quant[tq][i] = int(d.tmp[i+1])
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}
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}
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if n != 0 {
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return FormatError("DQT has wrong length")
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}
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return nil
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}
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// makeImg allocates and initializes the destination image.
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func (d *decoder) makeImg(h0, v0, mxx, myy int) {
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if d.nComp == nGrayComponent {
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m := image.NewGray(image.Rect(0, 0, 8*mxx, 8*myy))
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d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
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return
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}
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var subsampleRatio image.YCbCrSubsampleRatio
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switch h0 * v0 {
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case 1:
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subsampleRatio = image.YCbCrSubsampleRatio444
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case 2:
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subsampleRatio = image.YCbCrSubsampleRatio422
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case 4:
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subsampleRatio = image.YCbCrSubsampleRatio420
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default:
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panic("unreachable")
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}
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m := image.NewYCbCr(image.Rect(0, 0, 8*h0*mxx, 8*v0*myy), subsampleRatio)
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d.img3 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.YCbCr)
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}
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// Specified in section B.2.3.
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func (d *decoder) processSOS(n int) error {
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if d.nComp == 0 {
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return FormatError("missing SOF marker")
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}
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if n != 4+2*d.nComp {
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return UnsupportedError("SOS has wrong length")
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}
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_, err := io.ReadFull(d.r, d.tmp[0:4+2*d.nComp])
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if err != nil {
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return err
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}
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if int(d.tmp[0]) != d.nComp {
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return UnsupportedError("SOS has wrong number of image components")
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}
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var scan [nColorComponent]struct {
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td uint8 // DC table selector.
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ta uint8 // AC table selector.
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}
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for i := 0; i < d.nComp; i++ {
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cs := d.tmp[1+2*i] // Component selector.
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if cs != d.comp[i].c {
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return UnsupportedError("scan components out of order")
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}
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scan[i].td = d.tmp[2+2*i] >> 4
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scan[i].ta = d.tmp[2+2*i] & 0x0f
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}
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// mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
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h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components.
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mxx := (d.width + 8*h0 - 1) / (8 * h0)
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myy := (d.height + 8*v0 - 1) / (8 * v0)
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if d.img1 == nil && d.img3 == nil {
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d.makeImg(h0, v0, mxx, myy)
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}
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mcu, expectedRST := 0, uint8(rst0Marker)
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var (
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b block
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dc [nColorComponent]int
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)
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for my := 0; my < myy; my++ {
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for mx := 0; mx < mxx; mx++ {
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for i := 0; i < d.nComp; i++ {
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qt := &d.quant[d.comp[i].tq]
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for j := 0; j < d.comp[i].h*d.comp[i].v; j++ {
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// TODO(nigeltao): make this a "var b block" once the compiler's escape
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// analysis is good enough to allocate it on the stack, not the heap.
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b = block{}
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// Decode the DC coefficient, as specified in section F.2.2.1.
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value, err := d.decodeHuffman(&d.huff[dcTable][scan[i].td])
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if err != nil {
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return err
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}
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if value > 16 {
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return UnsupportedError("excessive DC component")
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}
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dcDelta, err := d.receiveExtend(value)
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if err != nil {
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return err
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}
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dc[i] += dcDelta
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b[0] = dc[i] * qt[0]
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// Decode the AC coefficients, as specified in section F.2.2.2.
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for k := 1; k < blockSize; k++ {
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value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta])
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if err != nil {
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return err
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}
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val0 := value >> 4
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val1 := value & 0x0f
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if val1 != 0 {
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k += int(val0)
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if k > blockSize {
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return FormatError("bad DCT index")
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}
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ac, err := d.receiveExtend(val1)
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if err != nil {
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return err
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}
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b[unzig[k]] = ac * qt[k]
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} else {
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if val0 != 0x0f {
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break
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}
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k += 0x0f
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}
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}
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// Perform the inverse DCT and store the MCU component to the image.
