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-rw-r--r--src/pkg/image/jpeg/reader.go476
1 files changed, 476 insertions, 0 deletions
diff --git a/src/pkg/image/jpeg/reader.go b/src/pkg/image/jpeg/reader.go
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+// Copyright 2009 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 implements a JPEG image decoder and encoder.
+//
+// JPEG is defined in ITU-T T.81: http://www.w3.org/Graphics/JPEG/itu-t81.pdf.
+package jpeg
+
+import (
+ "bufio"
+ "image"
+ "image/ycbcr"
+ "io"
+ "os"
+)
+
+// TODO(nigeltao): fix up the doc comment style so that sentences start with
+// the name of the type or function that they annotate.
+
+// A FormatError reports that the input is not a valid JPEG.
+type FormatError string
+
+func (e FormatError) String() string { return "invalid JPEG format: " + string(e) }
+
+// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
+type UnsupportedError string
+
+func (e UnsupportedError) String() string { return "unsupported JPEG feature: " + string(e) }
+
+// Component specification, specified in section B.2.2.
+type component struct {
+ h int // Horizontal sampling factor.
+ v int // Vertical sampling factor.
+ c uint8 // Component identifier.
+ tq uint8 // Quantization table destination selector.
+}
+
+type block [blockSize]int
+
+const (
+ blockSize = 64 // A DCT block is 8x8.
+
+ dcTable = 0
+ acTable = 1
+ maxTc = 1
+ maxTh = 3
+ maxTq = 3
+
+ // A grayscale JPEG image has only a Y component.
+ nGrayComponent = 1
+ // A color JPEG image has Y, Cb and Cr components.
+ nColorComponent = 3
+
+ // We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and therefore the
+ // number of luma samples per chroma sample is at most 2 in the horizontal
+ // and 2 in the vertical direction.
+ maxH = 2
+ maxV = 2
+)
+
+const (
+ soiMarker = 0xd8 // Start Of Image.
+ eoiMarker = 0xd9 // End Of Image.
+ sof0Marker = 0xc0 // Start Of Frame (Baseline).
+ sof2Marker = 0xc2 // Start Of Frame (Progressive).
+ dhtMarker = 0xc4 // Define Huffman Table.
+ dqtMarker = 0xdb // Define Quantization Table.
+ sosMarker = 0xda // Start Of Scan.
+ driMarker = 0xdd // Define Restart Interval.
+ rst0Marker = 0xd0 // ReSTart (0).
+ rst7Marker = 0xd7 // ReSTart (7).
+ app0Marker = 0xe0 // APPlication specific (0).
+ app15Marker = 0xef // APPlication specific (15).
+ comMarker = 0xfe // COMment.
+)
+
+// Maps from the zig-zag ordering to the natural ordering.
+var unzig = [blockSize]int{
+ 0, 1, 8, 16, 9, 2, 3, 10,
+ 17, 24, 32, 25, 18, 11, 4, 5,
+ 12, 19, 26, 33, 40, 48, 41, 34,
+ 27, 20, 13, 6, 7, 14, 21, 28,
+ 35, 42, 49, 56, 57, 50, 43, 36,
+ 29, 22, 15, 23, 30, 37, 44, 51,
+ 58, 59, 52, 45, 38, 31, 39, 46,
+ 53, 60, 61, 54, 47, 55, 62, 63,
+}
+
+// If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering.
+type Reader interface {
+ io.Reader
+ ReadByte() (c byte, err os.Error)
+}
+
+type decoder struct {
+ r Reader
+ width, height int
+ img1 *image.Gray
+ img3 *ycbcr.YCbCr
+ ri int // Restart Interval.
+ nComp int
+ comp [nColorComponent]component
+ huff [maxTc + 1][maxTh + 1]huffman
+ quant [maxTq + 1]block
+ b bits
+ tmp [1024]byte
+}
+
+// Reads and ignores the next n bytes.
+func (d *decoder) ignore(n int) os.Error {
+ for n > 0 {
+ m := len(d.tmp)
+ if m > n {
+ m = n
+ }
+ _, err := io.ReadFull(d.r, d.tmp[0:m])
+ if err != nil {
+ return err
+ }
+ n -= m
+ }
+ return nil
+}
+
+// Specified in section B.2.2.
+func (d *decoder) processSOF(n int) os.Error {
+ switch n {
+ case 6 + 3*nGrayComponent:
+ d.nComp = nGrayComponent
+ case 6 + 3*nColorComponent:
+ d.nComp = nColorComponent
+ default:
+ return UnsupportedError("SOF has wrong length")
+ }
+ _, err := io.ReadFull(d.r, d.tmp[:n])
+ if err != nil {
+ return err
+ }
+ // We only support 8-bit precision.
