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-rw-r--r--src/image/jpeg/reader.go527
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diff --git a/src/image/jpeg/reader.go b/src/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 (
+ "image"
+ "image/color"
+ "io"
+)
+
+// 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) Error() 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) Error() 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.
+}
+
+const (
+ 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, 4:4:0, 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.
+)
+
+// unzig maps from the zig-zag ordering to the natural ordering. For example,
+// unzig[3] is the column and row of the fourth element in zig-zag order. The
+// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
+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,
+}
+
+// Reader is deprecated.
+type Reader interface {
+ io.ByteReader
+ io.Reader
+}
+
+// bits holds the unprocessed bits that have been taken from the byte-stream.
+// The n least significant bits of a form the unread bits, to be read in MSB to
+// LSB order.
+type bits struct {
+ a uint32 // accumulator.
+ m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
+ n int32 // the number of unread bits in a.
+}
+
+type decoder struct {
+ r io.Reader
+ bits bits
+ // bytes is a byte buffer, similar to a bufio.Reader, except that it
+ // has to be able to unread more than 1 byte, due to byte stuffing.
+ // Byte stuffing is specified in section F.1.2.3.
+ bytes struct {
+ // buf[i:j] are the buffered bytes read from the underlying
+ // io.Reader that haven't yet been passed further on.
+ buf [4096]byte
+ i, j int
+ // nUnreadable is the number of bytes to back up i after
+ // overshooting. It can be 0, 1 or 2.
+ nUnreadable int
+ }
+ width, height int
+ img1 *image.Gray
+ img3 *image.YCbCr
+ ri int // Restart Interval.
+ nComp int
+ progressive bool
+ eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
+ comp [nColorComponent]component
+ progCoeffs [nColorComponent][]block // Saved state between progressive-mode scans.
+ huff [maxTc + 1][maxTh + 1]huffman
+ quant [maxTq + 1]block // Quantization tables, in zig-zag order.
+ tmp [blockSize + 1]byte
+}
+
+// fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
+// should only be called when there are no unread bytes in d.bytes.
+func (d *decoder) fill() error {
+ if d.bytes.i != d.bytes.j {
+ panic("jpeg: fill called when unread bytes exist")
+ }
+ // Move the last 2 bytes to the start of the buffer, in case we need
+ // to call unreadByteStuffedByte.
+ if d.bytes.j > 2 {
+ d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
+ d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
+ d.bytes.i, d.bytes.j = 2, 2
+ }
+ // Fill in the rest of the buffer.
+ n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
+ d.bytes.j += n
+ if n > 0 {
+ err = nil
+ }
+ return err
+}
+
+// unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
+// giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
+// requires at least 8 bits for look-up, which means that Huffman decoding can
+// sometimes overshoot and read one or two too many bytes. Two-byte overshoot
+// can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
+func (d *decoder) unreadByteStuffedByte() {
+ if d.bytes.nUnreadable == 0 {
+ panic("jpeg: unreadByteStuffedByte call cannot be fulfilled")
+ }
+ d.bytes.i -= d.bytes.nUnreadable
+ d.bytes.nUnreadable = 0
+ if d.bits.n >= 8 {
+ d.bits.a >>= 8
+ d.bits.n -= 8
+ d.bits.m >>= 8
+ }
+}
+
+// readByte returns the next byte, whether buffered or not buffered. It does
+// not care about byte stuffing.
+func (d *decoder) readByte() (x byte, err error) {
+ for d.bytes.i == d.bytes.j {
+ if err = d.fill(); err != nil {
+ return 0, err
+ }
+ }
+ x = d.bytes.buf[d.bytes.i]
+ d.bytes.i++
+ d.bytes.nUnreadable = 0
+ return x, nil
+}
+
+// errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
+// marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
+var errMissingFF00 = FormatError("missing 0xff00 sequence")
+
+// readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
+func (d *decoder) readByteStuffedByte() (x byte, err error) {
+ // Take the fast path if d.bytes.buf contains at least two bytes.
