// 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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