+ if d.tmp[0] != 8 {
+ return UnsupportedError("precision")
+ }
+ d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
+ d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
+ if int(d.tmp[5]) != d.nComp {
+ return UnsupportedError("SOF has wrong number of image components")
+ }
+ for i := 0; i < d.nComp; i++ {
+ hv := d.tmp[7+3*i]
+ d.comp[i].h = int(hv >> 4)
+ d.comp[i].v = int(hv & 0x0f)
+ d.comp[i].c = d.tmp[6+3*i]
+ d.comp[i].tq = d.tmp[8+3*i]
+ if d.nComp == nGrayComponent {
+ continue
+ }
+ // For color images, we only support 4:4:4, 4:2:2 or 4:2:0 chroma
+ // downsampling ratios. This implies that the (h, v) values for the Y
+ // component are either (1, 1), (2, 1) or (2, 2), and the (h, v)
+ // values for the Cr and Cb components must be (1, 1).
+ if i == 0 {
+ if hv != 0x11 && hv != 0x21 && hv != 0x22 {
+ return UnsupportedError("luma downsample ratio")
+ }
+ } else if hv != 0x11 {
+ return UnsupportedError("chroma downsample ratio")
+ }
+ }
+ return nil
+}
+
+// Specified in section B.2.4.1.
+func (d *decoder) processDQT(n int) os.Error {
+ const qtLength = 1 + blockSize
+ for ; n >= qtLength; n -= qtLength {
+ _, err := io.ReadFull(d.r, d.tmp[0:qtLength])
+ if err != nil {
+ return err
+ }
+ pq := d.tmp[0] >> 4
+ if pq != 0 {
+ return UnsupportedError("bad Pq value")
+ }
+ tq := d.tmp[0] & 0x0f
+ if tq > maxTq {
+ return FormatError("bad Tq value")
+ }
+ for i := range d.quant[tq] {
+ d.quant[tq][i] = int(d.tmp[i+1])
+ }
+ }
+ if n != 0 {
+ return FormatError("DQT has wrong length")
+ }
+ return nil
+}
+
+// makeImg allocates and initializes the destination image.
+func (d *decoder) makeImg(h0, v0, mxx, myy int) {
+ if d.nComp == nGrayComponent {
+ m := image.NewGray(8*mxx, 8*myy)
+ d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
+ return
+ }
+ var subsampleRatio ycbcr.SubsampleRatio
+ n := h0 * v0
+ switch n {
+ case 1:
+ subsampleRatio = ycbcr.SubsampleRatio444
+ case 2:
+ subsampleRatio = ycbcr.SubsampleRatio422
+ case 4:
+ subsampleRatio = ycbcr.SubsampleRatio420
+ default:
+ panic("unreachable")
+ }
+ b := make([]byte, mxx*myy*(1*8*8*n+2*8*8))
+ d.img3 = &ycbcr.YCbCr{
+ Y: b[mxx*myy*(0*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+0*8*8)],
+ Cb: b[mxx*myy*(1*8*8*n+0*8*8) : mxx*myy*(1*8*8*n+1*8*8)],
+ Cr: b[mxx*myy*(1*8*8*n+1*8*8) : mxx*myy*(1*8*8*n+2*8*8)],
+ SubsampleRatio: subsampleRatio,
+ YStride: mxx * 8 * h0,
+ CStride: mxx * 8,
+ Rect: image.Rect(0, 0, d.width, d.height),
+ }
+}
+
+// Specified in section B.2.3.
+func (d *decoder) processSOS(n int) os.Error {
+ if d.nComp == 0 {
+ return FormatError("missing SOF marker")
+ }
+ if n != 4+2*d.nComp {
+ return UnsupportedError("SOS has wrong length")
+ }
+ _, err := io.ReadFull(d.r, d.tmp[0:4+2*d.nComp])
+ if err != nil {
+ return err
+ }
+ if int(d.tmp[0]) != d.nComp {
+ return UnsupportedError("SOS has wrong number of image components")
+ }
+ var scan [nColorComponent]struct {
+ td uint8 // DC table selector.
+ ta uint8 // AC table selector.
+ }
+ for i := 0; i < d.nComp; i++ {
+ cs := d.tmp[1+2*i] // Component selector.
+ if cs != d.comp[i].c {
+ return UnsupportedError("scan components out of order")
+ }
+ scan[i].td = d.tmp[2+2*i] >> 4
+ scan[i].ta = d.tmp[2+2*i] & 0x0f
+ }
+ // 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)
+ }
+
+ mcu, expectedRST := 0, uint8(rst0Marker)
+ var (
+ b block
+ dc [nColorComponent]int
+ )
+ for my := 0; my < myy; my++ {
+ for mx := 0; mx < mxx; mx++ {
+ for i := 0; i < d.nComp; i++ {
+ qt := &d.quant[d.comp[i].tq]
+ for j := 0; j < d.comp[i].h*d.comp[i].v; j++ {
+ // TODO(nigeltao): make this a "var b block" once the compiler's escape
+ // analysis is good enough to allocate it on the stack, not the heap.
+ b = block{}
+
+ // 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[i] += dcDelta
+ b[0] = dc[i] * qt[0]
+
+ // Decode the AC coefficients, as specified in section F.2.2.2.
