diff --git a/src/pkg/Makefile b/src/pkg/Makefile index db33ab26299..f37502d58d4 100644 --- a/src/pkg/Makefile +++ b/src/pkg/Makefile @@ -72,6 +72,7 @@ DIRS=\ hash/crc32\ http\ image\ + image/jpeg\ image/png\ io\ io/ioutil\ @@ -117,6 +118,7 @@ NOTEST=\ go/token\ hash\ image\ + image/jpeg\ malloc\ rand\ runtime\ diff --git a/src/pkg/image/jpeg/Makefile b/src/pkg/image/jpeg/Makefile new file mode 100644 index 00000000000..c84811d6ada --- /dev/null +++ b/src/pkg/image/jpeg/Makefile @@ -0,0 +1,13 @@ +# Copyright 2009 The Go Authors. All rights reserved. +# Use of this source code is governed by a BSD-style +# license that can be found in the LICENSE file. + +include $(GOROOT)/src/Make.$(GOARCH) + +TARG=image/jpeg +GOFILES=\ + huffman.go\ + idct.go\ + reader.go\ + +include $(GOROOT)/src/Make.pkg diff --git a/src/pkg/image/jpeg/huffman.go b/src/pkg/image/jpeg/huffman.go new file mode 100644 index 00000000000..0d03a7317e2 --- /dev/null +++ b/src/pkg/image/jpeg/huffman.go @@ -0,0 +1,190 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package jpeg + +import ( + "io" + "os" +) + +// Each code is at most 16 bits long. +const maxCodeLength = 16 + +// Each decoded value is a uint8, so there are at most 256 such values. +const maxNumValues = 256 + +// Bit stream for the Huffman decoder. +// The n least significant bits of a form the unread bits, to be read in MSB to LSB order. +type bits struct { + a int // accumulator. + n int // the number of unread bits in a. + m int // mask. m==1<<(n-1) when n>0, with m==0 when n==0. +} + +// Huffman table decoder, specified in section C. +type huffman struct { + l [maxCodeLength]int + length int // sum of l[i]. + val [maxNumValues]uint8 // the decoded values, as sorted by their encoding. + size [maxNumValues]int // size[i] is the number of bits to encode val[i]. + code [maxNumValues]int // code[i] is the encoding of val[i]. + minCode [maxCodeLength]int // min codes of length i, or -1 if no codes of that length. + maxCode [maxCodeLength]int // max codes of length i, or -1 if no codes of that length. + valIndex [maxCodeLength]int // index into val of minCode[i]. +} + +// Reads bytes from the io.Reader to ensure that bits.n is at least n. +func (d *decoder) ensureNBits(n int) os.Error { + for d.b.n < n { + c, err := d.r.ReadByte() + if err != nil { + return err + } + d.b.a = d.b.a<<8 | int(c) + d.b.n += 8 + if d.b.m == 0 { + d.b.m = 1 << 7 + } else { + d.b.m <<= 8 + } + // Byte stuffing, specified in section F.1.2.3. + if c == 0xff { + c, err = d.r.ReadByte() + if err != nil { + return err + } + if c != 0x00 { + return FormatError("missing 0xff00 sequence") + } + } + } + return nil +} + +// The composition of RECEIVE and EXTEND, specified in section F.2.2.1. +func (d *decoder) receiveExtend(t uint8) (int, os.Error) { + err := d.ensureNBits(int(t)) + if err != nil { + return 0, err + } + d.b.n -= int(t) + d.b.m >>= t + s := 1 << t + x := (d.b.a >> uint8(d.b.n)) & (s - 1) + if x < s>>1 { + x += ((-1) << t) + 1 + } + return x, nil +} + +// Processes a Define Huffman Table marker, and initializes a huffman struct from its contents. +// Specified in section B.2.4.2. +func (d *decoder) processDHT(n int) os.Error { + for n > 0 { + if n < 17 { + return FormatError("DHT has wrong length") + } + _, err := io.ReadFull(d.r, d.tmp[0:17]) + if err != nil { + return err + } + tc := d.tmp[0] >> 4 + if tc > maxTc { + return FormatError("bad Tc value") + } + th := d.tmp[0] & 0x0f + const isBaseline = true // Progressive mode is not yet