+ if d.bytes.i+2 <= d.bytes.j {
+ x = d.bytes.buf[d.bytes.i]
+ d.bytes.i++
+ d.bytes.nUnreadable = 1
+ if x != 0xff {
+ return x, err
+ }
+ if d.bytes.buf[d.bytes.i] != 0x00 {
+ return 0, errMissingFF00
+ }
+ d.bytes.i++
+ d.bytes.nUnreadable = 2
+ return 0xff, nil
+ }
+
+ x, err = d.readByte()
+ if err != nil {
+ return 0, err
+ }
+ if x != 0xff {
+ d.bytes.nUnreadable = 1
+ return x, nil
+ }
+
+ x, err = d.readByte()
+ if err != nil {
+ d.bytes.nUnreadable = 1
+ return 0, err
+ }
+ d.bytes.nUnreadable = 2
+ if x != 0x00 {
+ return 0, errMissingFF00
+ }
+ return 0xff, nil
+}
+
+// readFull reads exactly len(p) bytes into p. It does not care about byte
+// stuffing.
+func (d *decoder) readFull(p []byte) error {
+ // Unread the overshot bytes, if any.
+ if d.bytes.nUnreadable != 0 {
+ if d.bits.n >= 8 {
+ d.unreadByteStuffedByte()
+ }
+ d.bytes.nUnreadable = 0
+ }
+
+ for {
+ n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
+ p = p[n:]
+ d.bytes.i += n
+ if len(p) == 0 {
+ break
+ }
+ if err := d.fill(); err != nil {
+ if err == io.EOF {
+ err = io.ErrUnexpectedEOF
+ }
+ return err
+ }
+ }
+ return nil
+}
+
+// ignore ignores the next n bytes.
+func (d *decoder) ignore(n int) error {
+ // Unread the overshot bytes, if any.
+ if d.bytes.nUnreadable != 0 {
+ if d.bits.n >= 8 {
+ d.unreadByteStuffedByte()
+ }
+ d.bytes.nUnreadable = 0
+ }
+
+ for {
+ m := d.bytes.j - d.bytes.i
+ if m > n {
+ m = n
+ }
+ d.bytes.i += m
+ n -= m
+ if n == 0 {
+ break
+ }
+ if err := d.fill(); err != nil {
+ if err == io.EOF {
+ err = io.ErrUnexpectedEOF
+ }
+ return err
+ }
+ }
+ return nil
+}
+
+// Specified in section B.2.2.
+func (d *decoder) processSOF(n int) error {
+ switch n {
+ case 6 + 3*nGrayComponent:
+ d.nComp = nGrayComponent
+ case 6 + 3*nColorComponent:
+ d.nComp = nColorComponent
+ default:
+ return UnsupportedError("SOF has wrong length")
+ }
+ if err := d.readFull(d.tmp[:n]); 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++ {
+ d.comp[i].c = d.tmp[6+3*i]
+ d.comp[i].tq = d.tmp[8+3*i]
+ if d.nComp == nGrayComponent {
+ // If a JPEG image has only one component, section A.2 says "this data
+ // is non-interleaved by definition" and section A.2.2 says "[in this
+ // case...] the order of data units within a scan shall be left-to-right
+ // and top-to-bottom... regardless of the values of H_1 and V_1". Section
+ // 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
+ // one data unit". Similarly, section A.1.1 explains that it is the ratio
+ // of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
+ // images, H_1 is the maximum H_j for all components j, so that ratio is
+ // always 1. The component's (h, v) is effectively always (1, 1): even if
+ // the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
+ // MCUs, not two 16x8 MCUs.
+ d.comp[i].h = 1
+ d.comp[i].v = 1
+ continue
+ }
+ hv := d.tmp[7+3*i]
+ d.comp[i].h = int(hv >> 4)
+ d.comp[i].v = int(hv & 0x0f)
+ // For color images, we only support 4:4:4, 4:4:0, 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), (1, 2), (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 && hv != 0x12 {
+ return UnsupportedError("luma/chroma downsample ratio")
+ }
+ } else if hv != 0x11 {
+ return UnsupportedError("luma/chroma downsample ratio")
+ }
+ }
+ return nil
+}
+
+// Specified in section B.2.4.1.
