// Copyright 2012 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 ( "image" "io" ) // makeImg allocates and initializes the destination image. func (d *decoder) makeImg(h0, v0, mxx, myy int) { if d.nComp == nGrayComponent { m := image.NewGray(image.Rect(0, 0, 8*mxx, 8*myy)) d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray) return } var subsampleRatio image.YCbCrSubsampleRatio switch { case h0 == 1 && v0 == 1: subsampleRatio = image.YCbCrSubsampleRatio444 case h0 == 1 && v0 == 2: subsampleRatio = image.YCbCrSubsampleRatio440 case h0 == 2 && v0 == 1: subsampleRatio = image.YCbCrSubsampleRatio422 case h0 == 2 && v0 == 2: subsampleRatio = image.YCbCrSubsampleRatio420 default: panic("unreachable") } m := image.NewYCbCr(image.Rect(0, 0, 8*h0*mxx, 8*v0*myy), subsampleRatio) d.img3 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.YCbCr) } // Specified in section B.2.3. func (d *decoder) processSOS(n int) error { if d.nComp == 0 { return FormatError("missing SOF marker") } if n < 6 || 4+2*d.nComp < n || n%2 != 0 { return FormatError("SOS has wrong length") } _, err := io.ReadFull(d.r, d.tmp[:n]) if err != nil { return err } nComp := int(d.tmp[0]) if n != 4+2*nComp { return FormatError("SOS length inconsistent with number of components") } var scan [nColorComponent]struct { compIndex uint8 td uint8 // DC table selector. ta uint8 // AC table selector. } for i := 0; i < nComp; i++ { cs := d.tmp[1+2*i] // Component selector. compIndex := -1 for j, comp := range d.comp { if cs == comp.c { compIndex = j } } if compIndex < 0 { return FormatError("unknown component selector") } scan[i].compIndex = uint8(compIndex) scan[i].td = d.tmp[2+2*i] >> 4 scan[i].ta = d.tmp[2+2*i] & 0x0f } // zigStart and zigEnd are the spectral selection bounds. // ah and al are the successive approximation high and low values. // The spec calls these values Ss, Se, Ah and Al. // // For progressive JPEGs, these are the two more-or-less independent // aspects of progression. Spectral selection progression is when not // all of a block's 64 DCT coefficients are transmitted in one pass. // For example, three passes could transmit coefficient 0 (the DC // component), coefficients 1-5, and coefficients 6-63, in zig-zag // order. Successive approximation is when not all of the bits of a // band of coefficients are transmitted in one pass. For example, // three passes could transmit the 6 most significant bits, followed // by the second-least significant bit, followed by the least // significant bit. // // For baseline JPEGs, these parameters are hard-coded to 0/63/0/0. zigStart, zigEnd, ah, al := int32(0), int32(blockSize-1), uint32(0), uint32(0) if d.progressive { zigStart = int32(d.tmp[1+2*nComp]) zigEnd = int32(d.tmp[2+2*nComp]) ah = uint32(d.tmp[3+2*nComp] >> 4) al = uint32(d.tmp[3+2*nComp] & 0x0f) if (zigStart == 0 && zigEnd != 0) || zigStart > zigEnd || blockSize <= zigEnd { return FormatError("bad spectral selection bounds") } if zigStart != 0 && nComp != 1 { return FormatError("progressive AC coefficients for more than one component") } if ah != 0 && ah != al+1 { return FormatError("bad successive approximation values") } } // mxx and myy are the number of MCUs (Minimum Coded Units) in the image. h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components. mxx := (d.width + 8*h0 - 1) / (8 * h0) myy := (d.height + 8*v0 - 1) / (8 * v0) if d.img1 == nil && d.img3 == nil { d.makeImg(h0, v0, mxx, myy) if d.progressive { for i := 0; i < nComp; i++ { compIndex := scan[i].compIndex d.progCoeffs[compIndex] = make([]block, mxx*myy*d.comp[compIndex].h*d.comp[compIndex].v) } } } d.b = bits{} mcu, expectedRST := 0, uint8(rst0Marker) var ( // b is the decoded coefficients, in natural (not zig-zag) order. b block dc [nColorComponent]int32 // mx0 and my0 are the location of the current (in terms of 8x8 blocks). // For example, with 4:2:0 chroma subsampling, the block whose top left // pixel co-ordinates are (16, 8) is the third block in the first row: // mx0 is 2 and my0 is 0, even though the pixel is in the second MCU. // TODO(nigeltao): rename mx0 and my0 to bx and by? mx0, my0 int blockCount int ) for my := 0; my < myy; my++ { for mx := 0; mx < mxx; mx++ { for i := 0; i < nComp; i++ { compIndex := scan[i].compIndex qt := &d.quant[d.comp[compIndex].tq] for j := 0; j < d.comp[compIndex].h*d.comp[compIndex].v; j++ { // The blocks are traversed one MCU at a time. For 4:2:0 chroma // subsampling, there are four Y 8x8 blocks in every 16x16 MCU. // For a baseline 32x16 pixel image, the Y blocks visiting order is: // 0 1 4 5 // 2 3 6 7 // // For progressive images, the DC data blocks (zigStart == 0) are traversed // as above, but AC data blocks are traversed left to right, top to bottom: // 0 1 2 3 // 4 5 6 7 // // To further complicate matters, there is no AC data for any blocks that // are inside the image at the MCU level but outside the image at the pixel // level. For example, a 24x16 pixel 4:2:0 progressive image consists of // two 16x16 MCUs. The earlier scans will process 8 Y blocks: // 0 1 4 5 // 2 3 6 7 // The later scans will process only 6 Y blocks: // 0 1 2 // 3 4 5 if zigStart == 0 { mx0, my0 = d.comp[compIndex].h*mx, d.comp[compIndex].v*my if h0 == 1 { my0 += j } else { mx0 += j % 2 my0 += j / 2 } } else { q := mxx * d.comp[compIndex].h mx0 = blockCount % q my0 = blockCount / q blockCount++ if mx0*8 >= d.width || my0*8 >= d.height { continue } } // Load the previous partially decoded coefficients, if applicable. if d.progressive { b = d.progCoeffs[compIndex][my0*mxx*d.comp[compIndex].h+mx0] } else { b = block{} } if ah != 0 { if err := d.refine(&b, &d.huff[acTable][scan[i].ta], zigStart, zigEnd, 1< 16 { return UnsupportedError("excessive DC component") } dcDelta, err := d.receiveExtend(value) if err != nil { return err } dc[compIndex] += dcDelta b[0] = dc[compIndex] << al } if zig <= zigEnd && d.eobRun > 0 { d.eobRun-- } else { // Decode the AC coefficients, as specified in section F.2.2.2. for ; zig <= zigEnd; zig++ { value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta]) if err != nil { return err } val0 := value >> 4 val1 := value & 0x0f if val1 != 0 { zig += int32(val0) if zig > zigEnd { break } ac, err := d.receiveExtend(val1) if err != nil { return err } b[unzig[zig]] = ac << al } else { if val0 != 0x0f { d.eobRun = uint16(1 << val0) if val0 != 0 { bits, err := d.decodeBits(int(val0)) if err != nil { return err } d.eobRun |= uint16(bits) } d.eobRun-- break } zig += 0x0f } } } } if d.progressive { if zigEnd != blockSize-1 || al != 0 { // We haven't completely decoded this 8x8 block. Save the coefficients. d.progCoeffs[compIndex][my0*mxx*d.comp[compIndex].h+mx0] = b // At this point, we could execute the rest of the loop body to dequantize and // perform the inverse DCT, to save early stages of a progressive image to the // *image.YCbCr buffers (the whole point of progressive encoding), but in Go, // the jpeg.Decode