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if d.nComp == nGrayComponent {
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idct(d.img1.Pix[8*(my*d.img1.Stride+mx):], d.img1.Stride, &b)
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} else {
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switch i {
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case 0:
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mx0 := h0*mx + (j % 2)
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my0 := v0*my + (j / 2)
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idct(d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride, &b)
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case 1:
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idct(d.img3.Cb[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
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case 2:
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idct(d.img3.Cr[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
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}
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}
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} // for j
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} // for i
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mcu++
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if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
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// A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
|
| 330 |
|
|
// but this one assumes well-formed input, and hence the restart marker follows immediately.
|
| 331 |
|
|
_, err := io.ReadFull(d.r, d.tmp[0:2])
|
| 332 |
|
|
if err != nil {
|
| 333 |
|
|
return err
|
| 334 |
|
|
}
|
| 335 |
|
|
if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
|
| 336 |
|
|
return FormatError("bad RST marker")
|
| 337 |
|
|
}
|
| 338 |
|
|
expectedRST++
|
| 339 |
|
|
if expectedRST == rst7Marker+1 {
|
| 340 |
|
|
expectedRST = rst0Marker
|
| 341 |
|
|
}
|
| 342 |
|
|
// Reset the Huffman decoder.
|
| 343 |
|
|
d.b = bits{}
|
| 344 |
|
|
// Reset the DC components, as per section F.2.1.3.1.
|
| 345 |
|
|
dc = [nColorComponent]int{}
|
| 346 |
|
|
}
|
| 347 |
|
|
} // for mx
|
| 348 |
|
|
} // for my
|
| 349 |
|
|
|
| 350 |
|
|
return nil
|
| 351 |
|
|
}
|
| 352 |
|
|
|
| 353 |
|
|
// Specified in section B.2.4.4.
|
| 354 |
|
|
func (d *decoder) processDRI(n int) error {
|
| 355 |
|
|
if n != 2 {
|
| 356 |
|
|
return FormatError("DRI has wrong length")
|
| 357 |
|
|
}
|
| 358 |
|
|
_, err := io.ReadFull(d.r, d.tmp[0:2])
|
| 359 |
|
|
if err != nil {
|
| 360 |
|
|
return err
|
| 361 |
|
|
}
|
| 362 |
|
|
d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
|
| 363 |
|
|
return nil
|
| 364 |
|
|
}
|
| 365 |
|
|
|
| 366 |
|
|
// decode reads a JPEG image from r and returns it as an image.Image.
|
| 367 |
|
|
func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
|
| 368 |
|
|
if rr, ok := r.(Reader); ok {
|
| 369 |
|
|
d.r = rr
|
| 370 |
|
|
} else {
|
| 371 |
|
|
d.r = bufio.NewReader(r)
|
| 372 |
|
|
}
|
| 373 |
|
|
|
| 374 |
|
|
// Check for the Start Of Image marker.
|
| 375 |
|
|
_, err := io.ReadFull(d.r, d.tmp[0:2])
|
| 376 |
|
|
if err != nil {
|
| 377 |
|
|
return nil, err
|
| 378 |
|
|
}
|
| 379 |
|
|
if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
|
| 380 |
|
|
return nil, FormatError("missing SOI marker")
|
| 381 |
|
|
}
|
| 382 |
|
|
|
| 383 |
|
|
// Process the remaining segments until the End Of Image marker.
|
| 384 |
|
|
for {
|
| 385 |
|
|
_, err := io.ReadFull(d.r, d.tmp[0:2])
|
| 386 |
|
|
if err != nil {
|
| 387 |
|
|
return nil, err
|
| 388 |
|
|
}
|
| 389 |
|
|
if d.tmp[0] != 0xff {
|
| 390 |
|
|
return nil, FormatError("missing 0xff marker start")
|
| 391 |
|
|
}
|
| 392 |
|
|
marker := d.tmp[1]
|
| 393 |
|
|
if marker == eoiMarker { // End Of Image.
|
| 394 |
|
|
break
|
| 395 |
|
|
}
|
| 396 |
|
|
|
| 397 |
|
|
// Read the 16-bit length of the segment. The value includes the 2 bytes for the
|
| 398 |
|
|
// length itself, so we subtract 2 to get the number of remaining bytes.
