272 lines
6.9 KiB
Go
272 lines
6.9 KiB
Go
// Copyright 2011 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 webp
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import (
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"bytes"
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"errors"
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"image"
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"image/color"
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"io"
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"golang.org/x/image/riff"
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"golang.org/x/image/vp8"
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"golang.org/x/image/vp8l"
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)
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var errInvalidFormat = errors.New("webp: invalid format")
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var (
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fccALPH = riff.FourCC{'A', 'L', 'P', 'H'}
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fccVP8 = riff.FourCC{'V', 'P', '8', ' '}
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fccVP8L = riff.FourCC{'V', 'P', '8', 'L'}
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fccVP8X = riff.FourCC{'V', 'P', '8', 'X'}
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fccWEBP = riff.FourCC{'W', 'E', 'B', 'P'}
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)
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func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
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formType, riffReader, err := riff.NewReader(r)
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if err != nil {
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return nil, image.Config{}, err
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}
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if formType != fccWEBP {
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return nil, image.Config{}, errInvalidFormat
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}
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var (
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alpha []byte
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alphaStride int
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wantAlpha bool
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widthMinusOne uint32
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heightMinusOne uint32
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buf [10]byte
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)
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for {
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chunkID, chunkLen, chunkData, err := riffReader.Next()
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if err == io.EOF {
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err = errInvalidFormat
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}
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if err != nil {
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return nil, image.Config{}, err
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}
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switch chunkID {
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case fccALPH:
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if !wantAlpha {
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return nil, image.Config{}, errInvalidFormat
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}
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wantAlpha = false
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// Read the Pre-processing | Filter | Compression byte.
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if _, err := io.ReadFull(chunkData, buf[:1]); err != nil {
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if err == io.EOF {
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err = errInvalidFormat
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}
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return nil, image.Config{}, err
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}
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alpha, alphaStride, err = readAlpha(chunkData, widthMinusOne, heightMinusOne, buf[0]&0x03)
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if err != nil {
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return nil, image.Config{}, err
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}
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unfilterAlpha(alpha, alphaStride, (buf[0]>>2)&0x03)
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case fccVP8:
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if wantAlpha || int32(chunkLen) < 0 {
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return nil, image.Config{}, errInvalidFormat
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}
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d := vp8.NewDecoder()
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d.Init(chunkData, int(chunkLen))
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fh, err := d.DecodeFrameHeader()
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if err != nil {
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return nil, image.Config{}, err
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}
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if configOnly {
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return nil, image.Config{
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ColorModel: color.YCbCrModel,
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Width: fh.Width,
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Height: fh.Height,
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}, nil
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}
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m, err := d.DecodeFrame()
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if err != nil {
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return nil, image.Config{}, err
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}
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if alpha != nil {
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return &image.NYCbCrA{
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YCbCr: *m,
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A: alpha,
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AStride: alphaStride,
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}, image.Config{}, nil
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}
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return m, image.Config{}, nil
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case fccVP8L:
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if wantAlpha || alpha != nil {
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return nil, image.Config{}, errInvalidFormat
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}
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if configOnly {
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c, err := vp8l.DecodeConfig(chunkData)
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return nil, c, err
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}
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m, err := vp8l.Decode(chunkData)
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return m, image.Config{}, err
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case fccVP8X:
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if chunkLen != 10 {
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return nil, image.Config{}, errInvalidFormat
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}
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if _, err := io.ReadFull(chunkData, buf[:10]); err != nil {
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return nil, image.Config{}, err
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}
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const (
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animationBit = 1 << 1
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xmpMetadataBit = 1 << 2
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exifMetadataBit = 1 << 3
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alphaBit = 1 << 4
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iccProfileBit = 1 << 5
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)
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wantAlpha = (buf[0] & alphaBit) != 0
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widthMinusOne = uint32(buf[4]) | uint32(buf[5])<<8 | uint32(buf[6])<<16
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heightMinusOne = uint32(buf[7]) | uint32(buf[8])<<8 | uint32(buf[9])<<16
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if configOnly {
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if wantAlpha {
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return nil, image.Config{
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ColorModel: color.NYCbCrAModel,
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Width: int(widthMinusOne) + 1,
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Height: int(heightMinusOne) + 1,
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}, nil
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}
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return nil, image.Config{
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ColorModel: color.YCbCrModel,
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Width: int(widthMinusOne) + 1,
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Height: int(heightMinusOne) + 1,
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}, nil
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}
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}
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}
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}
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func readAlpha(chunkData io.Reader, widthMinusOne, heightMinusOne uint32, compression byte) (
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alpha []byte, alphaStride int, err error) {
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switch compression {
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case 0:
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w := int(widthMinusOne) + 1
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h := int(heightMinusOne) + 1
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alpha = make([]byte, w*h)
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if _, err := io.ReadFull(chunkData, alpha); err != nil {
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return nil, 0, err
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}
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return alpha, w, nil
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case 1:
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// Read the VP8L-compressed alpha values. First, synthesize a 5-byte VP8L header:
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// a 1-byte magic number, a 14-bit widthMinusOne, a 14-bit heightMinusOne,
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// a 1-bit (ignored, zero) alphaIsUsed and a 3-bit (zero) version.
