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+// 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 vp8 implements a decoder for the VP8 lossy image format.
+//
+// The VP8 specification is RFC 6386.
+package vp8 // import "golang.org/x/image/vp8"
+
+// This file implements the top-level decoding algorithm.
+
+import (
+ "errors"
+ "image"
+ "io"
+)
+
+// limitReader wraps an io.Reader to read at most n bytes from it.
+type limitReader struct {
+ r io.Reader
+ n int
+}
+
+// ReadFull reads exactly len(p) bytes into p.
+func (r *limitReader) ReadFull(p []byte) error {
+ if len(p) > r.n {
+ return io.ErrUnexpectedEOF
+ }
+ n, err := io.ReadFull(r.r, p)
+ r.n -= n
+ return err
+}
+
+// FrameHeader is a frame header, as specified in section 9.1.
+type FrameHeader struct {
+ KeyFrame bool
+ VersionNumber uint8
+ ShowFrame bool
+ FirstPartitionLen uint32
+ Width int
+ Height int
+ XScale uint8
+ YScale uint8
+}
+
+const (
+ nSegment = 4
+ nSegmentProb = 3
+)
+
+// segmentHeader holds segment-related header information.
+type segmentHeader struct {
+ useSegment bool
+ updateMap bool
+ relativeDelta bool
+ quantizer [nSegment]int8
+ filterStrength [nSegment]int8
+ prob [nSegmentProb]uint8
+}
+
+const (
+ nRefLFDelta = 4
+ nModeLFDelta = 4
+)
+
+// filterHeader holds filter-related header information.
+type filterHeader struct {
+ simple bool
+ level int8
+ sharpness uint8
+ useLFDelta bool
+ refLFDelta [nRefLFDelta]int8
+ modeLFDelta [nModeLFDelta]int8
+ perSegmentLevel [nSegment]int8
+}
+
+// mb is the per-macroblock decode state. A decoder maintains mbw+1 of these
+// as it is decoding macroblocks left-to-right and top-to-bottom: mbw for the
+// macroblocks in the row above, and one for the macroblock to the left.
+type mb struct {
+ // pred is the predictor mode for the 4 bottom or right 4x4 luma regions.
+ pred [4]uint8
+ // nzMask is a mask of 8 bits: 4 for the bottom or right 4x4 luma regions,
+ // and 2 + 2 for the bottom or right 4x4 chroma regions. A 1 bit indicates
+ // that that region has non-zero coefficients.
+ nzMask uint8
+ // nzY16 is a 0/1 value that is 1 if the macroblock used Y16 prediction and
+ // had non-zero coefficients.
+ nzY16 uint8
+}
+
+// Decoder decodes VP8 bitstreams into frames. Decoding one frame consists of
+// calling Init, DecodeFrameHeader and then DecodeFrame in that order.
+// A Decoder can be re-used to decode multiple frames.
+type Decoder struct {
+ // r is the input bitsream.
+ r limitReader
+ // scratch is a scratch buffer.
+ scratch [8]byte
+ // img is the YCbCr image to decode into.
+ img *image.YCbCr
+ // mbw and mbh are the number of 16x16 macroblocks wide and high the image is.
+ mbw, mbh int
+ // frameHeader is the frame header. When decoding multiple frames,
+ // frames that aren't key frames will inherit the Width, Height,
+ // XScale and YScale of the most recent key frame.
+ frameHeader FrameHeader
+ // Other headers.
+ segmentHeader segmentHeader
+ filterHeader filterHeader
+ // The image data is divided into a number of independent partitions.
+ // There is 1 "first partition" and between 1 and 8 "other partitions"
+ // for coefficient data.
+ fp partition
+ op [8]partition
+ nOP int
+ // Quantization factors.
+ quant [nSegment]quant
+ // DCT/WHT coefficient decoding probabilities.
+ tokenProb [nPlane][nBand][nContext][nProb]uint8
+ useSkipProb bool
+ skipProb uint8
+ // Loop filter parameters.
