Convert .tgs with go libraries (and cgo) (telegram) (#1569)

This commit adds support for go/cgo tgs conversion when building with the -tags `cgo`
The default binaries are still "pure" go and uses the old way of converting.

* Move lottie_convert.py conversion code to its own file

* Add optional libtgsconverter

* Update vendor

* Apply suggestions from code review

* Update bridge/helper/libtgsconverter.go

Co-authored-by: Wim <wim@42.be>

1 files changed, 1141 insertions, 0 deletions

diff --git a/vendor/github.com/kettek/apng/reader.go b/vendor/github.com/kettek/apng/reader.go
new file mode 100644
index 00000000..fd6ccee3
--- /dev/null
+++ b/vendor/github.com/kettek/apng/reader.go
@@ -0,0 +1,1141 @@
+// Original PNG code Copyright 2009 The Go Authors.
+// Additional APNG enhancements Copyright 2018 Ketchetwahmeegwun
+// Tecumseh Southall / kts of kettek.
+// All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package apng implements an APNG image decoder.
+//
+// The PNG specification is at https://www.w3.org/TR/PNG/.
+// The APNG specification is at https://wiki.mozilla.org/APNG_Specification
+package apng
+
+import (
+	"compress/zlib"
+	"encoding/binary"
+	"fmt"
+	"hash"
+	"hash/crc32"
+	"image"
+	"image/color"
+	"io"
+)
+
+// Color type, as per the PNG spec.
+const (
+	ctGrayscale      = 0
+	ctTrueColor      = 2
+	ctPaletted       = 3
+	ctGrayscaleAlpha = 4
+	ctTrueColorAlpha = 6
+)
+
+// A cb is a combination of color type and bit depth.
+const (
+	cbInvalid = iota
+	cbG1
+	cbG2
+	cbG4
+	cbG8
+	cbGA8
+	cbTC8
+	cbP1
+	cbP2
+	cbP4
+	cbP8
+	cbTCA8
+	cbG16
+	cbGA16
+	cbTC16
+	cbTCA16
+)
+
+func cbPaletted(cb int) bool {
+	return cbP1 <= cb && cb <= cbP8
+}
+
+// Filter type, as per the PNG spec.
+const (
+	ftNone    = 0
+	ftSub     = 1
+	ftUp      = 2
+	ftAverage = 3
+	ftPaeth   = 4
+	nFilter   = 5
+)
+
+// Interlace type.
+const (
+	itNone  = 0
+	itAdam7 = 1
+)
+
+// interlaceScan defines the placement and size of a pass for Adam7 interlacing.
+type interlaceScan struct {
+	xFactor, yFactor, xOffset, yOffset int
+}
+
+// interlacing defines Adam7 interlacing, with 7 passes of reduced images.
+// See https://www.w3.org/TR/PNG/#8Interlace
+var interlacing = []interlaceScan{
+	{8, 8, 0, 0},
+	{8, 8, 4, 0},
+	{4, 8, 0, 4},
+	{4, 4, 2, 0},
+	{2, 4, 0, 2},
+	{2, 2, 1, 0},
+	{1, 2, 0, 1},
+}
+
+// Decoding stage.
+// The PNG specification says that the IHDR, PLTE (if present), tRNS (if
+// present), IDAT and IEND chunks must appear in that order. There may be
+// multiple IDAT chunks, and IDAT chunks must be sequential (i.e. they may not
+// have any other chunks between them).
+// https://www.w3.org/TR/PNG/#5ChunkOrdering
+const (
+	dsStart = iota
+	dsSeenIHDR
+	dsSeenPLTE
+	dsSeentRNS
+	dsSeenacTL
+	dsSeenIDAT
+	dsSeenfdAT
+	dsSeenIEND
+)
+
+const pngHeader = "\x89PNG\r\n\x1a\n"
+
+type decoder struct {
+	r             io.Reader
+	num_frames    uint32
+	a             APNG
+	frame_index   int
+	crc           hash.Hash32
+	width, height int
+	depth         int
+	palette       color.Palette
+	cb            int
+	stage         int
+	idatLength    uint32
+	tmp           [3 * 256]byte
+	interlace     int
+
+	// useTransparent and transparent are used for grayscale and truecolor
+	// transparency, as opposed to palette transparency.
+	useTransparent bool
+	transparent    [6]byte
+}
+
+// A FormatError reports that the input is not a valid PNG.