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// TODO(nigeltao): be more efficient than decoding an *image.NRGBA just to
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// extract the green values to a separately allocated []byte. Fixing this
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// will require changes to the vp8l package's API.
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if widthMinusOne > 0x3fff || heightMinusOne > 0x3fff {
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return nil, 0, errors.New("webp: invalid format")
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}
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alphaImage, err := vp8l.Decode(io.MultiReader(
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bytes.NewReader([]byte{
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0x2f, // VP8L magic number.
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uint8(widthMinusOne),
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uint8(widthMinusOne>>8) | uint8(heightMinusOne<<6),
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uint8(heightMinusOne >> 2),
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uint8(heightMinusOne >> 10),
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}),
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chunkData,
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))
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if err != nil {
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return nil, 0, err
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}
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// The green values of the inner NRGBA image are the alpha values of the
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// outer NYCbCrA image.
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pix := alphaImage.(*image.NRGBA).Pix
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alpha = make([]byte, len(pix)/4)
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for i := range alpha {
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alpha[i] = pix[4*i+1]
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}
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return alpha, int(widthMinusOne) + 1, nil
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}
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return nil, 0, errInvalidFormat
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}
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func unfilterAlpha(alpha []byte, alphaStride int, filter byte) {
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if len(alpha) == 0 || alphaStride == 0 {
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return
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}
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switch filter {
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case 1: // Horizontal filter.
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for i := 1; i < alphaStride; i++ {
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alpha[i] += alpha[i-1]
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}
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for i := alphaStride; i < len(alpha); i += alphaStride {
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// The first column is equivalent to the vertical filter.
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alpha[i] += alpha[i-alphaStride]
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for j := 1; j < alphaStride; j++ {
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alpha[i+j] += alpha[i+j-1]
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}
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}
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case 2: // Vertical filter.
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// The first row is equivalent to the horizontal filter.
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for i := 1; i < alphaStride; i++ {
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alpha[i] += alpha[i-1]
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}
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for i := alphaStride; i < len(alpha); i++ {
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alpha[i] += alpha[i-alphaStride]
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}
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case 3: // Gradient filter.
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// The first row is equivalent to the horizontal filter.
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for i := 1; i < alphaStride; i++ {
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alpha[i] += alpha[i-1]
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}
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for i := alphaStride; i < len(alpha); i += alphaStride {
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// The first column is equivalent to the vertical filter.
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alpha[i] += alpha[i-alphaStride]
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// The interior is predicted on the three top/left pixels.
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for j := 1; j < alphaStride; j++ {
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c := int(alpha[i+j-alphaStride-1])
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b := int(alpha[i+j-alphaStride])
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a := int(alpha[i+j-1])
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x := a + b - c
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if x < 0 {
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x = 0
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} else if x > 255 {
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x = 255
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}
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alpha[i+j] += uint8(x)
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}
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}
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}
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}
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// Decode reads a WEBP image from r and returns it as an image.Image.
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func Decode(r io.Reader) (image.Image, error) {
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m, _, err := decode(r, false)
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if err != nil {
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return nil, err
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}
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return m, err
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}
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// DecodeConfig returns the color model and dimensions of a WEBP image without
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// decoding the entire image.
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func DecodeConfig(r io.Reader) (image.Config, error) {
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_, c, err := decode(r, true)
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return c, err
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}
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func init() {
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image.RegisterFormat("webp", "RIFF????WEBPVP8", Decode, DecodeConfig)
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}
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