+ filterParams [nSegment][2]filterParam
+ perMBFilterParams []filterParam
+
+ // The eight fields below relate to the current macroblock being decoded.
+ //
+ // Segment-based adjustments.
+ segment int
+ // Per-macroblock state for the macroblock immediately left of and those
+ // macroblocks immediately above the current macroblock.
+ leftMB mb
+ upMB []mb
+ // Bitmasks for which 4x4 regions of coeff contain non-zero coefficients.
+ nzDCMask, nzACMask uint32
+ // Predictor modes.
+ usePredY16 bool // The libwebp C code calls this !is_i4x4_.
+ predY16 uint8
+ predC8 uint8
+ predY4 [4][4]uint8
+
+ // The two fields below form a workspace for reconstructing a macroblock.
+ // Their specific sizes are documented in reconstruct.go.
+ coeff [1*16*16 + 2*8*8 + 1*4*4]int16
+ ybr [1 + 16 + 1 + 8][32]uint8
+}
+
+// NewDecoder returns a new Decoder.
+func NewDecoder() *Decoder {
+ return &Decoder{}
+}
+
+// Init initializes the decoder to read at most n bytes from r.
+func (d *Decoder) Init(r io.Reader, n int) {
+ d.r = limitReader{r, n}
+}
+
+// DecodeFrameHeader decodes the frame header.
+func (d *Decoder) DecodeFrameHeader() (fh FrameHeader, err error) {
+ // All frame headers are at least 3 bytes long.
+ b := d.scratch[:3]
+ if err = d.r.ReadFull(b); err != nil {
+ return
+ }
+ d.frameHeader.KeyFrame = (b[0] & 1) == 0
+ d.frameHeader.VersionNumber = (b[0] >> 1) & 7
+ d.frameHeader.ShowFrame = (b[0]>>4)&1 == 1
+ d.frameHeader.FirstPartitionLen = uint32(b[0])>>5 | uint32(b[1])<<3 | uint32(b[2])<<11
+ if !d.frameHeader.KeyFrame {
+ return d.frameHeader, nil
+ }
+ // Frame headers for key frames are an additional 7 bytes long.
+ b = d.scratch[:7]
+ if err = d.r.ReadFull(b); err != nil {
+ return
+ }
+ // Check the magic sync code.
+ if b[0] != 0x9d || b[1] != 0x01 || b[2] != 0x2a {
+ err = errors.New("vp8: invalid format")
+ return
+ }
+ d.frameHeader.Width = int(b[4]&0x3f)<<8 | int(b[3])
+ d.frameHeader.Height = int(b[6]&0x3f)<<8 | int(b[5])
+ d.frameHeader.XScale = b[4] >> 6
+ d.frameHeader.YScale = b[6] >> 6
+ d.mbw = (d.frameHeader.Width + 0x0f) >> 4
+ d.mbh = (d.frameHeader.Height + 0x0f) >> 4
+ d.segmentHeader = segmentHeader{
+ prob: [3]uint8{0xff, 0xff, 0xff},
+ }
+ d.tokenProb = defaultTokenProb
+ d.segment = 0
+ return d.frameHeader, nil
+}
+
+// ensureImg ensures that d.img is large enough to hold the decoded frame.
+func (d *Decoder) ensureImg() {
+ if d.img != nil {
+ p0, p1 := d.img.Rect.Min, d.img.Rect.Max
+ if p0.X == 0 && p0.Y == 0 && p1.X >= 16*d.mbw && p1.Y >= 16*d.mbh {
+ return
+ }
+ }
+ m := image.NewYCbCr(image.Rect(0, 0, 16*d.mbw, 16*d.mbh), image.YCbCrSubsampleRatio420)
+ d.img = m.SubImage(image.Rect(0, 0, d.frameHeader.Width, d.frameHeader.Height)).(*image.YCbCr)
+ d.perMBFilterParams = make([]filterParam, d.mbw*d.mbh)
+ d.upMB = make([]mb, d.mbw)
+}
+
+// parseSegmentHeader parses the segment header, as specified in section 9.3.