+func (d *decoder) processDQT(n int) error {
+ const qtLength = 1 + blockSize
+ for ; n >= qtLength; n -= qtLength {
+ if err := d.readFull(d.tmp[:qtLength]); 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] = int32(d.tmp[i+1])
+ }
+ }
+ if n != 0 {
+ return FormatError("DQT has wrong length")
+ }
+ return nil
+}
+
+// Specified in section B.2.4.4.
+func (d *decoder) processDRI(n int) error {
+ if n != 2 {
+ return FormatError("DRI has wrong length")
+ }
+ if err := d.readFull(d.tmp[:2]); 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, error) {
+ d.r = r
+
+ // Check for the Start Of Image marker.
+ if err := d.readFull(d.tmp[:2]); 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 := d.readFull(d.tmp[:2])
+ if err != nil {
+ return nil, err
+ }
+ for d.tmp[0] != 0xff {
+ // Strictly speaking, this is a format error. However, libjpeg is
+ // liberal in what it accepts. As of version 9, next_marker in
+ // jdmarker.c treats this as a warning (JWRN_EXTRANEOUS_DATA) and
+ // continues to decode the stream. Even before next_marker sees
+ // extraneous data, jpeg_fill_bit_buffer in jdhuff.c reads as many
+ // bytes as it can, possibly past the end of a scan's data. It
+ // effectively puts back any markers that it overscanned (e.g. an
+ // "\xff\xd9" EOI marker), but it does not put back non-marker data,
+ // and thus it can silently ignore a small number of extraneous
+ // non-marker bytes before next_marker has a chance to see them (and
+ // print a warning).
+ //
+ // We are therefore also liberal in what we accept. Extraneous data
+ // is silently ignored.
+ //
+ // This is similar to, but not exactly the same as, the restart
+ // mechanism within a scan (the RST[0-7] markers).
+ //
+ // Note that extraneous 0xff bytes in e.g. SOS data are escaped as
+ // "\xff\x00", and so are detected a little further down below.
+ d.tmp[0] = d.tmp[1]
+ d.tmp[1], err = d.readByte()
+ if err != nil {
+ return nil, err
+ }
+ }
+ marker := d.tmp[1]
+ if marker == 0 {
+ // Treat "\xff\x00" as extraneous data.
+ continue
+ }
+ for marker == 0xff {
+ // Section B.1.1.2 says, "Any marker may optionally be preceded by any
+ // number of fill bytes, which are bytes assigned code X'FF'".
+ marker, err = d.readByte()
+ if err != nil {
+ return nil, err
+ }
+ }
+ if marker == eoiMarker { // End Of Image.
+ break
+ }
+ if rst0Marker <= marker && marker <= rst7Marker {
+ // Figures B.2 and B.16 of the specification suggest that restart markers should
+ // only occur between Entropy Coded Segments and not after the final ECS.
+ // However, some encoders may generate incorrect JPEGs with a final restart
+ // marker. That restart marker will be seen here instead of inside the processSOS
+ // method, and is ignored as a harmless error. Restart markers have no extra data,
+ // so we check for this before we read the 16-bit length of the segment.
+ continue
+ }
+
+ // 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.
+ if err = d.readFull(d.tmp[:2]); 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 || marker == sof2Marker: // Start Of Frame.
+ d.progressive = marker == sof2Marker
+ err = d.processSOF(n)
+ if configOnly {
+ return nil, err
+ }
+ 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 app0Marker <= marker && 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, 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, error) {
+ var d decoder
+ if _, err := d.decode(r, true); err != nil {
+ return image.Config{}, err
+ }
+ switch d.nComp {
+ case nGrayComponent:
+ return image.Config{
+ ColorModel: color.GrayModel,
+ Width: d.width,
+ Height: d.height,
+ }, nil
+ case nColorComponent:
+ return image.Config{
+ ColorModel: color.YCbCrModel,
+ Width: d.width,
+ Height: d.height,
+ }, nil
+ }
+ return image.Config{}, FormatError("missing SOF marker")
+}
+
+func init() {
+ image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
+}
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