| // 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" |
| ) |
| |
| // 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") |
| } |
| if err := d.readFull(d.tmp[:n]); 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 |
| if scan[i].td > maxTh { |
| return FormatError("bad Td value") |
| } |
| scan[i].ta = d.tmp[2+2*i] & 0x0f |
| if scan[i].ta > maxTh { |
| return FormatError("bad Ta value") |
| } |
| } |
| |
| // 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 |
| if d.progCoeffs[compIndex] == nil { |
| d.progCoeffs[compIndex] = make([]block, mxx*myy*d.comp[compIndex].h*d.comp[compIndex].v) |
| } |
| } |
| } |
| |
| d.bits = bits{} |
| mcu, expectedRST := 0, uint8(rst0Marker) |
| var ( |
| // b is the decoded coefficients, in natural (not zig-zag) order. |
| b block |
| dc [nColorComponent]int32 |
| // bx and by 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: |
| // bx is 2 and by is 0, even though the pixel is in the second MCU. |
| bx, by 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 interleaved scans (those with nComp > 1) |
| // are traversed as above, but non-interleaved scans are traversed left |
| // to right, top to bottom: |
| // 0 1 2 3 |
| // 4 5 6 7 |
| // Only DC scans (zigStart == 0) can be interleaved. AC scans must have |
| // only one component. |
| // |
| // To further complicate matters, for non-interleaved scans, there is no |
| // 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 interleaved scans |
| // will process 8 Y blocks: |
| // 0 1 4 5 |
| // 2 3 6 7 |
| // The non-interleaved scans will process only 6 Y blocks: |
| // 0 1 2 |
| // 3 4 5 |
| if nComp != 1 { |
| bx, by = d.comp[compIndex].h*mx, d.comp[compIndex].v*my |
| if h0 == 1 { |
| by += j |
| } else { |
| bx += j % 2 |
| by += j / 2 |
| } |
| } else { |
| q := mxx * d.comp[compIndex].h |
| bx = blockCount % q |
| by = blockCount / q |
| blockCount++ |
| if bx*8 >= d.width || by*8 >= d.height { |
| continue |
| } |
| } |
| |
| // Load the previous partially decoded coefficients, if applicable. |
| if d.progressive { |
| b = d.progCoeffs[compIndex][by*mxx*d.comp[compIndex].h+bx] |
| } else { |
| b = block{} |
| } |
| |
| if ah != 0 { |
| if err := d.refine(&b, &d.huff[acTable][scan[i].ta], zigStart, zigEnd, 1<<al); err != nil { |
| return err |
| } |
| } else { |
| zig := zigStart |
| if zig == 0 { |
| zig++ |
| // Decode the DC coefficient, as specified in section F.2.2.1. |
| value, err := d.decodeHuffman(&d.huff[dcTable][scan[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[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. |
| huff := &d.huff[acTable][scan[i].ta] |
| for ; zig <= zigEnd; zig++ { |
| value, err := d.decodeHuffman(huff) |
| 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(int32(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][by*mxx*d.comp[compIndex].h+bx] = 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*(by*d.img1.Stride+bx):], d.img1.Stride |
| } else { |
| switch compIndex { |
| case 0: |
| dst, stride = d.img3.Y[8*(by*d.img3.YStride+bx):], d.img3.YStride |
| case 1: |
| dst, stride = d.img3.Cb[8*(by*d.img3.CStride+bx):], d.img3.CStride |
| case 2: |
| dst, stride = d.img3.Cr[8*(by*d.img3.CStride+bx):], 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. |
| if err := d.readFull(d.tmp[:2]); 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.bits = 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(int32(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 |
| } |
