go.image/webp: implement alpha filtering and uncompressed alpha.
This passes the four new alpha_*.webp conformance tests referred to in https://golang.org/cl/162850043/#msg7 LGTM=pascal.massimino, r R=r, pascal.massimino CC=golang-codereviews https://golang.org/cl/163630043
This commit is contained in:
Nigel Tao
parent
013424077b
commit
c087821dc6
141
webp/decode.go
141
webp/decode.go
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@ -70,45 +70,11 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
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}
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return nil, image.Config{}, err
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}
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filter := (buf[0] >> 2) & 0x03
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if filter != 0 {
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return nil, image.Config{}, errors.New(
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"webp: VP8X Alpha filtering != 0 is not implemented")
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}
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compression := buf[0] & 0x03
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if compression != 1 {
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return nil, image.Config{}, errors.New(
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"webp: VP8X Alpha compression != 1 is not implemented")
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}
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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, image.Config{}, errors.New("webp: invalid format")
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}
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buf[0] = 0x2f // VP8L magic number.
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buf[1] = uint8(widthMinusOne)
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buf[2] = uint8(widthMinusOne>>8) | uint8(heightMinusOne<<6)
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buf[3] = uint8(heightMinusOne >> 2)
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buf[4] = uint8(heightMinusOne >> 10)
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alphaImage, err := vp8l.Decode(io.MultiReader(
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bytes.NewReader(buf[:5]),
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chunkData,
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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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// 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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alphaStride = int(widthMinusOne) + 1
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unfilterAlpha(alpha, alphaStride, (buf[0]>>2)&0x03)
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case fccVP8:
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if wantAlpha {
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@ -182,6 +148,109 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
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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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