示例#1
0
func readAlpha(chunkData io.Reader, widthMinusOne, heightMinusOne uint32, compression byte) (
	alpha []byte, alphaStride int, err error) {

	switch compression {
	case 0:
		w := int(widthMinusOne) + 1
		h := int(heightMinusOne) + 1
		alpha = make([]byte, w*h)
		if _, err := io.ReadFull(chunkData, alpha); err != nil {
			return nil, 0, err
		}
		return alpha, w, nil

	case 1:
		// 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, 0, errors.New("webp: invalid format")
		}
		alphaImage, err := vp8l.Decode(io.MultiReader(
			bytes.NewReader([]byte{
				0x2f, // VP8L magic number.
				uint8(widthMinusOne),
				uint8(widthMinusOne>>8) | uint8(heightMinusOne<<6),
				uint8(heightMinusOne >> 2),
				uint8(heightMinusOne >> 10),
			}),
			chunkData,
		))
		if err != nil {
			return nil, 0, 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]
		}
		return alpha, int(widthMinusOne) + 1, nil
	}
	return nil, 0, errInvalidFormat
}
示例#2
0
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
			}
			alpha, alphaStride, err = readAlpha(chunkData, widthMinusOne, heightMinusOne, buf[0]&0x03)
			if err != nil {
				return nil, image.Config{}, err
			}
			unfilterAlpha(alpha, alphaStride, (buf[0]>>2)&0x03)

		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
		}
	}
}