golang-image/webp/decode.go

// 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 at:
// https://developers.google.com/speed/webp/docs/riff_container
package webp

import (
	"bytes"
	"errors"
	"image"
	"image/color"
	"io"

	"code.google.com/p/go.image/riff"
	"code.google.com/p/go.image/vp8"
	"code.google.com/p/go.image/vp8l"
	"code.google.com/p/go.image/webp/nycbcra"
)

var errInvalidFormat = errors.New("webp: invalid format")

var (
	fccALPH = riff.FourCC{'A', 'L', 'P', 'H'}
	fccVP8  = riff.FourCC{'V', 'P', '8', ' '}
	fccVP8L = riff.FourCC{'V', 'P', '8', 'L'}
	fccVP8X = riff.FourCC{'V', 'P', '8', 'X'}
	fccWEBP = riff.FourCC{'W', 'E', 'B', 'P'}
)

func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
	formType, riffReader, err := riff.NewReader(r)
	if err != nil {
		return nil, image.Config{}, err
	}
	if formType != fccWEBP {
		return nil, image.Config{}, errInvalidFormat
	}

	var (
		alpha          []byte
		alphaStride    int
		wantAlpha      bool
		widthMinusOne  uint32
		heightMinusOne uint32
		buf            [10]byte
	)
	for {
		chunkID, chunkLen, chunkData, err := riffReader.Next()
		if err == io.EOF {
			err = errInvalidFormat
		}
		if err != nil {
			return nil, image.Config{}, err
		}

		switch chunkID {
		case fccALPH:
			if !wantAlpha {
				return nil, image.Config{}, errInvalidFormat
			}
			wantAlpha = false
			// Read the Pre-processing | Filter | Compression byte.
			if _, err := io.ReadFull(chunkData, buf[:1]); err != nil {
				if err == io.EOF {
					err = errInvalidFormat
				}
				return nil, image.Config{}, err
			}
			filter := (buf[0] >> 2) & 0x03
			if filter != 0 {
				return nil, image.Config{}, errors.New(
					"webp: VP8X Alpha filtering != 0 is not implemented")
			}
			compression := buf[0] & 0x03
			if compression != 1 {
				return nil, image.Config{}, errors.New(
					"webp: VP8X Alpha compression != 1 is not implemented")
			}
			// Read the VP8L-compressed alpha values. First, synthesize a 5-byte VP8L header:
			// a 1-byte magic number, a 14-bit widthMinusOne, a 14-bit heightMinusOne,
			// a 1-bit (ignored, zero) alphaIsUsed and a 3-bit (zero) version.
			// TODO(nigeltao): be more efficient than decoding an *image.NRGBA just to
			// extract the green values to a separately allocated []byte. Fixing this
			// will require changes to the vp8l package's API.
			if widthMinusOne > 0x3fff || heightMinusOne > 0x3fff {
				return nil, image.Config{}, errors.New("webp: invalid format")
			}
			buf[0] = 0x2f // VP8L magic number.
			buf[1] = uint8(widthMinusOne)
			buf[2] = uint8(widthMinusOne>>8) | uint8(heightMinusOne<<6)
			buf[3] = uint8(heightMinusOne >> 2)
			buf[4] = uint8(heightMinusOne >> 10)
			alphaImage, err := vp8l.Decode(io.MultiReader(
				bytes.NewReader(buf[:5]),
				chunkData,
			))
			if err != nil {
				return nil, image.Config{}, err
			}
			// The green values of the inner NRGBA image are the alpha values of the
			// outer NYCbCrA image.
			pix := alphaImage.(*image.NRGBA).Pix
			alpha = make([]byte, len(pix)/4)
			for i := range alpha {
				alpha[i] = pix[4*i+1]
			}
			alphaStride = int(widthMinusOne) + 1

		case fccVP8:
			if wantAlpha {
				return nil, image.Config{}, errInvalidFormat
			}
			d := vp8.NewDecoder()
			d.Init(chunkData, int(chunkLen))
			fh, err := d.DecodeFrameHeader()
			if err != nil {
				return nil, image.Config{}, err
			}
			if configOnly {
				return nil, image.Config{
					ColorModel: color.YCbCrModel,
					Width:      fh.Width,
					Height:     fh.Height,
				}, nil
			}
			m, err := d.DecodeFrame()
			if err != nil {
				return nil, image.Config{}, err
			}
			if alpha != nil {
				return &nycbcra.Image{
					YCbCr:   *m,
					A:       alpha,
					AStride: alphaStride,
				}, image.Config{}, nil
			}
			return m, image.Config{}, nil

		case fccVP8L:
			if wantAlpha || alpha != nil {
				return nil, image.Config{}, errInvalidFormat
			}
			if configOnly {
				c, err := vp8l.DecodeConfig(chunkData)
				return nil, c, err
			}
			m, err := vp8l.Decode(chunkData)
			return m, image.Config{}, err

		case fccVP8X:
			if chunkLen != 10 {
				return nil, image.Config{}, errInvalidFormat
			}
			if _, err := io.ReadFull(chunkData, buf[:10]); err != nil {
				return nil, image.Config{}, err
			}
			const (
				animationBit    = 1 << 1
				xmpMetadataBit  = 1 << 2
				exifMetadataBit = 1 << 3
				alphaBit        = 1 << 4
				iccProfileBit   = 1 << 5
			)
			if buf[0] != alphaBit {
				return nil, image.Config{}, errors.New("webp: non-Alpha VP8X is not implemented")
			}
			widthMinusOne = uint32(buf[4]) | uint32(buf[5])<<8 | uint32(buf[6])<<16
			heightMinusOne = uint32(buf[7]) | uint32(buf[8])<<8 | uint32(buf[9])<<16
			if configOnly {
				return nil, image.Config{
					ColorModel: nycbcra.ColorModel,
					Width:      int(widthMinusOne) + 1,
					Height:     int(heightMinusOne) + 1,
				}, nil
			}
			wantAlpha = true
		}
	}
}

// Decode reads a WEBP image from r and returns it as an image.Image.
func Decode(r io.Reader) (image.Image, error) {
	m, _, err := decode(r, false)
	if err != nil {
		return nil, err
	}
	return m, err
}

// DecodeConfig returns the color model and dimensions of a WEBP image without
// decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, error) {
	_, c, err := decode(r, true)
	return c, err
}

func init() {
	image.RegisterFormat("webp", "RIFF????WEBPVP8", Decode, DecodeConfig)
}