/third_party/gofrontend/libgo/go/image/jpeg/reader.go
Go | 802 lines | 596 code | 61 blank | 145 comment | 216 complexity | 01330512674a56234cbf9b3c28d0634a MD5 | raw file
Possible License(s): BSD-3-Clause, MIT
- // 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"
- "image/internal/imageutil"
- "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) }
- var errUnsupportedSubsamplingRatio = UnsupportedError("luma/chroma subsampling ratio")
- // 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
- maxComponents = 4
- )
- const (
- sof0Marker = 0xc0 // Start Of Frame (Baseline).
- sof1Marker = 0xc1 // Start Of Frame (Extended Sequential).
- sof2Marker = 0xc2 // Start Of Frame (Progressive).
- dhtMarker = 0xc4 // Define Huffman Table.
- rst0Marker = 0xd0 // ReSTart (0).
- rst7Marker = 0xd7 // ReSTart (7).
- soiMarker = 0xd8 // Start Of Image.
- eoiMarker = 0xd9 // End Of Image.
- sosMarker = 0xda // Start Of Scan.
- dqtMarker = 0xdb // Define Quantization Table.
- driMarker = 0xdd // Define Restart Interval.
- comMarker = 0xfe // COMment.
- // "APPlication specific" markers aren't part of the JPEG spec per se,
- // but in practice, their use is described at
- // http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html
- app0Marker = 0xe0
- app14Marker = 0xee
- app15Marker = 0xef
- )
- // See http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
- const (
- adobeTransformUnknown = 0
- adobeTransformYCbCr = 1
- adobeTransformYCbCrK = 2
- )
- // 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,
- }
- // Deprecated: 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
- blackPix []byte
- blackStride int
- ri int // Restart Interval.
- nComp int
- progressive bool
- jfif bool
- adobeTransformValid bool
- adobeTransform uint8
- eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
- comp [maxComponents]component
- progCoeffs [maxComponents][]block // Saved state between progressive-mode scans.
- huff [maxTc + 1][maxTh + 1]huffman
- quant [maxTq + 1]block // Quantization tables, in zig-zag order.
- tmp [2 * blockSize]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() {
- 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
- }
- d.bytes.nUnreadable = 0
- x, err = d.readByte()
- if err != nil {
- return 0, err
- }
- d.bytes.nUnreadable = 1
- if x != 0xff {
- return x, nil
- }
- x, err = d.readByte()
- if err != nil {
- 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 {
- if d.nComp != 0 {
- return FormatError("multiple SOF markers")
- }
- switch n {
- case 6 + 3*1: // Grayscale image.
- d.nComp = 1
- case 6 + 3*3: // YCbCr or RGB image.
- d.nComp = 3
- case 6 + 3*4: // YCbCrK or CMYK image.
- d.nComp = 4
- default:
- return UnsupportedError("number of components")
- }
- 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 FormatError("SOF has wrong length")
- }
- for i := 0; i < d.nComp; i++ {
- d.comp[i].c = d.tmp[6+3*i]
- // Section B.2.2 states that "the value of C_i shall be different from
- // the values of C_1 through C_(i-1)".
- for j := 0; j < i; j++ {
- if d.comp[i].c == d.comp[j].c {
- return FormatError("repeated component identifier")
- }
- }
- d.comp[i].tq = d.tmp[8+3*i]
- if d.comp[i].tq > maxTq {
- return FormatError("bad Tq value")
- }
- hv := d.tmp[7+3*i]
- h, v := int(hv>>4), int(hv&0x0f)
- if h < 1 || 4 < h || v < 1 || 4 < v {
- return FormatError("luma/chroma subsampling ratio")
- }
- if h == 3 || v == 3 {
- return errUnsupportedSubsamplingRatio
- }
- switch d.nComp {
- case 1:
- // 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.
- h, v = 1, 1
- case 3:
- // For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
- // 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
- // (h, v) values for the Y component are either (1, 1), (1, 2),
- // (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
- // must be a multiple of the Cb and Cr component's values. We also
- // assume that the two chroma components have the same subsampling
- // ratio.
- switch i {
- case 0: // Y.
- // We have already verified, above, that h and v are both
- // either 1, 2 or 4, so invalid (h, v) combinations are those
- // with v == 4.
- if v == 4 {
- return errUnsupportedSubsamplingRatio
- }
- case 1: // Cb.
- if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
- return errUnsupportedSubsamplingRatio
- }
- case 2: // Cr.