+type FormatError string
+
+func (e FormatError) Error() string { return "apng: invalid format: " + string(e) }
+
+var chunkOrderError = FormatError("chunk out of order")
+
+// An UnsupportedError reports that the input uses a valid but unimplemented PNG feature.
+type UnsupportedError string
+
+func (e UnsupportedError) Error() string { return "apng: unsupported feature: " + string(e) }
+
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
+func (d *decoder) parseIHDR(length uint32) error {
+	if length != 13 {
+		return FormatError("bad IHDR length")
+	}
+	if _, err := io.ReadFull(d.r, d.tmp[:13]); err != nil {
+		return err
+	}
+	d.crc.Write(d.tmp[:13])
+	if d.tmp[10] != 0 {
+		return UnsupportedError("compression method")
+	}
+	if d.tmp[11] != 0 {
+		return UnsupportedError("filter method")
+	}
+	if d.tmp[12] != itNone && d.tmp[12] != itAdam7 {
+		return FormatError("invalid interlace method")
+	}
+	d.interlace = int(d.tmp[12])
+
+	w := int32(binary.BigEndian.Uint32(d.tmp[0:4]))
+	h := int32(binary.BigEndian.Uint32(d.tmp[4:8]))
+	if w <= 0 || h <= 0 {
+		return FormatError("non-positive dimension")
+	}
+	nPixels := int64(w) * int64(h)
+	if nPixels != int64(int(nPixels)) {
+		return UnsupportedError("dimension overflow")
+	}
+	// There can be up to 8 bytes per pixel, for 16 bits per channel RGBA.
+	if nPixels != (nPixels*8)/8 {
+		return UnsupportedError("dimension overflow")
+	}
+
+	d.cb = cbInvalid
+	d.depth = int(d.tmp[8])
+	switch d.depth {
+	case 1:
+		switch d.tmp[9] {
+		case ctGrayscale:
+			d.cb = cbG1
+		case ctPaletted:
+			d.cb = cbP1
+		}
+	case 2:
+		switch d.tmp[9] {
+		case ctGrayscale:
+			d.cb = cbG2
+		case ctPaletted:
+			d.cb = cbP2
+		}
+	case 4:
+		switch d.tmp[9] {
+		case ctGrayscale:
+			d.cb = cbG4
+		case ctPaletted:
+			d.cb = cbP4
+		}
+	case 8:
+		switch d.tmp[9] {
+		case ctGrayscale:
+			d.cb = cbG8
+		case ctTrueColor:
+			d.cb = cbTC8
+		case ctPaletted:
+			d.cb = cbP8
+		case ctGrayscaleAlpha:
+			d.cb = cbGA8
+		case ctTrueColorAlpha:
+			d.cb = cbTCA8
+		}
+	case 16:
+		switch d.tmp[9] {
+		case ctGrayscale:
+			d.cb = cbG16
+		case ctTrueColor:
+			d.cb = cbTC16
+		case ctGrayscaleAlpha:
+			d.cb = cbGA16
+		case ctTrueColorAlpha:
+			d.cb = cbTCA16
+		}
+	}
+	if d.cb == cbInvalid {
+		return UnsupportedError(fmt.Sprintf("bit depth %d, color type %d", d.tmp[8], d.tmp[9]))
+	}
+	d.a.Frames[0].width, d.a.Frames[0].height = int(w), int(h)
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parsePLTE(length uint32) error {
+	np := int(length / 3) // The number of palette entries.
+	if length%3 != 0 || np <= 0 || np > 256 || np > 1<<uint(d.depth) {
+		return FormatError("bad PLTE length")
+	}
+	n, err := io.ReadFull(d.r, d.tmp[:3*np])
+	if err != nil {
+		return err
+	}
+	d.crc.Write(d.tmp[:n])
+	switch d.cb {
+	case cbP1, cbP2, cbP4, cbP8:
+		d.palette = make(color.Palette, 256)
+		for i := 0; i < np; i++ {
+			d.palette[i] = color.RGBA{d.tmp[3*i+0], d.tmp[3*i+1], d.tmp[3*i+2], 0xff}
+		}
+		for i := np; i < 256; i++ {
+			// Initialize the rest of the palette to opaque black. The spec (section
+			// 11.2.3) says that "any out-of-range pixel value found in the image data
+			// is an error", but some real-world PNG files have out-of-range pixel
+			// values. We fall back to opaque black, the same as libpng 1.5.13;
+			// ImageMagick 6.5.7 returns an error.
