815 lines
23 KiB
Go
815 lines
23 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package jpeg implements a JPEG image decoder and encoder.
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//
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// JPEG is defined in ITU-T T.81: https://www.w3.org/Graphics/JPEG/itu-t81.pdf.
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package jpeg
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import (
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"image"
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"image/color"
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"image/internal/imageutil"
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"io"
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)
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// A FormatError reports that the input is not a valid JPEG.
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type FormatError string
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func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }
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// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
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type UnsupportedError string
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func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }
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var errUnsupportedSubsamplingRatio = UnsupportedError("luma/chroma subsampling ratio")
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// Component specification, specified in section B.2.2.
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type component struct {
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h int // Horizontal sampling factor.
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v int // Vertical sampling factor.
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c uint8 // Component identifier.
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tq uint8 // Quantization table destination selector.
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}
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const (
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dcTable = 0
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acTable = 1
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maxTc = 1
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maxTh = 3
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maxTq = 3
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maxComponents = 4
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)
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const (
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sof0Marker = 0xc0 // Start Of Frame (Baseline Sequential).
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sof1Marker = 0xc1 // Start Of Frame (Extended Sequential).
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sof2Marker = 0xc2 // Start Of Frame (Progressive).
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dhtMarker = 0xc4 // Define Huffman Table.
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rst0Marker = 0xd0 // ReSTart (0).
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rst7Marker = 0xd7 // ReSTart (7).
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soiMarker = 0xd8 // Start Of Image.
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eoiMarker = 0xd9 // End Of Image.
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sosMarker = 0xda // Start Of Scan.
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dqtMarker = 0xdb // Define Quantization Table.
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driMarker = 0xdd // Define Restart Interval.
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comMarker = 0xfe // COMment.
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// "APPlication specific" markers aren't part of the JPEG spec per se,
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// but in practice, their use is described at
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// https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html
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app0Marker = 0xe0
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app14Marker = 0xee
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app15Marker = 0xef
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)
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// See https://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html#Adobe
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const (
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adobeTransformUnknown = 0
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adobeTransformYCbCr = 1
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adobeTransformYCbCrK = 2
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)
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// unzig maps from the zig-zag ordering to the natural ordering. For example,
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// unzig[3] is the column and row of the fourth element in zig-zag order. The
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// value is 16, which means first column (16%8 == 0) and third row (16/8 == 2).
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var unzig = [blockSize]int{
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0, 1, 8, 16, 9, 2, 3, 10,
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17, 24, 32, 25, 18, 11, 4, 5,
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12, 19, 26, 33, 40, 48, 41, 34,
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27, 20, 13, 6, 7, 14, 21, 28,
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35, 42, 49, 56, 57, 50, 43, 36,
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29, 22, 15, 23, 30, 37, 44, 51,
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58, 59, 52, 45, 38, 31, 39, 46,
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53, 60, 61, 54, 47, 55, 62, 63,
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}
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// Deprecated: Reader is not used by the image/jpeg package and should
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// not be used by others. It is kept for compatibility.
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type Reader interface {
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io.ByteReader
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io.Reader
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}
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// bits holds the unprocessed bits that have been taken from the byte-stream.
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// The n least significant bits of a form the unread bits, to be read in MSB to
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// LSB order.
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type bits struct {
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a uint32 // accumulator.
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m uint32 // mask. m==1<<(n-1) when n>0, with m==0 when n==0.
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n int32 // the number of unread bits in a.
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}
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type decoder struct {
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r io.Reader
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bits bits
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// bytes is a byte buffer, similar to a bufio.Reader, except that it
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// has to be able to unread more than 1 byte, due to byte stuffing.
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// Byte stuffing is specified in section F.1.2.3.
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bytes struct {
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// buf[i:j] are the buffered bytes read from the underlying
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// io.Reader that haven't yet been passed further on.
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buf [4096]byte
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i, j int
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// nUnreadable is the number of bytes to back up i after
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// overshooting. It can be 0, 1 or 2.
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nUnreadable int
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}
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width, height int
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img1 *image.Gray
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img3 *image.YCbCr
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blackPix []byte
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blackStride int
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ri int // Restart Interval.