- if d.comp[1].h != h || d.comp[1].v != v {
- return errUnsupportedSubsamplingRatio
- }
- }
- case 4:
- // For 4-component images (either CMYK or YCbCrK), we only support two
- // hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
- // Theoretically, 4-component JPEG images could mix and match hv values
- // but in practice, those two combinations are the only ones in use,
- // and it simplifies the applyBlack code below if we can assume that:
- // - for CMYK, the C and K channels have full samples, and if the M
- // and Y channels subsample, they subsample both horizontally and
- // vertically.
- // - for YCbCrK, the Y and K channels have full samples.
- switch i {
- case 0:
- if hv != 0x11 && hv != 0x22 {
- return errUnsupportedSubsamplingRatio
- }
- case 1, 2:
- if hv != 0x11 {
- return errUnsupportedSubsamplingRatio
- }
- case 3:
- if d.comp[0].h != h || d.comp[0].v != v {
- return errUnsupportedSubsamplingRatio
- }
- }
- }
- d.comp[i].h = h
- d.comp[i].v = v
- }
- return nil
- }
- // Specified in section B.2.4.1.
- func (d *decoder) processDQT(n int) error {
- loop:
- for n > 0 {
- n--
- x, err := d.readByte()
- if err != nil {
- return err
- }
- tq := x & 0x0f
- if tq > maxTq {
- return FormatError("bad Tq value")
- }
- switch x >> 4 {
- default:
- return FormatError("bad Pq value")
- case 0:
- if n < blockSize {
- break loop
- }
- n -= blockSize
- if err := d.readFull(d.tmp[:blockSize]); err != nil {
- return err
- }
- for i := range d.quant[tq] {
- d.quant[tq][i] = int32(d.tmp[i])
- }
- case 1:
- if n < 2*blockSize {
- break loop
- }
- n -= 2 * blockSize
- if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
- return err
- }
- for i := range d.quant[tq] {
- d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*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
- }
- func (d *decoder) processApp0Marker(n int) error {
- if n < 5 {
- return d.ignore(n)
- }
- if err := d.readFull(d.tmp[:5]); err != nil {
- return err
- }
- n -= 5
- d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
- if n > 0 {
- return d.ignore(n)
- }
- return nil
- }
- func (d *decoder) processApp14Marker(n int) error {
- if n < 12 {
- return d.ignore(n)
- }
- if err := d.readFull(d.tmp[:12]); err != nil {
- return err
- }
- n -= 12
- if d.tmp[0] == 'A' && d.tmp[1] == 'd' && d.tmp[2] == 'o' && d.tmp[3] == 'b' && d.tmp[4] == 'e' {
- d.adobeTransformValid = true
- d.adobeTransform = d.tmp[11]
- }
- if n > 0 {
- return d.ignore(n)
- }
- 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 marker {
- case sof0Marker, sof1Marker, sof2Marker:
- d.progressive = marker == sof2Marker
- err = d.processSOF(n)
- if configOnly && d.jfif {
- return nil, err
- }
- case dhtMarker:
- if configOnly {
- err = d.ignore(n)
- } else {
- err = d.processDHT(n)
- }
- case dqtMarker:
- if configOnly {
- err = d.ignore(n)
- } else {
- err = d.processDQT(n)
- }
- case sosMarker:
- if configOnly {
- return nil, nil
- }
- err = d.processSOS(n)
- case driMarker:
- if configOnly {
- err = d.ignore(n)
- } else {
- err = d.processDRI(n)
- }
- case app0Marker:
- err = d.processApp0Marker(n)
- case app14Marker:
- err = d.processApp14Marker(n)
- default:
- if app0Marker <= marker && marker <= app15Marker || marker == comMarker {
- err = d.ignore(n)
- } else if marker < 0xc0 { // See Table B.1 "Marker code assignments".
- err = FormatError("unknown marker")
- } else {
- err = UnsupportedError("unknown marker")
- }
- }
- if err != nil {
- return nil, err
- }
- }
- if d.img1 != nil {
- return d.img1, nil
- }
- if d.img3 != nil {
- if d.blackPix != nil {
- return d.applyBlack()
- } else if d.isRGB() {
- return d.convertToRGB()
- }
- return d.img3, nil
- }
- return nil, FormatError("missing SOS marker")
- }
- // applyBlack combines d.img3 and d.blackPix into a CMYK image. The formula
- // used depends on whether the JPEG image is stored as CMYK or YCbCrK,
- // indicated by the APP14 (Adobe) metadata.
- //
- // Adobe CMYK JPEG images are inverted, where 255 means no ink instead of full
- // ink, so we apply "v = 255 - v" at various points. Note that a double
- // inversion is a no-op, so inversions might be implicit in the code below.
