462 lines
12 KiB
Go
462 lines
12 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/ycbcr"
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"io"
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"os"
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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) String() 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) String() 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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dcTableClass = 0
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acTableClass = 1
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maxTc = 1
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maxTh = 3
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maxTq = 3
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// We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and assume that the components are Y, Cb, Cr.
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nComponent = 3
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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 os.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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img *ycbcr.YCbCr
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ri int // Restart Interval.
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comps [nComponent]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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blocks [nComponent][maxH * maxV]block
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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) os.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) os.Error {
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if n != 6+3*nComponent {
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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[0:6+3*nComponent])
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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 d.tmp[5] != nComponent {
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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 < nComponent; i++ {
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hv := d.tmp[7+3*i]
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d.comps[i].h = int(hv >> 4)
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d.comps[i].v = int(hv & 0x0f)
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d.comps[i].c = d.tmp[6+3*i]
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d.comps[i].tq = d.tmp[8+3*i]
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// We only support YCbCr images, and 4:4:4, 4:2:2 or 4:2:0 chroma downsampling ratios. This implies that
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// the (h, v) values for the Y component are either (1, 1), (2, 1) or (2, 2), and the
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// (h, v) 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 {
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if hv != 0x11 {
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return UnsupportedError("chroma downsample ratio")
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}
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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) os.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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// Clip x to the range [0, 255] inclusive.
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func clip(x int) uint8 {
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if x < 0 {
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return 0
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}
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if x > 255 {
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return 255
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}
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return uint8(x)
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}
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// Store the MCU to the image.
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func (d *decoder) storeMCU(mx, my int) {
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h0, v0 := d.comps[0].h, d.comps[0].v
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// Store the luma blocks.
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for v := 0; v < v0; v++ {
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for h := 0; h < h0; h++ {
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p := 8 * ((v0*my+v)*d.img.YStride + (h0*mx + h))
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for y := 0; y < 8; y++ {
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for x := 0; x < 8; x++ {
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d.img.Y[p] = clip(d.blocks[0][h0*v+h][8*y+x])
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p++
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}
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p += d.img.YStride - 8
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}
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}
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}
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// Store the chroma blocks.
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p := 8 * (my*d.img.CStride + mx)
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for y := 0; y < 8; y++ {
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for x := 0; x < 8; x++ {
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d.img.Cb[p] = clip(d.blocks[1][0][8*y+x])
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d.img.Cr[p] = clip(d.blocks[2][0][8*y+x])
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p++
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}
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p += d.img.CStride - 8
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}
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}
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// Specified in section B.2.3.
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func (d *decoder) processSOS(n int) os.Error {
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if n != 4+2*nComponent {
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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*nComponent])
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if err != nil {
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return err
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}
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if d.tmp[0] != nComponent {
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return UnsupportedError("SOS has wrong number of image components")
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}
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var scanComps [nComponent]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 < nComponent; i++ {
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cs := d.tmp[1+2*i] // Component selector.
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if cs != d.comps[i].c {
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return UnsupportedError("scan components out of order")
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}
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scanComps[i].td = d.tmp[2+2*i] >> 4
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scanComps[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.comps[0].h, d.comps[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.img == nil {
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var subsampleRatio ycbcr.SubsampleRatio
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n := h0 * v0
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switch n {
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case 1:
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subsampleRatio = ycbcr.SubsampleRatio444
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case 2:
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subsampleRatio = ycbcr.SubsampleRatio422
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case 4:
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subsampleRatio = ycbcr.SubsampleRatio420
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default:
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panic("unreachable")
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}
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b := make([]byte, mxx*myy*(1*8*8*n+2*8*8))
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d.img = &ycbcr.YCbCr{
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Y: b[mxx*myy*(0*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+0*8*8)],
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Cb: b[mxx*myy*(1*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+1*8*8)],
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Cr: b[mxx*myy*(1*8*8*n+1*8*8) : mxx*myy*(1*8*8*n+2*8*8)],
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SubsampleRatio: subsampleRatio,
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YStride: mxx * 8 * h0,
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CStride: mxx * 8,
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Rect: image.Rect(0, 0, d.width, d.height),
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}
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}
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mcu, expectedRST := 0, uint8(rst0Marker)
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var allZeroes block
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var dc [nComponent]int
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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 < nComponent; i++ {
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qt := &d.quant[d.comps[i].tq]
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for j := 0; j < d.comps[i].h*d.comps[i].v; j++ {
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d.blocks[i][j] = allZeroes
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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[dcTableClass][scanComps[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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d.blocks[i][j][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[acTableClass][scanComps[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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d.blocks[i][j][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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idct(&d.blocks[i][j])
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} // for j
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} // for i
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d.storeMCU(mx, my)
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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,
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// but this one assumes well-formed input, and hence the restart marker follows immediately.
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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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if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
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return FormatError("bad RST marker")
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}
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expectedRST++
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if expectedRST == rst7Marker+1 {
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expectedRST = rst0Marker
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}
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// Reset the Huffman decoder.
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d.b = bits{}
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// Reset the DC components, as per section F.2.1.3.1.
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for i := 0; i < nComponent; i++ {
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dc[i] = 0
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}
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}
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} // for mx
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} // for my
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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) os.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, os.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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if d.tmp[0] != 0xff {
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return nil, FormatError("missing 0xff marker start")
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}
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marker := d.tmp[1]
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if marker == eoiMarker { // End Of Image.
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break
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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: // Start Of Frame (Baseline).
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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 == sof2Marker: // Start Of Frame (Progressive).
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err = UnsupportedError("progressive mode")
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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 marker >= app0Marker && 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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return d.img, 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 Decode(r io.Reader) (image.Image, os.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, os.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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return image.Config{image.RGBAColorModel, d.width, d.height}, nil
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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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