+func (d *Decoder) parseSegmentHeader() {
+ d.segmentHeader.useSegment = d.fp.readBit(uniformProb)
+ if !d.segmentHeader.useSegment {
+ d.segmentHeader.updateMap = false
+ return
+ }
+ d.segmentHeader.updateMap = d.fp.readBit(uniformProb)
+ if d.fp.readBit(uniformProb) {
+ d.segmentHeader.relativeDelta = !d.fp.readBit(uniformProb)
+ for i := range d.segmentHeader.quantizer {
+ d.segmentHeader.quantizer[i] = int8(d.fp.readOptionalInt(uniformProb, 7))
+ }
+ for i := range d.segmentHeader.filterStrength {
+ d.segmentHeader.filterStrength[i] = int8(d.fp.readOptionalInt(uniformProb, 6))
+ }
+ }
+ if !d.segmentHeader.updateMap {
+ return
+ }
+ for i := range d.segmentHeader.prob {
+ if d.fp.readBit(uniformProb) {
+ d.segmentHeader.prob[i] = uint8(d.fp.readUint(uniformProb, 8))
+ } else {
+ d.segmentHeader.prob[i] = 0xff
+ }
+ }
+}
+
+// parseFilterHeader parses the filter header, as specified in section 9.4.
+func (d *Decoder) parseFilterHeader() {
+ d.filterHeader.simple = d.fp.readBit(uniformProb)
+ d.filterHeader.level = int8(d.fp.readUint(uniformProb, 6))
+ d.filterHeader.sharpness = uint8(d.fp.readUint(uniformProb, 3))
+ d.filterHeader.useLFDelta = d.fp.readBit(uniformProb)
+ if d.filterHeader.useLFDelta && d.fp.readBit(uniformProb) {
+ for i := range d.filterHeader.refLFDelta {
+ d.filterHeader.refLFDelta[i] = int8(d.fp.readOptionalInt(uniformProb, 6))
+ }
+ for i := range d.filterHeader.modeLFDelta {
+ d.filterHeader.modeLFDelta[i] = int8(d.fp.readOptionalInt(uniformProb, 6))
+ }
+ }
+ if d.filterHeader.level == 0 {
+ return
+ }
+ if d.segmentHeader.useSegment {
+ for i := range d.filterHeader.perSegmentLevel {
+ strength := d.segmentHeader.filterStrength[i]
+ if d.segmentHeader.relativeDelta {
+ strength += d.filterHeader.level
+ }
+ d.filterHeader.perSegmentLevel[i] = strength
+ }
+ } else {
+ d.filterHeader.perSegmentLevel[0] = d.filterHeader.level
+ }
+ d.computeFilterParams()
+}
+
+// parseOtherPartitions parses the other partitions, as specified in section 9.5.
+func (d *Decoder) parseOtherPartitions() error {
+ const maxNOP = 1 << 3
+ var partLens [maxNOP]int
+ d.nOP = 1 << d.fp.readUint(uniformProb, 2)
+
+ // The final partition length is implied by the the remaining chunk data
+ // (d.r.n) and the other d.nOP-1 partition lengths. Those d.nOP-1 partition
+ // lengths are stored as 24-bit uints, i.e. up to 16 MiB per partition.
+ n := 3 * (d.nOP - 1)
+ partLens[d.nOP-1] = d.r.n - n
+ if partLens[d.nOP-1] < 0 {
+ return io.ErrUnexpectedEOF
+ }
+ if n > 0 {
+ buf := make([]byte, n)
+ if err := d.r.ReadFull(buf); err != nil {
+ return err
+ }
+ for i := 0; i < d.nOP-1; i++ {
+ pl := int(buf[3*i+0]) | int(buf[3*i+1])<<8 | int(buf[3*i+2])<<16
+ if pl > partLens[d.nOP-1] {
+ return io.ErrUnexpectedEOF
+ }
+ partLens[i] = pl
+ partLens[d.nOP-1] -= pl
+ }
+ }
+
+ // We check if the final partition length can also fit into a 24-bit uint.
