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Diffstat (limited to 'src/pkg/image/jpeg/reader.go')
-rw-r--r--src/pkg/image/jpeg/reader.go524
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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 (
- "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
- 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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