supported. + if th > maxTh || isBaseline && th > 1 { + return FormatError("bad Th value") + } + h := &d.huff[tc][th] + + // Read l and val (and derive length). + h.length = 0 + for i := 0; i < maxCodeLength; i++ { + h.l[i] = int(d.tmp[i+1]) + h.length += h.l[i] + } + if h.length == 0 { + return FormatError("Huffman table has zero length") + } + if h.length > maxNumValues { + return FormatError("Huffman table has excessive length") + } + n -= h.length + 17 + if n < 0 { + return FormatError("DHT has wrong length") + } + _, err = io.ReadFull(d.r, h.val[0:h.length]) + if err != nil { + return err + } + + // Derive size. + k := 0 + for i := 0; i < maxCodeLength; i++ { + for j := 0; j < h.l[i]; j++ { + h.size[k] = i + 1 + k++ + } + } + + // Derive code. + code := 0 + size := h.size[0] + for i := 0; i < h.length; i++ { + if size != h.size[i] { + code <<= uint8(h.size[i] - size) + size = h.size[i] + } + h.code[i] = code + code++ + } + + // Derive minCode, maxCode, and valIndex. + k = 0 + index := 0 + for i := 0; i < maxCodeLength; i++ { + if h.l[i] == 0 { + h.minCode[i] = -1 + h.maxCode[i] = -1 + h.valIndex[i] = -1 + } else { + h.minCode[i] = k + h.maxCode[i] = k + h.l[i] - 1 + h.valIndex[i] = index + k += h.l[i] + index += h.l[i] + } + k <<= 1 + } + } + return nil +} + +// Returns the next Huffman-coded value from the bit stream, decoded according to h. +// TODO(nigeltao): This decoding algorithm is simple, but slow. A lookahead table, instead of always +// peeling off only 1 bit at at time, ought to be faster. +func (d *decoder) decodeHuffman(h *huffman) (uint8, os.Error) { + if h.length == 0 { + return 0, FormatError("uninitialized Huffman table") + } + for i, code := 0, 0; i < maxCodeLength; i++ { + err := d.ensureNBits(1) + if err != nil { + return 0, err + } + if d.b.a&d.b.m != 0 { + code |= 1 + } + d.b.n-- + d.b.m >>= 1 + if code <= h.maxCode[i] { + return h.val[h.valIndex[i]+code-h.minCode[i]], nil + } + code <<= 1 + } + return 0, FormatError("bad Huffman code") +} diff --git a/src/pkg/image/jpeg/idct.go b/src/pkg/image/jpeg/idct.go new file mode 100644 index 00000000000..5189931105b --- /dev/null +++ b/src/pkg/image/jpeg/idct.go @@ -0,0 +1,190 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// This is a Go translation of idct.c from +// +// http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/verifier/mpeg2decode_960109.tar.gz +// +// which carries the following notice: + +/* Copyright (C) 1996, MPEG Software Simulation Group. All Rights Reserved. */ + +/* + * Disclaimer of Warranty + * + * These software programs are available to the user without any license fee or + * royalty on an "as is" basis. The MPEG Software Simulation Group disclaims + * any and all warranties, whether express, implied, or statuary, including any + * implied warranties or merchantability or of fitness for a particular + * purpose. In no event shall the copyright-holder be liable for any + * incidental, punitive, or consequential damages of any kind whatsoever + * arising from the use of these programs. + * + * This disclaimer of warranty extends to the user of these programs and user's + * customers, employees, agents, transferees, successors, and assigns. + * + * The MPEG Software Simulation Group does not represent or warrant that the + * programs furnished hereunder are free of infringement of any third-party + * patents. + * + * Commercial implementations of MPEG-1 and MPEG-2 video, including shareware, + * are subject to royalty fees to patent