// Copyright 2014 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 vp8l implements a decoder for the VP8L lossless image format. // // The VP8L specification is at: // https://developers.google.com/speed/webp/docs/riff_container package vp8l // import "golang.org/x/image/vp8l" import ( "bufio" "errors" "image" "image/color" "io" ) var ( errInvalidCodeLengths = errors.New("vp8l: invalid code lengths") errInvalidHuffmanTree = errors.New("vp8l: invalid Huffman tree") ) // colorCacheMultiplier is the multiplier used for the color cache hash // function, specified in section 4.2.3. const colorCacheMultiplier = 0x1e35a7bd // distanceMapTable is the look-up table for distanceMap. var distanceMapTable = [120]uint8{ 0x18, 0x07, 0x17, 0x19, 0x28, 0x06, 0x27, 0x29, 0x16, 0x1a, 0x26, 0x2a, 0x38, 0x05, 0x37, 0x39, 0x15, 0x1b, 0x36, 0x3a, 0x25, 0x2b, 0x48, 0x04, 0x47, 0x49, 0x14, 0x1c, 0x35, 0x3b, 0x46, 0x4a, 0x24, 0x2c, 0x58, 0x45, 0x4b, 0x34, 0x3c, 0x03, 0x57, 0x59, 0x13, 0x1d, 0x56, 0x5a, 0x23, 0x2d, 0x44, 0x4c, 0x55, 0x5b, 0x33, 0x3d, 0x68, 0x02, 0x67, 0x69, 0x12, 0x1e, 0x66, 0x6a, 0x22, 0x2e, 0x54, 0x5c, 0x43, 0x4d, 0x65, 0x6b, 0x32, 0x3e, 0x78, 0x01, 0x77, 0x79, 0x53, 0x5d, 0x11, 0x1f, 0x64, 0x6c, 0x42, 0x4e, 0x76, 0x7a, 0x21, 0x2f, 0x75, 0x7b, 0x31, 0x3f, 0x63, 0x6d, 0x52, 0x5e, 0x00, 0x74, 0x7c, 0x41, 0x4f, 0x10, 0x20, 0x62, 0x6e, 0x30, 0x73, 0x7d, 0x51, 0x5f, 0x40, 0x72, 0x7e, 0x61, 0x6f, 0x50, 0x71, 0x7f, 0x60, 0x70, } // distanceMap maps a LZ77 backwards reference distance to a two-dimensional // pixel offset, specified in section 4.2.2. func distanceMap(w int32, code uint32) int32 { if int32(code) > int32(len(distanceMapTable)) { return int32(code) - int32(len(distanceMapTable)) } distCode := int32(distanceMapTable[code-1]) yOffset := distCode >> 4 xOffset := 8 - distCode&0xf if d := yOffset*w + xOffset; d >= 1 { return d } return 1 } // decoder holds the bit-stream for a VP8L image. type decoder struct { r io.ByteReader bits uint32 nBits uint32 } // read reads the next n bits from the decoder's bit-stream. func (d *decoder) read(n uint32) (uint32, error) { for d.nBits < n { c, err := d.r.ReadByte() if err != nil { if err == io.EOF { err = io.ErrUnexpectedEOF } return 0, err } d.bits |= uint32(c) << d.nBits d.nBits += 8 } u := d.bits & (1<>= n d.nBits -= n return u, nil } // decodeTransform decodes the next transform and the width of the image after // transformation (or equivalently, before inverse transformation), specified // in section 3. func (d *decoder) decodeTransform(w int32, h int32) (t transform, newWidth int32, err error) { t.oldWidth = w t.transformType, err = d.read(2) if err != nil { return transform{}, 0, err } switch t.transformType { case transformTypePredictor, transformTypeCrossColor: t.bits, err = d.read(3) if err != nil { return transform{}, 0, err } t.bits += 2 t.pix, err = d.decodePix(nTiles(w, t.bits), nTiles(h, t.bits), 0, false) if err != nil { return transform{}, 0, err } case transformTypeSubtractGreen: // No-op. case transformTypeColorIndexing: nColors, err := d.read(8) if err != nil { return transform{}, 0, err } nColors++ t.bits = 0 switch { case nColors <= 2: t.bits = 3 case nColors <= 4: t.bits = 2 case nColors <= 16: t.bits = 1 } w = nTiles(w, t.bits) pix, err := d.decodePix(int32(nColors), 1, 4*256, false) if err != nil { return