378 lines
11 KiB
Go
378 lines
11 KiB
Go
// 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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const (
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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, 4:4:0, 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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// unzig maps from the zig-zag ordering to the natural ordering. For example,
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// unzig[3] is the column and row of the fourth element in zig-zag order. The
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// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
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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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b bits
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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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progressive bool
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eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
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comp [nColorComponent]component
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progCoeffs [nColorComponent][]block // Saved state between progressive-mode scans.
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huff [maxTc + 1][maxTh + 1]huffman
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quant [maxTq + 1]block // Quantization tables, in zig-zag order.
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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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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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// If a JPEG image has only one component, section A.2 says "this data
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// is non-interleaved by definition" and section A.2.2 says "[in this
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// case...] the order of data units within a scan shall be left-to-right
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// and top-to-bottom... regardless of the values of H_1 and V_1". Section
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// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
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// one data unit". Similarly, section A.1.1 explains that it is the ratio
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// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
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// images, H_1 is the maximum H_j for all components j, so that ratio is
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// always 1. The component's (h, v) is effectively always (1, 1): even if
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// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
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// MCUs, not two 16x8 MCUs.
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d.comp[i].h = 1
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d.comp[i].v = 1
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continue
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}
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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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// For color images, we only support 4:4:4, 4:4:0, 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), (1, 2), (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 && hv != 0x12 {
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return UnsupportedError("luma/chroma downsample ratio")
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}
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} else if hv != 0x11 {
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return UnsupportedError("luma/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] = int32(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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// Specified in section B.2.4.4.
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func (d *decoder) processDRI(n int) error {
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if n != 2 {
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return FormatError("DRI has wrong length")
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}
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_, err := io.ReadFull(d.r, d.tmp[0:2])
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if err != nil {
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return err
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}
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d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
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return nil
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}
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// decode reads a JPEG image from r and returns it as an image.Image.
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func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
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if rr, ok := r.(Reader); ok {
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d.r = rr
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} else {
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d.r = bufio.NewReader(r)
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}
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// Check for the Start Of Image marker.
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_, err := io.ReadFull(d.r, d.tmp[0:2])
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if err != nil {
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return nil, err
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}
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if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
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return nil, FormatError("missing SOI marker")
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}
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// Process the remaining segments until the End Of Image marker.
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for {
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_, err := io.ReadFull(d.r, d.tmp[0:2])
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if err != nil {
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return nil, err
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}
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for d.tmp[0] != 0xff {
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// Strictly speaking, this is a format error. However, libjpeg is
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// liberal in what it accepts. As of version 9, next_marker in
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// jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
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// continues to decode the stream. Even before next_marker sees
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// extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
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// bytes as it can, possibly past the end of a scan's data. It
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// effectively puts back any markers that it overscanned (e.g. an
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// "\xff\xd9" EOI marker), but it does not put back non-marker data,
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// and thus it can silently ignore a small number of extraneous
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// non-marker bytes before next_marker has a chance to see them (and
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// print a warning).
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//
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// We are therefore also liberal in what we accept. Extraneous data
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// is silently ignored.
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//
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// This is similar to, but not exactly the same as, the restart
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// mechanism within a scan (the RST[0-7] markers).
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//
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// Note that extraneous 0xff bytes in e.g. SOS data are escaped as
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// "\xff\x00", and so are detected a little further down below.
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d.tmp[0] = d.tmp[1]
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d.tmp[1], err = d.r.ReadByte()
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if err != nil {
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return nil, err
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}
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}
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marker := d.tmp[1]
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if marker == 0 {
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// Treat "\xff\x00" as extraneous data.
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continue
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}
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for marker == 0xff {
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// Section B.1.1.2 says, "Any marker may optionally be preceded by any
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// number of fill bytes, which are bytes assigned code X'FF'".
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marker, err = d.r.ReadByte()
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if err != nil {
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return nil, err
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}
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}
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if marker == eoiMarker { // End Of Image.
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break
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}
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if rst0Marker <= marker && marker <= rst7Marker {
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// Figures B.2 and B.16 of the specification suggest that restart markers should
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// only occur between Entropy Coded Segments and not after the final ECS.
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// However, some encoders may generate incorrect JPEGs with a final restart
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// marker. That restart marker will be seen here instead of inside the processSOS
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// method, and is ignored as a harmless error. Restart markers have no extra data,
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// so we check for this before we read the 16-bit length of the segment.
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continue
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}
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// Read the 16-bit length of the segment. The value includes the 2 bytes for the
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// length itself, so we subtract 2 to get the number of remaining bytes.
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_, err = io.ReadFull(d.r, d.tmp[0:2])
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if err != nil {
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return nil, err
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}
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n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
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if n < 0 {
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return nil, FormatError("short segment length")
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}
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switch {
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case marker == sof0Marker || marker == sof2Marker: // Start Of Frame.
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d.progressive = marker == sof2Marker
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err = d.processSOF(n)
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if configOnly {
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return nil, err
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}
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case marker == dhtMarker: // Define Huffman Table.
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err = d.processDHT(n)
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case marker == dqtMarker: // Define Quantization Table.
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err = d.processDQT(n)
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case marker == sosMarker: // Start Of Scan.
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err = d.processSOS(n)
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case marker == driMarker: // Define Restart Interval.
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err = d.processDRI(n)
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case app0Marker <= marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
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err = d.ignore(n)
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default:
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err = UnsupportedError("unknown marker")
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}
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if err != nil {
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return nil, err
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}
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}
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if d.img1 != nil {
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return d.img1, nil
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}
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if d.img3 != nil {
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return d.img3, nil
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}
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return nil, FormatError("missing SOS marker")
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}
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// Decode reads a JPEG image from r and returns it as an image.Image.
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func Decode(r io.Reader) (image.Image, error) {
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var d decoder
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return d.decode(r, false)
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}
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// DecodeConfig returns the color model and dimensions of a JPEG image without
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// decoding the entire image.
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func DecodeConfig(r io.Reader) (image.Config, error) {
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var d decoder
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if _, err := d.decode(r, true); err != nil {
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return image.Config{}, err
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}
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switch d.nComp {
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case nGrayComponent:
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return image.Config{
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ColorModel: color.GrayModel,
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Width: d.width,
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Height: d.height,
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}, nil
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case nColorComponent:
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return image.Config{
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ColorModel: color.YCbCrModel,
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Width: d.width,
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Height: d.height,
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}, nil
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}
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return image.Config{}, FormatError("missing SOF marker")
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}
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func init() {
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image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
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}
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