+			d.palette[i] = color.RGBA{0x00, 0x00, 0x00, 0xff}
+		}
+		d.palette = d.palette[:np]
+	case cbTC8, cbTCA8, cbTC16, cbTCA16:
+		// As per the PNG spec, a PLTE chunk is optional (and for practical purposes,
+		// ignorable) for the ctTrueColor and ctTrueColorAlpha color types (section 4.1.2).
+	default:
+		return FormatError("PLTE, color type mismatch")
+	}
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parsetRNS(length uint32) error {
+	switch d.cb {
+	case cbG1, cbG2, cbG4, cbG8, cbG16:
+		if length != 2 {
+			return FormatError("bad tRNS length")
+		}
+		n, err := io.ReadFull(d.r, d.tmp[:length])
+		if err != nil {
+			return err
+		}
+		d.crc.Write(d.tmp[:n])
+
+		copy(d.transparent[:], d.tmp[:length])
+		switch d.cb {
+		case cbG1:
+			d.transparent[1] *= 0xff
+		case cbG2:
+			d.transparent[1] *= 0x55
+		case cbG4:
+			d.transparent[1] *= 0x11
+		}
+		d.useTransparent = true
+
+	case cbTC8, cbTC16:
+		if length != 6 {
+			return FormatError("bad tRNS length")
+		}
+		n, err := io.ReadFull(d.r, d.tmp[:length])
+		if err != nil {
+			return err
+		}
+		d.crc.Write(d.tmp[:n])
+
+		copy(d.transparent[:], d.tmp[:length])
+		d.useTransparent = true
+
+	case cbP1, cbP2, cbP4, cbP8:
+		if length > 256 {
+			return FormatError("bad tRNS length")
+		}
+		n, err := io.ReadFull(d.r, d.tmp[:length])
+		if err != nil {
+			return err
+		}
+		d.crc.Write(d.tmp[:n])
+
+		if len(d.palette) < n {
+			d.palette = d.palette[:n]
+		}
+		for i := 0; i < n; i++ {
+			rgba := d.palette[i].(color.RGBA)
+			d.palette[i] = color.NRGBA{rgba.R, rgba.G, rgba.B, d.tmp[i]}
+		}
+
+	default:
+		return FormatError("tRNS, color type mismatch")
+	}
+	return d.verifyChecksum()
+}
+
+// Read presents one or more IDAT chunks as one continuous stream (minus the
+// intermediate chunk headers and footers). If the PNG data looked like:
+//   ... len0 IDAT xxx crc0 len1 IDAT yy crc1 len2 IEND crc2
+// then this reader presents xxxyy. For well-formed PNG data, the decoder state
+// immediately before the first Read call is that d.r is positioned between the
+// first IDAT and xxx, and the decoder state immediately after the last Read
+// call is that d.r is positioned between yy and crc1.
+func (d *decoder) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	for d.idatLength == 0 {
+		// We have exhausted an IDAT chunk. Verify the checksum of that chunk.
+		if err := d.verifyChecksum(); err != nil {
+			return 0, err
+		}
+		// Read the length and chunk type of the next chunk, and check that
+		// it is an IDAT chunk.
+		if _, err := io.ReadFull(d.r, d.tmp[:8]); err != nil {
+			return 0, err
+		}
+
+		if d.stage < dsSeenfdAT {
+			d.idatLength = binary.BigEndian.Uint32(d.tmp[:4])
+			if string(d.tmp[4:8]) != "IDAT" {
+				return 0, FormatError(fmt.Sprintf("expected IDAT, found %s", string(d.tmp[4:8])))
+			}
+		} else {
+			d.idatLength = binary.BigEndian.Uint32(d.tmp[:4]) - 4
+			if string(d.tmp[4:8]) != "fdAT" {
+				return 0, FormatError(fmt.Sprintf("expected fdAT, found %s", string(d.tmp[4:8])))
+			}
+		}
+		d.crc.Reset()
+		d.crc.Write(d.tmp[4:8])
+		if d.stage >= dsSeenfdAT {
+			if _, err := io.ReadFull(d.r, d.tmp[:4]); err != nil {
+				return 0, err
+			}
+			d.crc.Write(d.tmp[:4])
+		}
+	}
+	if int(d.idatLength) < 0 {
+		return 0, UnsupportedError("IDAT/fdAT chunk length overflow")
+	}
+	n, err := d.r.Read(p[:min(len(p), int(d.idatLength))])
+	d.crc.Write(p[:n])
+	d.idatLength -= uint32(n)
+	return n, err
+}
+
+// decode decodes the IDAT data into an image.