+ // Strictly speaking, this isn't part of the spec, but it guards against a
+ // malicious WEBP image that is too large to ReadFull the encoded DCT
+ // coefficients into memory, whether that's because the actual WEBP file is
+ // too large, or whether its RIFF metadata lists too large a chunk.
+ if 1<<24 <= partLens[d.nOP-1] {
+ return errors.New("vp8: too much data to decode")
+ }
+
+ buf := make([]byte, d.r.n)
+ if err := d.r.ReadFull(buf); err != nil {
+ return err
+ }
+ for i, pl := range partLens {
+ if i == d.nOP {
+ break
+ }
+ d.op[i].init(buf[:pl])
+ buf = buf[pl:]
+ }
+ return nil
+}
+
+// parseOtherHeaders parses header information other than the frame header.
+func (d *Decoder) parseOtherHeaders() error {
+ // Initialize and parse the first partition.
+ firstPartition := make([]byte, d.frameHeader.FirstPartitionLen)
+ if err := d.r.ReadFull(firstPartition); err != nil {
+ return err
+ }
+ d.fp.init(firstPartition)
+ if d.frameHeader.KeyFrame {
+ // Read and ignore the color space and pixel clamp values. They are
+ // specified in section 9.2, but are unimplemented.
+ d.fp.readBit(uniformProb)
+ d.fp.readBit(uniformProb)
+ }
+ d.parseSegmentHeader()
+ d.parseFilterHeader()
+ if err := d.parseOtherPartitions(); err != nil {
+ return err
+ }
+ d.parseQuant()
+ if !d.frameHeader.KeyFrame {
+ // Golden and AltRef frames are specified in section 9.7.
+ // TODO(nigeltao): implement. Note that they are only used for video, not still images.
+ return errors.New("vp8: Golden / AltRef frames are not implemented")
+ }
+ // Read and ignore the refreshLastFrameBuffer bit, specified in section 9.8.
+ // It applies only to video, and not still images.
+ d.fp.readBit(uniformProb)
+ d.parseTokenProb()
+ d.useSkipProb = d.fp.readBit(uniformProb)
+ if d.useSkipProb {
+ d.skipProb = uint8(d.fp.readUint(uniformProb, 8))
+ }
+ if d.fp.unexpectedEOF {
+ return io.ErrUnexpectedEOF
+ }
+ return nil
+}
+
+// DecodeFrame decodes the frame and returns it as an YCbCr image.
+// The image's contents are valid up until the next call to Decoder.Init.
+func (d *Decoder) DecodeFrame() (*image.YCbCr, error) {
+ d.ensureImg()
+ if err := d.parseOtherHeaders(); err != nil {
+ return nil, err
+ }
+ // Reconstruct the rows.
+ for mbx := 0; mbx < d.mbw; mbx++ {
+ d.upMB[mbx] = mb{}
+ }
+ for mby := 0; mby < d.mbh; mby++ {
+ d.leftMB = mb{}
+ for mbx := 0; mbx < d.mbw; mbx++ {
+ skip := d.reconstruct(mbx, mby)
+ fs := d.filterParams[d.segment][btou(!d.usePredY16)]
+ fs.inner = fs.inner || !skip
+ d.perMBFilterParams[d.mbw*mby+mbx] = fs
+ }
+ }
+ if d.fp.unexpectedEOF {
+ return nil, io.ErrUnexpectedEOF
+ }
+ for i := 0; i < d.nOP; i++ {
+ if d.op[i].unexpectedEOF {
+ return nil, io.ErrUnexpectedEOF
+ }
+ }
+ // Apply the loop filter.
+ //
+ // Even if we are using per-segment levels, section 15 says that "loop
+ // filtering must be skipped entirely if loop_filter_level at either the
+ // frame header level or macroblock override level is 0".
+ if d.filterHeader.level != 0 {
+ if d.filterHeader.simple {
+ d.simpleFilter()
+ } else {
+ d.normalFilter()
+ }
+ }
+ return d.img, nil
+}