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nComp int
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// As per section 4.5, there are four modes of operation (selected by the
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// SOF? markers): sequential DCT, progressive DCT, lossless and
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// hierarchical, although this implementation does not support the latter
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// two non-DCT modes. Sequential DCT is further split into baseline and
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// extended, as per section 4.11.
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baseline bool
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progressive bool
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jfif bool
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adobeTransformValid bool
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adobeTransform uint8
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eobRun uint16 // End-of-Band run, specified in section G.1.2.2.
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comp [maxComponents]component
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progCoeffs [maxComponents][]block // Saved state between progressive-mode scans.
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huff [maxTc + 1][maxTh + 1]huffman
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quant [maxTq + 1]block // Quantization tables, in zig-zag order.
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tmp [2 * blockSize]byte
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}
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// fill fills up the d.bytes.buf buffer from the underlying io.Reader. It
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// should only be called when there are no unread bytes in d.bytes.
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func (d *decoder) fill() error {
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if d.bytes.i != d.bytes.j {
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panic("jpeg: fill called when unread bytes exist")
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}
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// Move the last 2 bytes to the start of the buffer, in case we need
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// to call unreadByteStuffedByte.
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if d.bytes.j > 2 {
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d.bytes.buf[0] = d.bytes.buf[d.bytes.j-2]
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d.bytes.buf[1] = d.bytes.buf[d.bytes.j-1]
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d.bytes.i, d.bytes.j = 2, 2
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}
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// Fill in the rest of the buffer.
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n, err := d.r.Read(d.bytes.buf[d.bytes.j:])
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d.bytes.j += n
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if n > 0 {
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err = nil
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}
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return err
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}
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// unreadByteStuffedByte undoes the most recent readByteStuffedByte call,
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// giving a byte of data back from d.bits to d.bytes. The Huffman look-up table
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// requires at least 8 bits for look-up, which means that Huffman decoding can
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// sometimes overshoot and read one or two too many bytes. Two-byte overshoot
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// can happen when expecting to read a 0xff 0x00 byte-stuffed byte.
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func (d *decoder) unreadByteStuffedByte() {
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d.bytes.i -= d.bytes.nUnreadable
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d.bytes.nUnreadable = 0
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if d.bits.n >= 8 {
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d.bits.a >>= 8
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d.bits.n -= 8
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d.bits.m >>= 8
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}
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}
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// readByte returns the next byte, whether buffered or not buffered. It does
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// not care about byte stuffing.
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func (d *decoder) readByte() (x byte, err error) {
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for d.bytes.i == d.bytes.j {
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if err = d.fill(); err != nil {
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return 0, err
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}
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}
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x = d.bytes.buf[d.bytes.i]
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d.bytes.i++
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d.bytes.nUnreadable = 0
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return x, nil
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}
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// errMissingFF00 means that readByteStuffedByte encountered an 0xff byte (a
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// marker byte) that wasn't the expected byte-stuffed sequence 0xff, 0x00.
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var errMissingFF00 = FormatError("missing 0xff00 sequence")
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// readByteStuffedByte is like readByte but is for byte-stuffed Huffman data.
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func (d *decoder) readByteStuffedByte() (x byte, err error) {
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// Take the fast path if d.bytes.buf contains at least two bytes.
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if d.bytes.i+2 <= d.bytes.j {
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x = d.bytes.buf[d.bytes.i]
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d.bytes.i++
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d.bytes.nUnreadable = 1
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if x != 0xff {
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return x, err
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}
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if d.bytes.buf[d.bytes.i] != 0x00 {
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return 0, errMissingFF00
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}
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d.bytes.i++
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d.bytes.nUnreadable = 2
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return 0xff, nil
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}
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d.bytes.nUnreadable = 0
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x, err = d.readByte()
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if err != nil {
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return 0, err
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}
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d.bytes.nUnreadable = 1
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if x != 0xff {
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return x, nil
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}
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x, err = d.readByte()
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if err != nil {
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return 0, err
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}
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d.bytes.nUnreadable = 2
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if x != 0x00 {
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return 0, errMissingFF00
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}
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return 0xff, nil
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}
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// readFull reads exactly len(p) bytes into p. It does not care about byte
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// stuffing.