+func (d *decoder) decode() (image.Image, error) {
+	r, err := zlib.NewReader(d)
+	if err != nil {
+		return nil, err
+	}
+	defer r.Close()
+	var img image.Image
+	if d.interlace == itNone {
+		img, err = d.readImagePass(r, 0, false)
+		if err != nil {
+			return nil, err
+		}
+	} else if d.interlace == itAdam7 {
+		// Allocate a blank image of the full size.
+		img, err = d.readImagePass(nil, 0, true)
+		if err != nil {
+			return nil, err
+		}
+		for pass := 0; pass < 7; pass++ {
+			imagePass, err := d.readImagePass(r, pass, false)
+			if err != nil {
+				return nil, err
+			}
+			if imagePass != nil {
+				d.mergePassInto(img, imagePass, pass)
+			}
+		}
+	}
+
+	// Check for EOF, to verify the zlib checksum.
+	n := 0
+	for i := 0; n == 0 && err == nil; i++ {
+		if i == 100 {
+			return nil, io.ErrNoProgress
+		}
+		n, err = r.Read(d.tmp[:1])
+	}
+	if err != nil && err != io.EOF {
+		return nil, FormatError(err.Error())
+	}
+	if n != 0 || d.idatLength != 0 {
+		return nil, FormatError("too much pixel data")
+	}
+
+	return img, nil
+}
+
+// readImagePass reads a single image pass, sized according to the pass number.
+func (d *decoder) readImagePass(r io.Reader, pass int, allocateOnly bool) (image.Image, error) {
+	bitsPerPixel := 0
+	pixOffset := 0
+	var (
+		gray     *image.Gray
+		rgba     *image.RGBA
+		paletted *image.Paletted
+		nrgba    *image.NRGBA
+		gray16   *image.Gray16
+		rgba64   *image.RGBA64
+		nrgba64  *image.NRGBA64
+		img      image.Image
+		width    int
+		height   int
+	)
+	width = d.a.Frames[d.frame_index].width
+	height = d.a.Frames[d.frame_index].height
+	if d.interlace == itAdam7 && !allocateOnly {
+		p := interlacing[pass]
+		// Add the multiplication factor and subtract one, effectively rounding up.
+		width = (width - p.xOffset + p.xFactor - 1) / p.xFactor
+		height = (height - p.yOffset + p.yFactor - 1) / p.yFactor
+		// A PNG image can't have zero width or height, but for an interlaced
+		// image, an individual pass might have zero width or height. If so, we
+		// shouldn't even read a per-row filter type byte, so return early.
+		if width == 0 || height == 0 {
+			return nil, nil
+		}
+	}
+	switch d.cb {
+	case cbG1, cbG2, cbG4, cbG8:
+		bitsPerPixel = d.depth
+		if d.useTransparent {
+			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
+			img = nrgba
+		} else {
+			gray = image.NewGray(image.Rect(0, 0, width, height))
+			img = gray
+		}
+	case cbGA8:
+		bitsPerPixel = 16
+		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
+		img = nrgba
+	case cbTC8:
+		bitsPerPixel = 24
+		if d.useTransparent {
+			nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
+			img = nrgba
+		} else {
+			rgba = image.NewRGBA(image.Rect(0, 0, width, height))
+			img = rgba
+		}
+	case cbP1, cbP2, cbP4, cbP8:
+		bitsPerPixel = d.depth
+		paletted = image.NewPaletted(image.Rect(0, 0, width, height), d.palette)
+		img = paletted
+	case cbTCA8:
+		bitsPerPixel = 32
+		nrgba = image.NewNRGBA(image.Rect(0, 0, width, height))
+		img = nrgba
+	case cbG16:
+		bitsPerPixel = 16
+		if d.useTransparent {
+			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
+			img = nrgba64
+		} else {
+			gray16 = image.NewGray16(image.Rect(0, 0, width, height))
+			img = gray16
+		}
+	case cbGA16:
+		bitsPerPixel = 32
+		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
+		img = nrgba64
+	case cbTC16:
+		bitsPerPixel = 48
+		if d.useTransparent {
+			nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
+			img = nrgba64
+		} else {
+			rgba64 = image.NewRGBA64(image.Rect(0, 0, width, height))
+			img = rgba64
+		}
+	case cbTCA16:
+		bitsPerPixel = 64
+		nrgba64 = image.NewNRGBA64(image.Rect(0, 0, width, height))
+		img = nrgba64
+	}
+	if allocateOnly {
+		return img, nil
+	}
+	bytesPerPixel := (bitsPerPixel + 7) / 8
+
+	// The +1 is for the per-row filter type, which is at cr[0].