- func (d *decoder) applyBlack() (image.Image, error) {
- if !d.adobeTransformValid {
- return nil, UnsupportedError("unknown color model: 4-component JPEG doesn't have Adobe APP14 metadata")
- }
- // If the 4-component JPEG image isn't explicitly marked as "Unknown (RGB
- // or CMYK)" as per
- // http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
- // we assume that it is YCbCrK. This matches libjpeg's jdapimin.c.
- if d.adobeTransform != adobeTransformUnknown {
- // Convert the YCbCr part of the YCbCrK to RGB, invert the RGB to get
- // CMY, and patch in the original K. The RGB to CMY inversion cancels
- // out the 'Adobe inversion' described in the applyBlack doc comment
- // above, so in practice, only the fourth channel (black) is inverted.
- bounds := d.img3.Bounds()
- img := image.NewRGBA(bounds)
- imageutil.DrawYCbCr(img, bounds, d.img3, bounds.Min)
- for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
- for i, x := iBase+3, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
- img.Pix[i] = 255 - d.blackPix[(y-bounds.Min.Y)*d.blackStride+(x-bounds.Min.X)]
- }
- }
- return &image.CMYK{
- Pix: img.Pix,
- Stride: img.Stride,
- Rect: img.Rect,
- }, nil
- }
- // The first three channels (cyan, magenta, yellow) of the CMYK
- // were decoded into d.img3, but each channel was decoded into a separate
- // []byte slice, and some channels may be subsampled. We interleave the
- // separate channels into an image.CMYK's single []byte slice containing 4
- // contiguous bytes per pixel.
- bounds := d.img3.Bounds()
- img := image.NewCMYK(bounds)
- translations := [4]struct {
- src []byte
- stride int
- }{
- {d.img3.Y, d.img3.YStride},
- {d.img3.Cb, d.img3.CStride},
- {d.img3.Cr, d.img3.CStride},
- {d.blackPix, d.blackStride},
- }
- for t, translation := range translations {
- subsample := d.comp[t].h != d.comp[0].h || d.comp[t].v != d.comp[0].v
- for iBase, y := 0, bounds.Min.Y; y < bounds.Max.Y; iBase, y = iBase+img.Stride, y+1 {
- sy := y - bounds.Min.Y
- if subsample {
- sy /= 2
- }
- for i, x := iBase+t, bounds.Min.X; x < bounds.Max.X; i, x = i+4, x+1 {
- sx := x - bounds.Min.X
- if subsample {
- sx /= 2
- }
- img.Pix[i] = 255 - translation.src[sy*translation.stride+sx]
- }
- }
- }
- return img, nil
- }
- func (d *decoder) isRGB() bool {
- if d.jfif {
- return false
- }
- if d.adobeTransformValid && d.adobeTransform == adobeTransformUnknown {
- // http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
- // says that 0 means Unknown (and in practice RGB) and 1 means YCbCr.
- return true
- }
- return d.comp[0].c == 'R' && d.comp[1].c == 'G' && d.comp[2].c == 'B'
- }
- func (d *decoder) convertToRGB() (image.Image, error) {
- cScale := d.comp[0].h / d.comp[1].h
- bounds := d.img3.Bounds()
- img := image.NewRGBA(bounds)
- for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
- po := img.PixOffset(bounds.Min.X, y)
- yo := d.img3.YOffset(bounds.Min.X, y)
- co := d.img3.COffset(bounds.Min.X, y)
- for i, iMax := 0, bounds.Max.X-bounds.Min.X; i < iMax; i++ {
- img.Pix[po+4*i+0] = d.img3.Y[yo+i]
- img.Pix[po+4*i+1] = d.img3.Cb[co+i/cScale]
- img.Pix[po+4*i+2] = d.img3.Cr[co+i/cScale]
- img.Pix[po+4*i+3] = 255
- }
- }
- return img, nil
- }
- // 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 1:
- return image.Config{
- ColorModel: color.GrayModel,
- Width: d.width,
- Height: d.height,
- }, nil
- case 3:
- cm := color.YCbCrModel
- if d.isRGB() {
- cm = color.RGBAModel
- }
- return image.Config{
- ColorModel: cm,
- Width: d.width,
- Height: d.height,
- }, nil
- case 4:
- return image.Config{
- ColorModel: color.CMYKModel,
- Width: d.width,
- Height: d.height,
- }, nil
- }
- return image.Config{}, FormatError("missing SOF marker")
- }
- func init() {
- image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
- }