holders. Many of these patents are + * general enough such that they are unavoidable regardless of implementation + * design. + * + */ + +package jpeg + +const ( + w1 = 2841 // 2048*sqrt(2)*cos(1*pi/16) + w2 = 2676 // 2048*sqrt(2)*cos(2*pi/16) + w3 = 2408 // 2048*sqrt(2)*cos(3*pi/16) + w5 = 1609 // 2048*sqrt(2)*cos(5*pi/16) + w6 = 1108 // 2048*sqrt(2)*cos(6*pi/16) + w7 = 565 // 2048*sqrt(2)*cos(7*pi/16) + + w1pw7 = w1 + w7 + w1mw7 = w1 - w7 + w2pw6 = w2 + w6 + w2mw6 = w2 - w6 + w3pw5 = w3 + w5 + w3mw5 = w3 - w5 + + r2 = 181 // 256/sqrt(2) +) + +// 2-D Inverse Discrete Cosine Transformation, followed by a +128 level shift. +// +// The input coefficients should already have been multiplied by the appropriate quantization table. +// We use fixed-point computation, with the number of bits for the fractional component varying over the +// intermediate stages. The final values are expected to range within [0, 255], after a +128 level shift. +// +// For more on the actual algorithm, see Z. Wang, "Fast algorithms for the discrete W transform and +// for the discrete Fourier transform", IEEE Trans. on ASSP, Vol. ASSP- 32, pp. 803-816, Aug. 1984. +func idct(b *[blockSize]int) { + // Horizontal 1-D IDCT. + for y := 0; y < 8; y++ { + // If all the AC components are zero, then the IDCT is trivial. + if b[y*8+1] == 0 && b[y*8+2] == 0 && b[y*8+3] == 0 && + b[y*8+4] == 0 && b[y*8+5] == 0 && b[y*8+6] == 0 && b[y*8+7] == 0 { + dc := b[y*8+0] << 3 + b[y*8+0] = dc + b[y*8+1] = dc + b[y*8+2] = dc + b[y*8+3] = dc + b[y*8+4] = dc + b[y*8+5] = dc + b[y*8+6] = dc + b[y*8+7] = dc + continue + } + + // Prescale. + x0 := (b[y*8+0] << 11) + 128 + x1 := b[y*8+4] << 11 + x2 := b[y*8+6] + x3 := b[y*8+2] + x4 := b[y*8+1] + x5 := b[y*8+7] + x6 := b[y*8+5] + x7 := b[y*8+3] + + // Stage 1. + x8 := w7 * (x4 + x5) + x4 = x8 + w1mw7*x4 + x5 = x8 - w1pw7*x5 + x8 = w3 * (x6 + x7) + x6 = x8 - w3mw5*x6 + x7 = x8 - w3pw5*x7 + + // Stage 2. + x8 = x0 + x1 + x0 -= x1 + x1 = w6 * (x3 + x2) + x2 = x1 - w2pw6*x2 + x3 = x1 + w2mw6*x3 + x1 = x4 + x6 + x4 -= x6 + x6 = x5 + x7 + x5 -= x7 + + // Stage 3. + x7 = x8 + x3 + x8 -= x3 + x3 = x0 + x2 + x0 -= x2 + x2 = (r2*(x4+x5) + 128) >> 8 + x4 = (r2*(x4-x5) + 128) >> 8 + + // Stage 4. + b[8*y+0] = (x7 + x1) >> 8 + b[8*y+1] = (x3 + x2) >> 8 + b[8*y+2] = (x0 + x4) >> 8 + b[8*y+3] = (x8 + x6) >> 8 + b[8*y+4] = (x8 - x6) >> 8 + b[8*y+5] = (x0 - x4) >> 8 + b[8*y+6] = (x3 - x2) >> 8 + b[8*y+7] = (x7 - x1) >> 8 + } + + // Vertical 1-D IDCT. + for x := 0; x < 8; x++ { + // Similar to the horizontal 1-D IDCT case, if all the AC components are zero, then the IDCT is trivial. + // However, after performing the horizontal 1-D IDCT, there are typically non-zero AC components, so + // we do not bother to check for the all-zero case. + + // Prescale. + y0 := (b[8*0+x] << 8) + 8192 + y1 := b[8*4+x] << 8 + y2 := b[8*6+x] + y3 := b[8*2+x] + y4 := b[8*1+x] + y5 := b[8*7+x] + y6 := b[8*5+x] + y7 := b[8*3+x] + + // Stage 1. + y8 := w7*(y4+y5) + 4 + y4 = (y8 + w1mw7*y4) >> 3 + y5 = (y8 - w1pw7*y5) >> 3 + y8 = w3*(y6+y7) + 4 + y6 = (y8 - w3mw5*y6) >> 3 + y7 = (y8 - w3pw5*y7) >> 3 + + // Stage 2. + y8 = y0 + y1 + y0 -= y1 + y1 = w6*(y3+y2) + 4 + y2 = (y1 - w2pw6*y2) >> 3 + y3 = (y1 + w2mw6*y3) >> 3 + y1 = y4 + y6 + y4 -= y6 + y6 = y5 + y7 + y5 -= y7 + + // Stage 3. + y7 = y8 + y3 + y8 -= y3 + y3 = y0 + y2 + y0 -= y2 + y2 = (r2*(y4+y5) + 128) >> 8 + y4 = (r2*(y4-y5) + 128) >> 8 + + // Stage 4. + b[8*0+x] = (y7 + y1) >> 14 + b[8*1+x] = (y3 + y2) >> 14 + b[8*2+x] = (y0 + y4) >> 14 + b[8*3+x] = (y8 + y6) >> 14 + b[8*4+x] = (y8 - y6) >> 14 + b[8*5+x] = (y0 - y4) >> 14 + b[8*6+x] = (y3 - y2) >> 14 + b[8*7+x] = (y7 - y1) >> 14 + } + + // Level shift. + for i := range *b { + b[i] += 128 + } +} diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go new file mode 100644 index 00000000000..7e640d9bac5 --- /dev/null +++ b/src/pkg/image/jpeg/reader.go @@ -0,0 +1,392 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +// The jpeg package implements a decoder for JPEG images, as defined in ITU-T T.81. +package jpeg + +// See http://www.w3.org/Graphics/JPEG/itu-t81.pdf + +import ( + "bufio" + "image" + "io" + "os" +) + +// A FormatError reports that the input is not a valid JPEG. +type FormatError string + +func (e FormatError) String() string { return "invalid JPEG format: " + string(e) } + +// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature. +type UnsupportedError string + +func (e UnsupportedError) String() string { return "unsupported JPEG feature: " + string(e) } + +// Component specification, specified in section B.2.2. +type component struct { + c uint8 // Component identifier. + h uint8 // Horizontal sampling factor. + v uint8 // Vertical sampling factor. + tq uint8 // Quantization table destination selector. +} + +const ( + blockSize = 64 // A DCT block is 8x8. + + dcTableClass = 0 + acTableClass = 1 + maxTc = 1 + maxTh = 3 + maxTq = 3 + + // We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and assume that the components are Y, Cb, Cr. + nComponent = 3 + maxH = 2 + maxV = 2 +) + +const ( + soiMarker = 0xd8 // Start Of Image. + eoiMarker = 0xd9 // End Of Image. + sof0Marker = 0xc0 // Start Of Frame (Baseline). + sof2Marker = 0xc2 // Start Of Frame (Progressive). + dhtMarker = 0xc4 // Define Huffman Table. + dqtMarker = 0xdb // Define Quantization Table. + sosMarker = 0xda // Start Of Scan. + app0Marker = 0xe0 // APPlication specific (0). + app15Marker = 0xef // APPlication specific (15). + comMarker = 0xfe // COMment. +) + +// Maps from the zig-zag ordering to the natural ordering. +var unzig = [blockSize]int{ + 0, 1, 8, 16, 9, 2, 3, 10, + 17, 24, 32, 25, 18, 11, 4, 5, + 12, 19, 26, 33, 40, 48, 41, 34, + 27, 20, 13, 6, 7, 14, 21, 28, + 35, 42, 49, 56, 57, 50, 43, 36, + 29, 22, 15, 23, 30, 37, 44, 51, + 58, 59, 52, 45, 38, 31, 39, 46, + 53, 60, 61, 54, 47, 55, 62, 63, +} + +// If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering. +type Reader interface { + io.Reader + ReadByte() (c byte, err os.Error) +} + +type decoder struct { + r Reader + width, height int + image *image.RGBA + comps [nComponent]component + huff [maxTc + 1][maxTh + 1]huffman + quant [maxTq + 1][blockSize]int + b bits + blocks [nComponent][maxH * maxV][blockSize]int + tmp [1024]byte +} + +// Reads and ignores the next n bytes. +func (d *decoder) ignore(n int) os.Error { + for n > 0 { + m := len(d.tmp) + if m > n { + m = n + } + _, err := io.ReadFull(d.r, d.tmp[0:m]) + if err != nil { + return err + } + n -= m + } + return nil +} + +// Specified in section B.2.2. +func (d *decoder) processSOF(n int) os.Error { + if n != 6+3*nComponent { + return UnsupportedError("SOF has wrong length") + } + _, err := io.ReadFull(d.r, d.tmp[0:6+3*nComponent]) + if err != nil { + return err + } + // We only support 8-bit precision. + if d.tmp[0] != 8 { + return UnsupportedError("precision") + } + d.height = int(d.tmp[1])<<8 + int(d.tmp[2]) + d.width = int(d.tmp[3])<<8 + int(d.tmp[4]) + if d.tmp[5] != nComponent { + return UnsupportedError("SOF