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func (d *decoder) readFull(p []byte) error {
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// Unread the overshot bytes, if any.
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if d.bytes.nUnreadable != 0 {
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if d.bits.n >= 8 {
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d.unreadByteStuffedByte()
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}
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d.bytes.nUnreadable = 0
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}
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for {
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n := copy(p, d.bytes.buf[d.bytes.i:d.bytes.j])
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p = p[n:]
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d.bytes.i += n
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if len(p) == 0 {
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break
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}
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if err := d.fill(); err != nil {
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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return err
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}
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}
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return nil
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}
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// ignore ignores the next n bytes.
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func (d *decoder) ignore(n int) error {
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// Unread the overshot bytes, if any.
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if d.bytes.nUnreadable != 0 {
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if d.bits.n >= 8 {
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d.unreadByteStuffedByte()
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}
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d.bytes.nUnreadable = 0
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}
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for {
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m := d.bytes.j - d.bytes.i
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if m > n {
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m = n
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}
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d.bytes.i += m
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n -= m
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if n == 0 {
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break
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}
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if err := d.fill(); err != nil {
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if err == io.EOF {
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err = io.ErrUnexpectedEOF
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}
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return err
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}
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}
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return nil
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}
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// Specified in section B.2.2.
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func (d *decoder) processSOF(n int) error {
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if d.nComp != 0 {
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return FormatError("multiple SOF markers")
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}
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switch n {
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case 6 + 3*1: // Grayscale image.
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d.nComp = 1
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case 6 + 3*3: // YCbCr or RGB image.
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d.nComp = 3
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case 6 + 3*4: // YCbCrK or CMYK image.
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d.nComp = 4
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default:
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return UnsupportedError("number of components")
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}
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if err := d.readFull(d.tmp[:n]); err != nil {
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return err
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}
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// We only support 8-bit precision.
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if d.tmp[0] != 8 {
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return UnsupportedError("precision")
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}
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d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
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d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
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if int(d.tmp[5]) != d.nComp {
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return FormatError("SOF has wrong length")
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}
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for i := 0; i < d.nComp; i++ {
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d.comp[i].c = d.tmp[6+3*i]
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// Section B.2.2 states that "the value of C_i shall be different from
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// the values of C_1 through C_(i-1)".
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for j := 0; j < i; j++ {
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if d.comp[i].c == d.comp[j].c {
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return FormatError("repeated component identifier")
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}
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}
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d.comp[i].tq = d.tmp[8+3*i]
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if d.comp[i].tq > maxTq {
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return FormatError("bad Tq value")
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}
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hv := d.tmp[7+3*i]
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h, v := int(hv>>4), int(hv&0x0f)
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if h < 1 || 4 < h || v < 1 || 4 < v {
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return FormatError("luma/chroma subsampling ratio")
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}
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if h == 3 || v == 3 {
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return errUnsupportedSubsamplingRatio
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}
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switch d.nComp {
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case 1:
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// If a JPEG image has only one component, section A.2 says "this data
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// is non-interleaved by definition" and section A.2.2 says "[in this
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// case...] the order of data units within a scan shall be left-to-right
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// and top-to-bottom... regardless of the values of H_1 and V_1". Section
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// 4.8.2 also says "[for non-interleaved data], the MCU is defined to be
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// one data unit". Similarly, section A.1.1 explains that it is the ratio
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// of H_i to max_j(H_j) that matters, and similarly for V. For grayscale
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// images, H_1 is the maximum H_j for all components j, so that ratio is
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// always 1. The component's (h, v) is effectively always (1, 1): even if
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// the nominal (h, v) is (2, 1), a 20x5 image is encoded in three 8x8
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// MCUs, not two 16x8 MCUs.