+	rowSize := 1 + (bitsPerPixel*width+7)/8
+	// cr and pr are the bytes for the current and previous row.
+	cr := make([]uint8, rowSize)
+	pr := make([]uint8, rowSize)
+
+	for y := 0; y < height; y++ {
+		// Read the decompressed bytes.
+		_, err := io.ReadFull(r, cr)
+		if err != nil {
+			if err == io.EOF || err == io.ErrUnexpectedEOF {
+				return nil, FormatError("not enough pixel data")
+			}
+			return nil, err
+		}
+
+		// Apply the filter.
+		cdat := cr[1:]
+		pdat := pr[1:]
+		switch cr[0] {
+		case ftNone:
+			// No-op.
+		case ftSub:
+			for i := bytesPerPixel; i < len(cdat); i++ {
+				cdat[i] += cdat[i-bytesPerPixel]
+			}
+		case ftUp:
+			for i, p := range pdat {
+				cdat[i] += p
+			}
+		case ftAverage:
+			// The first column has no column to the left of it, so it is a
+			// special case. We know that the first column exists because we
+			// check above that width != 0, and so len(cdat) != 0.
+			for i := 0; i < bytesPerPixel; i++ {
+				cdat[i] += pdat[i] / 2
+			}
+			for i := bytesPerPixel; i < len(cdat); i++ {
+				cdat[i] += uint8((int(cdat[i-bytesPerPixel]) + int(pdat[i])) / 2)
+			}
+		case ftPaeth:
+			filterPaeth(cdat, pdat, bytesPerPixel)
+		default:
+			return nil, FormatError("bad filter type")
+		}
+
+		// Convert from bytes to colors.
+		switch d.cb {
+		case cbG1:
+			if d.useTransparent {
+				ty := d.transparent[1]
+				for x := 0; x < width; x += 8 {
+					b := cdat[x/8]
+					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
+						ycol := (b >> 7) * 0xff
+						acol := uint8(0xff)
+						if ycol == ty {
+							acol = 0x00
+						}
+						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
+						b <<= 1
+					}
+				}
+			} else {
+				for x := 0; x < width; x += 8 {
+					b := cdat[x/8]
+					for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
+						gray.SetGray(x+x2, y, color.Gray{(b >> 7) * 0xff})
+						b <<= 1
+					}
+				}
+			}
+		case cbG2:
+			if d.useTransparent {
+				ty := d.transparent[1]
+				for x := 0; x < width; x += 4 {
+					b := cdat[x/4]
+					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
+						ycol := (b >> 6) * 0x55
+						acol := uint8(0xff)
+						if ycol == ty {
+							acol = 0x00
+						}
+						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
+						b <<= 2
+					}
+				}
+			} else {
+				for x := 0; x < width; x += 4 {
+					b := cdat[x/4]
+					for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
+						gray.SetGray(x+x2, y, color.Gray{(b >> 6) * 0x55})
+						b <<= 2
+					}
+				}
+			}
+		case cbG4:
+			if d.useTransparent {
+				ty := d.transparent[1]
+				for x := 0; x < width; x += 2 {
+					b := cdat[x/2]
+					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
+						ycol := (b >> 4) * 0x11
+						acol := uint8(0xff)
+						if ycol == ty {
+							acol = 0x00
+						}
+						nrgba.SetNRGBA(x+x2, y, color.NRGBA{ycol, ycol, ycol, acol})
+						b <<= 4
+					}
+				}
+			} else {
+				for x := 0; x < width; x += 2 {
+					b := cdat[x/2]
+					for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
+						gray.SetGray(x+x2, y, color.Gray{(b >> 4) * 0x11})
+						b <<= 4
+					}
+				}
+			}
+		case cbG8:
+			if d.useTransparent {
+				ty := d.transparent[1]
+				for x := 0; x < width; x++ {
+					ycol := cdat[x]
+					acol := uint8(0xff)
+					if ycol == ty {
+						acol = 0x00
+					}
+					nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, acol})
+				}
+			} else {
+				copy(gray.Pix[pixOffset:], cdat)
+				pixOffset += gray.Stride
+			}
+		case cbGA8:
+			for x := 0; x < width; x++ {
+				ycol := cdat[2*x+0]
+				nrgba.SetNRGBA(x, y, color.NRGBA{ycol, ycol, ycol, cdat[2*x+1]})
+			}
+		case cbTC8:
+			if d.useTransparent {
+				pix, i, j := nrgba.Pix, pixOffset, 0
+				tr, tg, tb := d.transparent[1], d.transparent[3], d.transparent[5]
+				for x := 0; x < width; x++ {
+					r := cdat[j+0]
+					g := cdat[j+1]
+					b := cdat[j+2]