has wrong number of image components") + } + for i := 0; i < nComponent; i++ { + hv := d.tmp[7+3*i] + d.comps[i].c = d.tmp[6+3*i] + d.comps[i].h = hv >> 4 + d.comps[i].v = hv & 0x0f + d.comps[i].tq = d.tmp[8+3*i] + // We only support YCbCr images, and 4:4:4, 4:2:2 or 4:2:0 chroma downsampling ratios. This implies that + // the (h, v) values for the Y component are either (1, 1), (2, 1) or (2, 2), and the + // (h, v) values for the Cr and Cb components must be (1, 1). + if i == 0 { + if hv != 0x11 && hv != 0x21 && hv != 0x22 { + return UnsupportedError("luma downsample ratio") + } + } else { + if hv != 0x11 { + return UnsupportedError("chroma downsample ratio") + } + } + } + d.image = image.NewRGBA(d.width, d.height) + return nil +} + +// Specified in section B.2.4.1. +func (d *decoder) processDQT(n int) os.Error { + const qtLength = 1 + blockSize + for ; n >= qtLength; n -= qtLength { + _, err := io.ReadFull(d.r, d.tmp[0:qtLength]) + if err != nil { + return err + } + pq := d.tmp[0] >> 4 + if pq != 0 { + return UnsupportedError("bad Pq value") + } + tq := d.tmp[0] & 0x0f + if tq > maxTq { + return FormatError("bad Tq value") + } + for i := range d.quant[tq] { + d.quant[tq][i] = int(d.tmp[i+1]) + } + } + if n != 0 { + return FormatError("DQT has wrong length") + } + return nil +} + +// Set the Pixel (px, py)'s RGB value, based on its YCbCr value. +func (d *decoder) calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex int) { + y, cb, cr := d.blocks[0][lumaBlock][lumaIndex], d.blocks[1][0][chromaIndex], d.blocks[2][0][chromaIndex] + // The JFIF specification (http://www.w3.org/Graphics/JPEG/jfif3.pdf, page 3) gives the formula + // for translating YCbCr to RGB as: + // R = Y + 1.402 (Cr-128) + // G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128) + // B = Y + 1.772 (Cb-128) + yPlusHalf := 100000*y + 50000 + cb -= 128 + cr -= 128 + r := (yPlusHalf + 140200*cr) / 100000 + g := (yPlusHalf - 34414*cb - 71414*cr) / 100000 + b := (yPlusHalf + 177200*cb) / 100000 + if r < 0 { + r = 0 + } else if r > 255 { + r = 255 + } + if g < 0 { + g = 0 + } else if g > 255 { + g = 255 + } + if b < 0 { + b = 0 + } else if b > 255 { + b = 255 + } + d.image.Pixel[py][px] = image.RGBAColor{uint8(r), uint8(g), uint8(b), 0xff} +} + +// Convert the MCU from YCbCr to RGB. +func (d *decoder) convertMCU(mx, my, h0, v0 int) { + lumaBlock := 0 + for v := 0; v < v0; v++ { + for h := 0; h < h0; h++ { + chromaBase := 8*4*v + 4*h + py := 8 * (v0*my + v) + for y := 0; y < 8 && py < d.height; y++ { + px := 8 * (h0*mx + h) + lumaIndex := 8 * y + chromaIndex := chromaBase + 8*(y/v0) + for x := 0; x < 8 && px < d.width; x++ { + d.calcPixel(px, py, lumaBlock, lumaIndex, chromaIndex) + if h0 == 1 { + chromaIndex += 1 + } else { + chromaIndex += x % 2 + } + lumaIndex++ + px++ + } + py++ + } + lumaBlock++ + } + } +} + +// Specified in section B.2.3. +func (d *decoder) processSOS(n int) os.Error { + if d.image == nil { + return FormatError("missing SOF segment") + } + if n != 4+2*nComponent { + return UnsupportedError("SOS has wrong length") + } + _, err := io.ReadFull(d.r, d.tmp[0:4+2*nComponent]) + if err != nil { + return err + } + if d.tmp[0] != nComponent { + return UnsupportedError("SOS has wrong number of image components") + } + var scanComps [nComponent]struct { + td uint8 // DC table selector. + ta uint8 // AC table selector. + } + h0, v0 := int(d.comps[0].h), int(d.comps[0].v) // The h and v values from the Y components. + for i := 0; i < nComponent; i++ { + cs := d.tmp[1+2*i] // Component selector. + if cs != d.comps[i].c { + return UnsupportedError("scan components out of order") + } + scanComps[i].td = d.tmp[2+2*i] >> 4 + scanComps[i].ta = d.tmp[2+2*i] & 0x0f + } + // mxx and myy are the number of MCUs (Minimum Coded Units) in the image. + mxx := (d.width + 8*int(h0) - 1) / (8 * int(h0)) + myy := (d.height + 8*int(v0) - 1) / (8 * int(v0)) + + var allZeroes [blockSize]int + var dc [nComponent]int + for my := 0; my < myy; my++ { + for mx := 0; mx < mxx; mx++ { + for i := 0; i < nComponent; i++ { + qt := &d.quant[d.comps[i].tq] + for j := 0; j < int(d.comps[i].h*d.comps[i].v); j++ { + d.blocks[i][j] = allZeroes + + // Decode the DC coefficient, as specified in section F.2.2.1. + value, err := d.decodeHuffman(&d.huff[dcTableClass][scanComps[i].td]) + if err != nil { + return err + } + if value > 16 { + return UnsupportedError("excessive DC component") + } + dcDelta, err := d.receiveExtend(value) + if err != nil { + return err + } + dc[i] += dcDelta + d.blocks[i][j][0] = dc[i] * qt[0] + + // Decode the AC coefficients, as specified in section F.2.2.2. + for k := 1; k < blockSize; k++ { + value, err := d.decodeHuffman(&d.huff[acTableClass][scanComps[i].ta]) + if err != nil { + return err + } + v0 := value >> 4 + v1 := value & 0x0f + if v1 != 0 { + k += int(v0) + if k > blockSize { + return FormatError("bad DCT index") + } + ac, err := d.receiveExtend(v1) + if err != nil { + return err + } + d.blocks[i][j][unzig[k]] = ac * qt[k] + } else { + if v0 != 0x0f { + break + } + k += 0x0f + } + } + + idct(&d.blocks[i][j]) + } // for j + } // for i + d.convertMCU(mx, my, int(d.comps[0].h), int(d.comps[0].v)) + } // for mx + } // for my + + return nil +} + +// Decode reads a JPEG formatted image from r and returns it as an image.Image. +func Decode(r io.Reader) (image.Image, os.Error) { + var d decoder + if rr, ok := r.(Reader); ok { + d.r = rr + } else { + d.r = bufio.NewReader(r) + } + + // Check for the Start Of Image marker. + _, err := io.ReadFull(r, d.tmp[0:2]) + if err != nil { + return nil, err + } + if d.tmp[0] != 0xff || d.tmp[1] != soiMarker { + return nil, FormatError("missing SOI marker") + } + + // Process the remaining segments until the End Of Image marker. + for { + _, err := io.ReadFull(r, d.tmp[0:2]) + if err != nil { + return nil, err + } + if d.tmp[0] != 0xff { + return nil, FormatError("missing 0xff marker start") + } + marker := d.tmp[1] + if marker == eoiMarker { // End Of Image. + break + } + + // Read the 16-bit length of the segment. The value includes the 2 bytes for the + // length itself, so we subtract 2 to get the number of remaining bytes. + _, err = io.ReadFull(r, d.tmp[0:2]) + if err != nil { + return nil, err + } + n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2 + if n < 0 { + return nil, FormatError("short segment length") + } + + switch { + case marker == sof0Marker: // Start Of Frame (Baseline). + err = d.processSOF(n) + case marker == sof2Marker: // Start Of Frame (Progressive). + err = UnsupportedError("progressive mode") + case marker == dhtMarker: // Define Huffman Table. + err = d.processDHT(n) + case marker == dqtMarker: // Define Quantization Table. + err = d.processDQT(n) + case marker == sosMarker: // Start Of Scan. + err = d.processSOS(n) + case marker >= app0Marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment. + err = d.ignore(n) + default: + err = UnsupportedError("unknown marker") + } + if err != nil { + return nil, err + } + } + return d.image, nil +}