|
| 399 |
|
|
_, err = io.ReadFull(d.r, d.tmp[0:2])
|
| 400 |
|
|
if err != nil {
|
| 401 |
|
|
return nil, err
|
| 402 |
|
|
}
|
| 403 |
|
|
n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
|
| 404 |
|
|
if n < 0 {
|
| 405 |
|
|
return nil, FormatError("short segment length")
|
| 406 |
|
|
}
|
| 407 |
|
|
|
| 408 |
|
|
switch {
|
| 409 |
|
|
case marker == sof0Marker: // Start Of Frame (Baseline).
|
| 410 |
|
|
err = d.processSOF(n)
|
| 411 |
|
|
if configOnly {
|
| 412 |
|
|
return nil, err
|
| 413 |
|
|
}
|
| 414 |
|
|
case marker == sof2Marker: // Start Of Frame (Progressive).
|
| 415 |
|
|
err = UnsupportedError("progressive mode")
|
| 416 |
|
|
case marker == dhtMarker: // Define Huffman Table.
|
| 417 |
|
|
err = d.processDHT(n)
|
| 418 |
|
|
case marker == dqtMarker: // Define Quantization Table.
|
| 419 |
|
|
err = d.processDQT(n)
|
| 420 |
|
|
case marker == sosMarker: // Start Of Scan.
|
| 421 |
|
|
err = d.processSOS(n)
|
| 422 |
|
|
case marker == driMarker: // Define Restart Interval.
|
| 423 |
|
|
err = d.processDRI(n)
|
| 424 |
|
|
case marker >= app0Marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
|
| 425 |
|
|
err = d.ignore(n)
|
| 426 |
|
|
default:
|
| 427 |
|
|
err = UnsupportedError("unknown marker")
|
| 428 |
|
|
}
|
| 429 |
|
|
if err != nil {
|
| 430 |
|
|
return nil, err
|
| 431 |
|
|
}
|
| 432 |
|
|
}
|
| 433 |
|
|
if d.img1 != nil {
|
| 434 |
|
|
return d.img1, nil
|
| 435 |
|
|
}
|
| 436 |
|
|
if d.img3 != nil {
|
| 437 |
|
|
return d.img3, nil
|
| 438 |
|
|
}
|
| 439 |
|
|
return nil, FormatError("missing SOS marker")
|
| 440 |
|
|
}
|
| 441 |
|
|
|
| 442 |
|
|
// Decode reads a JPEG image from r and returns it as an image.Image.
|
| 443 |
|
|
func Decode(r io.Reader) (image.Image, error) {
|
| 444 |
|
|
var d decoder
|
| 445 |
|
|
return d.decode(r, false)
|
| 446 |
|
|
}
|
| 447 |
|
|
|
| 448 |
|
|
// DecodeConfig returns the color model and dimensions of a JPEG image without
|
| 449 |
|
|
// decoding the entire image.
|
| 450 |
|
|
func DecodeConfig(r io.Reader) (image.Config, error) {
|
| 451 |
|
|
var d decoder
|
| 452 |
|
|
if _, err := d.decode(r, true); err != nil {
|
| 453 |
|
|
return image.Config{}, err
|
| 454 |
|
|
}
|
| 455 |
|
|
switch d.nComp {
|
| 456 |
|
|
case nGrayComponent:
|
| 457 |
|
|
return image.Config{
|
| 458 |
|
|
ColorModel: color.GrayModel,
|
| 459 |
|
|
Width: d.width,
|
| 460 |
|
|
Height: d.height,
|
| 461 |
|
|
}, nil
|
| 462 |
|
|
case nColorComponent:
|
| 463 |
|
|
return image.Config{
|
| 464 |
|
|
ColorModel: color.YCbCrModel,
|
| 465 |
|
|
Width: d.width,
|
| 466 |
|
|
Height: d.height,
|
| 467 |
|
|
}, nil
|
| 468 |
|
|
}
|
| 469 |
|
|
return image.Config{}, FormatError("missing SOF marker")
|
| 470 |
|
|
}
|
| 471 |
|
|
|
| 472 |
|
|
func init() {
|
| 473 |
|
|
image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
|
| 474 |
|
|
}
|
