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// 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 - 1)
d.bits >>= 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<<ccBits)
}
// Decode the Huffman groups.
hGroups, hPix, hBits, err := d.decodeHuffmanGroups(w, h, topLevel, ccBits)
if err != nil {
return nil, err
}
hMask, tilesPerRow := int32(0), int32(0)
if hBits != 0 {
hMask, tilesPerRow = 1<<hBits-1, nTiles(w, hBits)
}
// Decode the pixels.
if minCap < 4*w*h {
minCap = 4 * w * h
}
pix := make([]byte, 4*w*h, minCap)
p, cachedP := 0, 0
x, y := int32(0), int32(0)
hg, lookupHG := &hGroups[0], hMask != 0
for p < len(pix) {
if lookupHG {
i := 4 * (tilesPerRow*(y>>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
}
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