+ for k := 1; k < blockSize; k++ {
+ value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta])
+ if err != nil {
+ return err
+ }
+ val0 := value >> 4
+ val1 := value & 0x0f
+ if val1 != 0 {
+ k += int(val0)
+ if k > blockSize {
+ return FormatError("bad DCT index")
+ }
+ ac, err := d.receiveExtend(val1)
+ if err != nil {
+ return err
+ }
+ b[unzig[k]] = ac * qt[k]
+ } else {
+ if val0 != 0x0f {
+ break
+ }
+ k += 0x0f
+ }
+ }
+
+ // Perform the inverse DCT and store the MCU component to the image.
+ if d.nComp == nGrayComponent {
+ idct(d.img1.Pix[8*(my*d.img1.Stride+mx):], d.img1.Stride, &b)
+ } else {
+ switch i {
+ case 0:
+ mx0 := h0*mx + (j % 2)
+ my0 := v0*my + (j / 2)
+ idct(d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride, &b)
+ case 1:
+ idct(d.img3.Cb[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
+ case 2:
+ idct(d.img3.Cr[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
+ }
+ }
+ } // 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]int{}
+ }
+ } // for mx
+ } // for my
+
+ return nil
+}
+
+// Specified in section B.2.4.4.
+func (d *decoder) processDRI(n int) os.Error {
+ if n != 2 {
+ return FormatError("DRI has wrong length")
+ }
+ _, err := io.ReadFull(d.r, d.tmp[0:2])
+ if err != nil {
+ return err
+ }
+ d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
+ return nil
+}
+
+// decode reads a JPEG image from r and returns it as an image.Image.
+func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, os.Error) {
+ if rr, ok := r.(Reader); ok {
+ d.r = rr
+ } else {
+ d.r = bufio.NewReader(r)
+ }
+
+ // Check for the Start Of Image marker.
+ _, err := io.ReadFull(d.r, d.tmp[0:2])
+ if err != nil {
+ return nil, err
+ }
+ if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
+ return nil, FormatError("missing SOI marker")
+ }
+
+ // Process the remaining segments until the End Of Image marker.
+ for {
+ _, err := io.ReadFull(d.r, d.tmp[0:2])
+ if err != nil {
+ return nil, err
+ }
+ if d.tmp[0] != 0xff {
+ return nil, FormatError("missing 0xff marker start")
+ }
+ marker := d.tmp[1]
+ if marker == eoiMarker { // End Of Image.
+ break
+ }
+
+ // Read the 16-bit length of the segment. The value includes the 2 bytes for the
+ // length itself, so we subtract 2 to get the number of remaining bytes.
+ _, err = io.ReadFull(d.r, d.tmp[0:2])
+ if err != nil {
+ return nil, err
+ }
+ n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
+ if n < 0 {
+ return nil, FormatError("short segment length")
+ }
+
+ switch {
+ case marker == sof0Marker: // Start Of Frame (Baseline).
+ err = d.processSOF(n)
+ if configOnly {
+ return nil, err
+ }
+ case marker == sof2Marker: // Start Of Frame (Progressive).
+ err = UnsupportedError("progressive mode")
+ case marker == dhtMarker: // Define Huffman Table.
+ err = d.processDHT(n)
+ case marker == dqtMarker: // Define Quantization Table.
+ err = d.processDQT(n)
+ case marker == sosMarker: // Start Of Scan.
+ err = d.processSOS(n)
+ case marker == driMarker: // Define Restart Interval.
+ err = d.processDRI(n)
+ case marker >= app0Marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
+ err = d.ignore(n)
+ default:
+ err = UnsupportedError("unknown marker")
+ }
+ if err != nil {
+ return nil, err
+ }
+ }
+ if d.img1 != nil {
+ return d.img1, nil
+ }
+ if d.img3 != nil {
+ return d.img3, nil
+ }
+ return nil, FormatError("missing SOS marker")
+}
+
+// Decode reads a JPEG image from r and returns it as an image.Image.
+func Decode(r io.Reader) (image.Image, os.Error) {
+ var d decoder
+ return d.decode(r, false)
+}
+
+// DecodeConfig returns the color model and dimensions of a JPEG image without
+// decoding the entire image.
+func DecodeConfig(r io.Reader) (image.Config, os.Error) {
+ var d decoder
+ if _, err := d.decode(r, true); err != nil {
+ return image.Config{}, err
+ }
+ switch d.nComp {
+ case nGrayComponent:
+ return image.Config{image.GrayColorModel, d.width, d.height}, nil
+ case nColorComponent:
+ return image.Config{ycbcr.YCbCrColorModel, d.width, d.height}, nil
+ }
+ return image.Config{}, FormatError("missing SOF marker")
+}
+
+func init() {
+ image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
+}
generated by cgit v1.2.3 (git 2.39.1) at 2025-07-22 22:42:59 +0000