+					a := uint8(0xff)
+					if r == tr && g == tg && b == tb {
+						a = 0x00
+					}
+					pix[i+0] = r
+					pix[i+1] = g
+					pix[i+2] = b
+					pix[i+3] = a
+					i += 4
+					j += 3
+				}
+				pixOffset += nrgba.Stride
+			} else {
+				pix, i, j := rgba.Pix, pixOffset, 0
+				for x := 0; x < width; x++ {
+					pix[i+0] = cdat[j+0]
+					pix[i+1] = cdat[j+1]
+					pix[i+2] = cdat[j+2]
+					pix[i+3] = 0xff
+					i += 4
+					j += 3
+				}
+				pixOffset += rgba.Stride
+			}
+		case cbP1:
+			for x := 0; x < width; x += 8 {
+				b := cdat[x/8]
+				for x2 := 0; x2 < 8 && x+x2 < width; x2++ {
+					idx := b >> 7
+					if len(paletted.Palette) <= int(idx) {
+						paletted.Palette = paletted.Palette[:int(idx)+1]
+					}
+					paletted.SetColorIndex(x+x2, y, idx)
+					b <<= 1
+				}
+			}
+		case cbP2:
+			for x := 0; x < width; x += 4 {
+				b := cdat[x/4]
+				for x2 := 0; x2 < 4 && x+x2 < width; x2++ {
+					idx := b >> 6
+					if len(paletted.Palette) <= int(idx) {
+						paletted.Palette = paletted.Palette[:int(idx)+1]
+					}
+					paletted.SetColorIndex(x+x2, y, idx)
+					b <<= 2
+				}
+			}
+		case cbP4:
+			for x := 0; x < width; x += 2 {
+				b := cdat[x/2]
+				for x2 := 0; x2 < 2 && x+x2 < width; x2++ {
+					idx := b >> 4
+					if len(paletted.Palette) <= int(idx) {
+						paletted.Palette = paletted.Palette[:int(idx)+1]
+					}
+					paletted.SetColorIndex(x+x2, y, idx)
+					b <<= 4
+				}
+			}
+		case cbP8:
+			if len(paletted.Palette) != 255 {
+				for x := 0; x < width; x++ {
+					if len(paletted.Palette) <= int(cdat[x]) {
+						paletted.Palette = paletted.Palette[:int(cdat[x])+1]
+					}
+				}
+			}
+			copy(paletted.Pix[pixOffset:], cdat)
+			pixOffset += paletted.Stride
+		case cbTCA8:
+			copy(nrgba.Pix[pixOffset:], cdat)
+			pixOffset += nrgba.Stride
+		case cbG16:
+			if d.useTransparent {
+				ty := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
+				for x := 0; x < width; x++ {
+					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
+					acol := uint16(0xffff)
+					if ycol == ty {
+						acol = 0x0000
+					}
+					nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
+				}
+			} else {
+				for x := 0; x < width; x++ {
+					ycol := uint16(cdat[2*x+0])<<8 | uint16(cdat[2*x+1])
+					gray16.SetGray16(x, y, color.Gray16{ycol})
+				}
+			}
+		case cbGA16:
+			for x := 0; x < width; x++ {
+				ycol := uint16(cdat[4*x+0])<<8 | uint16(cdat[4*x+1])
+				acol := uint16(cdat[4*x+2])<<8 | uint16(cdat[4*x+3])
+				nrgba64.SetNRGBA64(x, y, color.NRGBA64{ycol, ycol, ycol, acol})
+			}
+		case cbTC16:
+			if d.useTransparent {
+				tr := uint16(d.transparent[0])<<8 | uint16(d.transparent[1])
+				tg := uint16(d.transparent[2])<<8 | uint16(d.transparent[3])
+				tb := uint16(d.transparent[4])<<8 | uint16(d.transparent[5])
+				for x := 0; x < width; x++ {
+					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
+					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
+					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
+					acol := uint16(0xffff)
+					if rcol == tr && gcol == tg && bcol == tb {
+						acol = 0x0000
+					}
+					nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
+				}
+			} else {
+				for x := 0; x < width; x++ {
+					rcol := uint16(cdat[6*x+0])<<8 | uint16(cdat[6*x+1])
+					gcol := uint16(cdat[6*x+2])<<8 | uint16(cdat[6*x+3])
+					bcol := uint16(cdat[6*x+4])<<8 | uint16(cdat[6*x+5])
+					rgba64.SetRGBA64(x, y, color.RGBA64{rcol, gcol, bcol, 0xffff})
+				}
+			}
+		case cbTCA16:
+			for x := 0; x < width; x++ {
+				rcol := uint16(cdat[8*x+0])<<8 | uint16(cdat[8*x+1])
+				gcol := uint16(cdat[8*x+2])<<8 | uint16(cdat[8*x+3])
+				bcol := uint16(cdat[8*x+4])<<8 | uint16(cdat[8*x+5])
+				acol := uint16(cdat[8*x+6])<<8 | uint16(cdat[8*x+7])
+				nrgba64.SetNRGBA64(x, y, color.NRGBA64{rcol, gcol, bcol, acol})
+			}
+		}
+
+		// The current row for y is the previous row for y+1.