transform{}, 0, err } for p := 4; p < len(pix); p += 4 { pix[p+0] += pix[p-4] pix[p+1] += pix[p-3] pix[p+2] += pix[p-2] pix[p+3] += pix[p-1] } // The spec says that "if the index is equal or larger than color_table_size, // the argb color value should be set to 0x00000000 (transparent black)." // We re-slice up to 256 4-byte pixels. t.pix = pix[:4*256] } return t, w, nil } // repeatsCodeLength is the minimum code length for repeated codes. const repeatsCodeLength = 16 // These magic numbers are specified at the end of section 5.2.2. // The 3-length arrays apply to code lengths >= repeatsCodeLength. var ( codeLengthCodeOrder = [19]uint8{ 17, 18, 0, 1, 2, 3, 4, 5, 16, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, } repeatBits = [3]uint8{2, 3, 7} repeatOffsets = [3]uint8{3, 3, 11} ) // decodeCodeLengths decodes a Huffman tree's code lengths which are themselves // encoded via a Huffman tree, specified in section 5.2.2. func (d *decoder) decodeCodeLengths(dst []uint32, codeLengthCodeLengths []uint32) error { h := hTree{} if err := h.build(codeLengthCodeLengths); err != nil { return err } maxSymbol := len(dst) useLength, err := d.read(1) if err != nil { return err } if useLength != 0 { n, err := d.read(3) if err != nil { return err } n = 2 + 2*n ms, err := d.read(n) if err != nil { return err } maxSymbol = int(ms) + 2 if maxSymbol > len(dst) { return errInvalidCodeLengths } } // The spec says that "if code 16 [meaning repeat] is used before // a non-zero value has been emitted, a value of 8 is repeated." prevCodeLength := uint32(8) for symbol := 0; symbol < len(dst); { if maxSymbol == 0 { break } maxSymbol-- codeLength, err := h.next(d) if err != nil { return err } if codeLength < repeatsCodeLength { dst[symbol] = codeLength symbol++ if codeLength != 0 { prevCodeLength = codeLength } continue } repeat, err := d.read(uint32(repeatBits[codeLength-repeatsCodeLength])) if err != nil { return err } repeat += uint32(repeatOffsets[codeLength-repeatsCodeLength]) if symbol+int(repeat) > len(dst) { return errInvalidCodeLengths } // A code length of 16 repeats the previous non-zero code. // A code length of 17 or 18 repeats zeroes. cl := uint32(0) if codeLength == 16 { cl = prevCodeLength } for ; repeat > 0; repeat-- { dst[symbol] = cl symbol++ } } return nil } // decodeHuffmanTree decodes a Huffman tree into h. func (d *decoder) decodeHuffmanTree(h *hTree, alphabetSize uint32) error { useSimple, err := d.read(1) if err != nil { return err } if useSimple != 0 { nSymbols, err := d.read(1) if err != nil { return err } nSymbols++ firstSymbolLengthCode, err := d.read(1) if err != nil { return err } firstSymbolLengthCode = 7*firstSymbolLengthCode + 1 var symbols [2]uint32 symbols[0], err = d.read(firstSymbolLengthCode) if err != nil { return err } if nSymbols == 2 { symbols[1], err = d.read(8) if err != nil { return err } } return h.buildSimple(nSymbols, symbols, alphabetSize) } nCodes, err := d.read(4) if err != nil { return err } nCodes += 4 if int(nCodes) > len(codeLengthCodeOrder) { return errInvalidHuffmanTree } codeLengthCodeLengths := [len(codeLengthCodeOrder)]uint32{} for i := uint32(0); i < nCodes; i++ { codeLengthCodeLengths[codeLengthCodeOrder[i]], err = d.read(3) if err != nil { return err } } codeLengths := make([]uint32, alphabetSize) if err = d.decodeCodeLengths(codeLengths, codeLengthCodeLengths[:]); err != nil { return err } return h.build(codeLengths) } const ( huffGreen = 0 huffRed = 1 huffBlue = 2 huffAlpha = 3 huffDistance = 4 nHuff = 5 ) // hGroup is an array of 5 Huffman trees. type hGroup [nHuff]hTree // decodeHuffmanGroups decodes the one or more hGroups used to decode the pixel // data. If one hGroup is used for the entire image, then hPix and hBits will // be zero. If more than one hGroup is used, then hPix contains the meta-image // that maps tiles to hGroup index, and hBits contains the log-2 tile size. func (d *decoder) decodeHuffmanGroups(w int32, h int32, topLevel bool, ccBits uint32) ( hGroups []hGroup, hPix []byte, hBits uint32, err error) { maxHGroupIndex := 0 if topLevel { useMeta, err := d.read(1) if err != nil { return nil, nil, 0, err } if useMeta != 0 { hBits, err = d.read(3) if err != nil { return nil, nil, 0, err } hBits += 2 hPix, err = d.decodePix(nTiles(w, hBits), nTiles(h, hBits), 0, false) if err != nil { return nil, nil, 0, err } for p := 0; p < len(hPix); p += 4 { i := int(hPix[p])<<8 | int(hPix[p+1]) if maxHGroupIndex < i { maxHGroupIndex = i } } } } hGroups = make([]hGroup, maxHGroupIndex+1) for i := range hGroups { for j, alphabetSize := range alphabetSizes { if j == 0 && ccBits > 0 { alphabetSize += 1 << ccBits } if err := d.decodeHuffmanTree(&hGroups[i][j], alphabetSize); err != nil { return nil, nil, 0, err } } } return hGroups, hPix, hBits, nil } const ( nLiteralCodes = 256 nLengthCodes = 24 nDistanceCodes = 40 ) var alphabetSizes = [nHuff]uint32{ nLiteralCodes + nLengthCodes, nLiteralCodes, nLiteralCodes, nLiteralCodes, nDistanceCodes, } // decodePix decodes pixel data, specified in section 5.2.2. func (d *decoder) decodePix(w int32, h int32, minCap int32, topLevel bool) ([]byte, error) { // Decode the color cache parameters. ccBits, ccShift, ccEntries := uint32(0), uint32(0), ([]uint32)(nil) useColorCache, err := d.read(1) if err != nil { return nil, err } if useColorCache != 0 { ccBits, err = d.read(4) if err != nil { return nil, err } if ccBits < 1 || 11 < ccBits { return nil, errors.New("vp8l: invalid color cache parameters") } ccShift = 32 - ccBits ccEntries = make([]uint32, 1<>hBits) + (x >> hBits)) hg = &hGroups[uint32(hPix[i])<<8|uint32(hPix[i+1])] } green, err := hg[huffGreen].next(d) if err != nil { return nil, err } switch { case green < nLiteralCodes: // We have a literal pixel. red, err := hg[huffRed].next(d) if err != nil { return nil, err } blue, err := hg[huffBlue].next(d) if err != nil { return nil, err } alpha, err := hg[huffAlpha].next(d) if err != nil { return nil, err } pix[p+0] = uint8(red) pix[p+1] = uint8(green) pix[p+2] = uint8(blue) pix[p+3] = uint8(alpha) p += 4 x++ if x == w { x, y = 0, y+1 } lookupHG = hMask != 0 && x&hMask == 0 case green < nLiteralCodes+nLengthCodes: // We have a LZ77 backwards reference. length, err := d.lz77Param(green - nLiteralCodes) if err != nil { return nil, err } distSym, err := hg[huffDistance].next(d) if err != nil { return nil, err } distCode, err := d.lz77Param(distSym) if err != nil { return nil, err } dist := distanceMap(w, distCode) pEnd := p + 4*int(length) q := p - 4*int(dist) qEnd := pEnd - 4*int(dist) if p < 0 || len(pix) < pEnd || q < 0 || len(pix) < qEnd { return nil, errors.New("vp8l: invalid LZ77 parameters") } for ; p < pEnd; p, q = p+1, q+1 { pix[p] = pix[q] } x += int32(length) for x >= w { x, y = x-w, y+1 } lookupHG = hMask != 0 default: // We have a color cache lookup. First, insert previous pixels // into the cache. Note that VP8L assumes ARGB order, but the // Go image.RGBA type is in RGBA order. for ; cachedP < p; cachedP += 4 { argb := uint32(pix[cachedP+0])<<16 | uint32(pix[cachedP+1])<<8 | uint32(pix[cachedP+2])<<0 | uint32(pix[cachedP+3])<<24 ccEntries[(argb*colorCacheMultiplier)>>ccShift] = argb } green -= nLiteralCodes + nLengthCodes if int(green) >= len(ccEntries) { return nil, errors.New("vp8l: invalid color cache index") } argb := ccEntries[green] pix[p+0] = uint8(argb >> 16) pix[p+1] = uint8(argb >> 8) pix[p+2] = uint8(argb >> 0) pix[p+3] = uint8(argb >> 24) p += 4 x++ if x == w { x, y = 0, y+1 } lookupHG = hMask != 0 && x&hMask == 0 } } return pix, nil } // lz77Param returns the next LZ77 parameter: a length or a distance, specified // in section 4.2.2. func (d *decoder) lz77Param(symbol uint32) (uint32, error) { if symbol < 4 { return symbol + 1, nil } extraBits := (symbol - 2) >> 1 offset := (2 + symbol&1) << extraBits n, err := d.read(extraBits) if err != nil { return 0, err } return offset + n + 1, nil } // decodeHeader decodes the VP8L header from r. func decodeHeader(r io.Reader) (d *decoder, w int32, h int32, err error) { rr, ok := r.(io.ByteReader) if !ok { rr = bufio.NewReader(r) } d = &decoder{r: rr} magic, err := d.read(8) if err != nil { return nil, 0, 0, err } if magic != 0x2f { return nil, 0, 0, errors.New("vp8l: invalid header") } width, err := d.read(14) if err != nil { return nil, 0, 0, err } width++ height, err := d.read(14) if err != nil { return nil, 0, 0, err } height++ _, err = d.read(1) // Read and ignore the hasAlpha hint. if err != nil { return nil, 0, 0, err } version, err := d.read(3) if err != nil { return nil, 0, 0, err } if version != 0 { return nil, 0, 0, errors.New("vp8l: invalid version") } return d, int32(width), int32(height), nil } // DecodeConfig decodes the color model and dimensions of a VP8L image from r. func DecodeConfig(r io.Reader) (image.Config, error) { _, w, h, err := decodeHeader(r) if err != nil { return image.Config{}, err } return image.Config{ ColorModel: color.NRGBAModel, Width: int(w), Height: int(h), }, nil } // Decode decodes a VP8L image from r. func Decode(r io.Reader) (image.Image, error) { d, w, h, err := decodeHeader(r) if err != nil { return nil, err } // Decode the transforms. var ( nTransforms int transforms [nTransformTypes]transform transformsSeen [nTransformTypes]bool originalW = w ) for { more, err := d.read(1) if err != nil { return nil, err } if more == 0 { break } var t transform t, w, err = d.decodeTransform(w, h) if err != nil { return nil, err } if transformsSeen[t.transformType] { return nil, errors.New("vp8l: repeated transform") } transformsSeen[t.transformType] = true transforms[nTransforms] = t nTransforms++ } // Decode the transformed pixels. pix, err := d.decodePix(w, h, 0, true) if err != nil { return nil, err } // Apply the inverse transformations. for i := nTransforms - 1; i >= 0; i-- { t := &transforms[i] pix = inverseTransforms[t.transformType](t, pix, h) } return &image.NRGBA{ Pix: pix, Stride: 4 * int(originalW), Rect: image.Rect(0, 0, int(originalW), int(h)), }, nil }