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