+		pr, cr = cr, pr
+	}
+
+	return img, nil
+}
+
+// mergePassInto merges a single pass into a full sized image.
+func (d *decoder) mergePassInto(dst image.Image, src image.Image, pass int) {
+	p := interlacing[pass]
+	var (
+		srcPix        []uint8
+		dstPix        []uint8
+		stride        int
+		rect          image.Rectangle
+		bytesPerPixel int
+	)
+	switch target := dst.(type) {
+	case *image.Alpha:
+		srcPix = src.(*image.Alpha).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 1
+	case *image.Alpha16:
+		srcPix = src.(*image.Alpha16).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 2
+	case *image.Gray:
+		srcPix = src.(*image.Gray).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 1
+	case *image.Gray16:
+		srcPix = src.(*image.Gray16).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 2
+	case *image.NRGBA:
+		srcPix = src.(*image.NRGBA).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 4
+	case *image.NRGBA64:
+		srcPix = src.(*image.NRGBA64).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 8
+	case *image.Paletted:
+		srcPix = src.(*image.Paletted).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 1
+	case *image.RGBA:
+		srcPix = src.(*image.RGBA).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 4
+	case *image.RGBA64:
+		srcPix = src.(*image.RGBA64).Pix
+		dstPix, stride, rect = target.Pix, target.Stride, target.Rect
+		bytesPerPixel = 8
+	}
+	s, bounds := 0, src.Bounds()
+	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
+		dBase := (y*p.yFactor+p.yOffset-rect.Min.Y)*stride + (p.xOffset-rect.Min.X)*bytesPerPixel
+		for x := bounds.Min.X; x < bounds.Max.X; x++ {
+			d := dBase + x*p.xFactor*bytesPerPixel
+			copy(dstPix[d:], srcPix[s:s+bytesPerPixel])
+			s += bytesPerPixel
+		}
+	}
+}
+
+func (d *decoder) parseacTL(length uint32) (err error) {
+	if length != 8 {
+		return FormatError("bad acTL length")
+	}
+	if _, err := io.ReadFull(d.r, d.tmp[:8]); err != nil {
+		return err
+	}
+
+	d.num_frames = binary.BigEndian.Uint32(d.tmp[:4])
+	d.a.LoopCount = uint(binary.BigEndian.Uint32(d.tmp[4:8]))
+
+	d.crc.Write(d.tmp[:8])
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parsefcTL(length uint32) (err error) {
+	if length != 26 {
+		return FormatError("bad fcTL length")
+	}
+	if _, err := io.ReadFull(d.r, d.tmp[:26]); err != nil {
+		return err
+	}
+
+	if d.frame_index >= len(d.a.Frames) {
+		d.a.Frames = append(d.a.Frames, Frame{})
+	}
+
+	d.a.Frames[d.frame_index].IsDefault = false
+	d.a.Frames[d.frame_index].width = int(int32(binary.BigEndian.Uint32(d.tmp[4:8])))
+	d.a.Frames[d.frame_index].height = int(int32(binary.BigEndian.Uint32(d.tmp[8:12])))
+	d.a.Frames[d.frame_index].XOffset = int(binary.BigEndian.Uint32(d.tmp[12:16]))
+	d.a.Frames[d.frame_index].YOffset = int(binary.BigEndian.Uint32(d.tmp[16:20]))
+	d.a.Frames[d.frame_index].DelayNumerator = binary.BigEndian.Uint16(d.tmp[20:22])
+	d.a.Frames[d.frame_index].DelayDenominator = binary.BigEndian.Uint16(d.tmp[22:24])
+	d.a.Frames[d.frame_index].DisposeOp = byte(d.tmp[24])
+	d.a.Frames[d.frame_index].BlendOp = byte(d.tmp[25])
+
+	d.crc.Write(d.tmp[:26])
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parsefdAT(length uint32) (err error) {
+	if _, err := io.ReadFull(d.r, d.tmp[:4]); err != nil {
+		return err
+	}
+	d.crc.Write(d.tmp[:4])
+	d.idatLength = length - 4
+	d.a.Frames[d.frame_index].Image, err = d.decode()
+	if err != nil {
+		return err
+	}
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parseIDAT(length uint32) (err error) {
+	d.idatLength = length
+	d.a.Frames[d.frame_index].Image, err = d.decode()
+	if err != nil {
+		return err
+	}
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parseIEND(length uint32) error {
+	if length != 0 {
+		return FormatError("bad IEND length")
+	}
+	return d.verifyChecksum()
+}
+
+func (d *decoder) parseChunk() error {
+	// Read the length and chunk type.