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h, v = 1, 1
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case 3:
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// For YCbCr images, we only support 4:4:4, 4:4:0, 4:2:2, 4:2:0,
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// 4:1:1 or 4:1:0 chroma subsampling ratios. This implies that the
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// (h, v) values for the Y component are either (1, 1), (1, 2),
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// (2, 1), (2, 2), (4, 1) or (4, 2), and the Y component's values
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// must be a multiple of the Cb and Cr component's values. We also
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// assume that the two chroma components have the same subsampling
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// ratio.
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switch i {
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case 0: // Y.
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// We have already verified, above, that h and v are both
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// either 1, 2 or 4, so invalid (h, v) combinations are those
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// with v == 4.
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if v == 4 {
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return errUnsupportedSubsamplingRatio
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}
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case 1: // Cb.
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if d.comp[0].h%h != 0 || d.comp[0].v%v != 0 {
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return errUnsupportedSubsamplingRatio
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}
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case 2: // Cr.
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if d.comp[1].h != h || d.comp[1].v != v {
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return errUnsupportedSubsamplingRatio
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}
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}
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case 4:
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// For 4-component images (either CMYK or YCbCrK), we only support two
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// hv vectors: [0x11 0x11 0x11 0x11] and [0x22 0x11 0x11 0x22].
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// Theoretically, 4-component JPEG images could mix and match hv values
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// but in practice, those two combinations are the only ones in use,
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// and it simplifies the applyBlack code below if we can assume that:
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// - for CMYK, the C and K channels have full samples, and if the M
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// and Y channels subsample, they subsample both horizontally and
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// vertically.
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// - for YCbCrK, the Y and K channels have full samples.
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switch i {
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case 0:
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if hv != 0x11 && hv != 0x22 {
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return errUnsupportedSubsamplingRatio
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}
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case 1, 2:
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if hv != 0x11 {
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return errUnsupportedSubsamplingRatio
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}
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case 3:
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if d.comp[0].h != h || d.comp[0].v != v {
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return errUnsupportedSubsamplingRatio
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}
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}
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}
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d.comp[i].h = h
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d.comp[i].v = v
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}
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return nil
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}
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// Specified in section B.2.4.1.
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func (d *decoder) processDQT(n int) error {
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loop:
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for n > 0 {
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n--
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x, err := d.readByte()
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if err != nil {
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return err
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}
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tq := x & 0x0f
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if tq > maxTq {
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return FormatError("bad Tq value")
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}
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switch x >> 4 {
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default:
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return FormatError("bad Pq value")
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case 0:
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if n < blockSize {
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break loop
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}
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n -= blockSize
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if err := d.readFull(d.tmp[:blockSize]); err != nil {
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return err
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}
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for i := range d.quant[tq] {
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d.quant[tq][i] = int32(d.tmp[i])
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}
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case 1:
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if n < 2*blockSize {
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break loop
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}
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n -= 2 * blockSize
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if err := d.readFull(d.tmp[:2*blockSize]); err != nil {
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return err
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}
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for i := range d.quant[tq] {
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d.quant[tq][i] = int32(d.tmp[2*i])<<8 | int32(d.tmp[2*i+1])
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}
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}
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}
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if n != 0 {
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return FormatError("DQT has wrong length")
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}
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return nil
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}
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// Specified in section B.2.4.4.
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func (d *decoder) processDRI(n int) error {
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if n != 2 {
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return FormatError("DRI has wrong length")
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}
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if err := d.readFull(d.tmp[:2]); err != nil {
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return err
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}
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d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
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return nil
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}
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func (d *decoder) processApp0Marker(n int) error {
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if n < 5 {
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return d.ignore(n)
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}
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if err := d.readFull(d.tmp[:5]); err != nil {
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return err
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}
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n -= 5
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d.jfif = d.tmp[0] == 'J' && d.tmp[1] == 'F' && d.tmp[2] == 'I' && d.tmp[3] == 'F' && d.tmp[4] == '\x00'
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if n > 0 {
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return d.ignore(n)
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}
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return nil
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}
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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.baseline = marker == sof0Marker
|
|
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.progressive {
|
|
if err := d.reconstructProgressiveImage(); 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
|
|
// https://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 {
|
|
// https://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)
|
|
}
|