golang-image/webp/decode.go

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// Copyright 2011 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 webp implements a decoder for WEBP images.
//
// WEBP is defined in the VP8 specification at:
// http://datatracker.ietf.org/doc/rfc6386/
package webp
import (
"errors"
"image"
"image/color"
"io"
"code.google.com/p/go.image/vp8"
)
func decode(r io.Reader) (d *vp8.Decoder, fh vp8.FrameHeader, err error) {
var b [20]byte
if _, err = io.ReadFull(r, b[:]); err != nil {
return
}
if string(b[0:4]) != "RIFF" || string(b[8:16]) != "WEBPVP8 " {
err = errors.New("webp: invalid format")
return
}
riffLen := uint32(b[4]) | uint32(b[5])<<8 | uint32(b[6])<<16 | uint32(b[7])<<24
dataLen := uint32(b[16]) | uint32(b[17])<<8 | uint32(b[18])<<16 | uint32(b[19])<<24
if riffLen < dataLen+12 {
err = errors.New("webp: invalid format")
return
}
if dataLen >= 1<<31 {
err = errors.New("webp: invalid format")
return
}
d = vp8.NewDecoder()
d.Init(r, int(dataLen))
fh, err = d.DecodeFrameHeader()
if err != nil {
d, fh = nil, vp8.FrameHeader{}
return
}
return
}
// Decode reads a WEBP image from r and returns it as an image.Image.
func Decode(r io.Reader) (image.Image, error) {
d, _, err := decode(r)
if err != nil {
return nil, err
}
return d.DecodeFrame()
}
// DecodeConfig returns the color model and dimensions of a WEBP image without
// decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, error) {
_, fh, err := decode(r)
if err != nil {
return image.Config{}, err
}
c := image.Config{
ColorModel: color.YCbCrModel,
Width: fh.Width,
Height: fh.Height,
}
return c, nil
}
func init() {
image.RegisterFormat("webp", "RIFF????WEBPVP8 ", Decode, DecodeConfig)
}