function does not return until the entire image is decoded, // so we "continue" here to avoid wasted computation. continue } } // Dequantize, perform the inverse DCT and store the block to the image. for zig := 0; zig < blockSize; zig++ { b[unzig[zig]] *= qt[zig] } idct(&b) dst, stride := []byte(nil), 0 if d.nComp == nGrayComponent { dst, stride = d.img1.Pix[8*(my0*d.img1.Stride+mx0):], d.img1.Stride } else { switch compIndex { case 0: dst, stride = d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride case 1: dst, stride = d.img3.Cb[8*(my0*d.img3.CStride+mx0):], d.img3.CStride case 2: dst, stride = d.img3.Cr[8*(my0*d.img3.CStride+mx0):], d.img3.CStride default: return UnsupportedError("too many components") } } // Level shift by +128, clip to [0, 255], and write to dst. for y := 0; y < 8; y++ { y8 := y * 8 yStride := y * stride for x := 0; x < 8; x++ { c := b[y8+x] if c < -128 { c = 0 } else if c > 127 { c = 255 } else { c += 128 } dst[yStride+x] = uint8(c) } } } // for j } // for i mcu++ if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy { // A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input, // but this one assumes well-formed input, and hence the restart marker follows immediately. _, err := io.ReadFull(d.r, d.tmp[0:2]) if err != nil { return err } if d.tmp[0] != 0xff || d.tmp[1] != expectedRST { return FormatError("bad RST marker") } expectedRST++ if expectedRST == rst7Marker+1 { expectedRST = rst0Marker } // Reset the Huffman decoder. d.b = bits{} // Reset the DC components, as per section F.2.1.3.1. dc = [nColorComponent]int32{} // Reset the progressive decoder state, as per section G.1.2.2. d.eobRun = 0 } } // for mx } // for my return nil } // refine decodes a successive approximation refinement block, as specified in // section G.1.2. func (d *decoder) refine(b *block, h *huffman, zigStart, zigEnd, delta int32) error { // Refining a DC component is trivial. if zigStart == 0 { if zigEnd != 0 { panic("unreachable") } bit, err := d.decodeBit() if err != nil { return err } if bit { b[0] |= delta } return nil } // Refining AC components is more complicated; see sections G.1.2.2 and G.1.2.3. zig := zigStart if d.eobRun == 0 { loop: for ; zig <= zigEnd; zig++ { z := int32(0) value, err := d.decodeHuffman(h) if err != nil { return err } val0 := value >> 4 val1 := value & 0x0f switch val1 { case 0: if val0 != 0x0f { d.eobRun = uint16(1 << val0) if val0 != 0 { bits, err := d.decodeBits(int(val0)) if err != nil { return err } d.eobRun |= uint16(bits) } break loop } case 1: z = delta bit, err := d.decodeBit() if err != nil { return err } if !bit { z = -z } default: return FormatError("unexpected Huffman code") } zig, err = d.refineNonZeroes(b, zig, zigEnd, int32(val0), delta) if err != nil { return err } if zig > zigEnd { return FormatError("too many coefficients") } if z != 0 { b[unzig[zig]] = z } } } if d.eobRun > 0 { d.eobRun-- if _, err := d.refineNonZeroes(b, zig, zigEnd, -1, delta); err != nil { return err } } return nil } // refineNonZeroes refines non-zero entries of b in zig-zag order. If nz >= 0, // the first nz zero entries are skipped over. func (d *decoder) refineNonZeroes(b *block, zig, zigEnd, nz, delta int32) (int32, error) { for ; zig <= zigEnd; zig++ { u := unzig[zig] if b[u] == 0 { if nz == 0 { break } nz-- continue } bit, err := d.decodeBit() if err != nil { return 0, err } if !bit { continue } if b[u] >= 0 { b[u] += delta } else { b[u] -= delta } } return zig, nil }