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Source file src/image/jpeg/scan.go

Documentation: image/jpeg

  // 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(mxx, myy int) {
  	if d.nComp == 1 {
  		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
  	}
  
  	h0 := d.comp[0].h
  	v0 := d.comp[0].v
  	hRatio := h0 / d.comp[1].h
  	vRatio := v0 / d.comp[1].v
  	var subsampleRatio image.YCbCrSubsampleRatio
  	switch hRatio<<4 | vRatio {
  	case 0x11:
  		subsampleRatio = image.YCbCrSubsampleRatio444
  	case 0x12:
  		subsampleRatio = image.YCbCrSubsampleRatio440
  	case 0x21:
  		subsampleRatio = image.YCbCrSubsampleRatio422
  	case 0x22:
  		subsampleRatio = image.YCbCrSubsampleRatio420
  	case 0x41:
  		subsampleRatio = image.YCbCrSubsampleRatio411
  	case 0x42:
  		subsampleRatio = image.YCbCrSubsampleRatio410
  	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)
  
  	if d.nComp == 4 {
  		h3, v3 := d.comp[3].h, d.comp[3].v
  		d.blackPix = make([]byte, 8*h3*mxx*8*v3*myy)
  		d.blackStride = 8 * h3 * mxx
  	}
  }
  
  // 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 [maxComponents]struct {
  		compIndex uint8
  		td        uint8 // DC table selector.
  		ta        uint8 // AC table selector.
  	}
  	totalHV := 0
  	for i := 0; i < nComp; i++ {
  		cs := d.tmp[1+2*i] // Component selector.
  		compIndex := -1
  		for j, comp := range d.comp[:d.nComp] {
  			if cs == comp.c {
  				compIndex = j
  			}
  		}
  		if compIndex < 0 {
  			return FormatError("unknown component selector")
  		}
  		scan[i].compIndex = uint8(compIndex)
  		// Section B.2.3 states that "the value of Cs_j shall be different from
  		// the values of Cs_1 through Cs_(j-1)". Since we have previously
  		// verified that a frame's component identifiers (C_i values in section
  		// B.2.2) are unique, it suffices to check that the implicit indexes
  		// into d.comp are unique.
  		for j := 0; j < i; j++ {
  			if scan[i].compIndex == scan[j].compIndex {
  				return FormatError("repeated component selector")
  			}
  		}
  		totalHV += d.comp[compIndex].h * d.comp[compIndex].v
  
  		// The baseline t <= 1 restriction is specified in table B.3.
  		scan[i].td = d.tmp[2+2*i] >> 4
  		if t := scan[i].td; t > maxTh || (d.baseline && t > 1) {
  			return FormatError("bad Td value")
  		}
  		scan[i].ta = d.tmp[2+2*i] & 0x0f
  		if t := scan[i].ta; t > maxTh || (d.baseline && t > 1) {
  			return FormatError("bad Ta value")
  		}
  	}
  	// Section B.2.3 states that if there is more than one component then the
  	// total H*V values in a scan must be <= 10.
  	if d.nComp > 1 && totalHV > 10 {
  		return FormatError("total sampling factors too large")
  	}
  
  	// 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 sequential JPEGs, these parameters are hard-coded to 0/63/0/0, as
  	// per table B.3.
  	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(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 [maxComponents]int32
  		// bx and by are the location of the current block, in units of 8x8
  		// blocks: the third block in the first row has (bx, by) = (2, 0).
  		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
  				hi := d.comp[compIndex].h
  				vi := d.comp[compIndex].v
  				for j := 0; j < hi*vi; 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 sequential 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 = hi*mx + j%hi
  						by = vi*my + j/hi
  					} else {
  						q := mxx * hi
  						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*hi+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 {
  						// Save the coefficients.
  						d.progCoeffs[compIndex][by*mxx*hi+bx] = b
  						// At this point, we could call reconstructBlock 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. Instead, reconstructBlock is called on each
  						// accumulated block by the reconstructProgressiveImage method after all of the
  						// SOS markers are processed.
  						continue
  					}
  					if err := d.reconstructBlock(&b, bx, by, int(compIndex)); err != nil {
  						return err
  					}
  				} // 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 = [maxComponents]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
  }
  
  func (d *decoder) reconstructProgressiveImage() error {
  	// The h0, mxx, by and bx variables have the same meaning as in the
  	// processSOS method.
  	h0 := d.comp[0].h
  	mxx := (d.width + 8*h0 - 1) / (8 * h0)
  	for i := 0; i < d.nComp; i++ {
  		if d.progCoeffs[i] == nil {
  			continue
  		}
  		v := 8 * d.comp[0].v / d.comp[i].v
  		h := 8 * d.comp[0].h / d.comp[i].h
  		stride := mxx * d.comp[i].h
  		for by := 0; by*v < d.height; by++ {
  			for bx := 0; bx*h < d.width; bx++ {
  				if err := d.reconstructBlock(&d.progCoeffs[i][by*stride+bx], bx, by, i); err != nil {
  					return err
  				}
  			}
  		}
  	}
  	return nil
  }
  
  // reconstructBlock dequantizes, performs the inverse DCT and stores the block
  // to the image.
  func (d *decoder) reconstructBlock(b *block, bx, by, compIndex int) error {
  	qt := &d.quant[d.comp[compIndex].tq]
  	for zig := 0; zig < blockSize; zig++ {
  		b[unzig[zig]] *= qt[zig]
  	}
  	idct(b)
  	dst, stride := []byte(nil), 0
  	if d.nComp == 1 {
  		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
  		case 3:
  			dst, stride = d.blackPix[8*(by*d.blackStride+bx):], d.blackStride
  		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)
  		}
  	}
  	return nil
  }
  

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