+	n, err := io.ReadFull(d.r, d.tmp[:8])
+	if err != nil {
+		return err
+	}
+	length := binary.BigEndian.Uint32(d.tmp[:4])
+	d.crc.Reset()
+	d.crc.Write(d.tmp[4:8])
+
+	// Read the chunk data.
+	switch string(d.tmp[4:8]) {
+	case "IHDR":
+		if d.stage != dsStart {
+			return chunkOrderError
+		}
+		d.stage = dsSeenIHDR
+		return d.parseIHDR(length)
+	case "PLTE":
+		if d.stage != dsSeenIHDR {
+			return chunkOrderError
+		}
+		d.stage = dsSeenPLTE
+		return d.parsePLTE(length)
+	case "tRNS":
+		if cbPaletted(d.cb) {
+			if d.stage != dsSeenPLTE {
+				return chunkOrderError
+			}
+		} else if d.stage != dsSeenIHDR {
+			return chunkOrderError
+		}
+		d.stage = dsSeentRNS
+		return d.parsetRNS(length)
+	case "acTL":
+		if d.stage >= dsSeenIDAT {
+			return chunkOrderError
+		}
+		return d.parseacTL(length)
+	case "fcTL":
+		if d.stage >= dsSeenIDAT {
+			d.frame_index = d.frame_index + 1
+		}
+		return d.parsefcTL(length)
+	case "fdAT":
+		if d.stage < dsSeenIDAT {
+			return chunkOrderError
+		}
+		d.stage = dsSeenfdAT
+		return d.parsefdAT(length)
+	case "IDAT":
+		if d.stage < dsSeenIHDR || d.stage > dsSeenIDAT || (d.stage == dsSeenIHDR && cbPaletted(d.cb)) {
+			return chunkOrderError
+		} else if d.stage == dsSeenIDAT {
+			// Ignore trailing zero-length or garbage IDAT chunks.
+			//
+			// This does not affect valid PNG images that contain multiple IDAT
+			// chunks, since the first call to parseIDAT below will consume all
+			// consecutive IDAT chunks required for decoding the image.
+			break
+		}
+		d.stage = dsSeenIDAT
+		return d.parseIDAT(length)
+	case "IEND":
+		if d.stage < dsSeenIDAT {
+			return chunkOrderError
+		}
+		d.stage = dsSeenIEND
+		return d.parseIEND(length)
+	}
+	if length > 0x7fffffff {
+		return FormatError(fmt.Sprintf("Bad chunk length: %d", length))
+	}
+	// Ignore this chunk (of a known length).
+	var ignored [4096]byte
+	for length > 0 {
+		n, err = io.ReadFull(d.r, ignored[:min(len(ignored), int(length))])
+		if err != nil {
+			return err
+		}
+		d.crc.Write(ignored[:n])
+		length -= uint32(n)
+	}
+	return d.verifyChecksum()
+}
+
+func (d *decoder) verifyChecksum() error {
+	if _, err := io.ReadFull(d.r, d.tmp[:4]); err != nil {
+		return err
+	}
+	if binary.BigEndian.Uint32(d.tmp[:4]) != d.crc.Sum32() {
+		return FormatError("invalid checksum")
+	}
+	return nil
+}
+
+func (d *decoder) checkHeader() error {
+	_, err := io.ReadFull(d.r, d.tmp[:len(pngHeader)])
+	if err != nil {
+		return err
+	}
+	if string(d.tmp[:len(pngHeader)]) != pngHeader {
+		return FormatError("not a PNG file")
+	}
+	return nil
+}
+
+// Decode reads an APNG file from r and returns it as an APNG
+// Type. If the first frame returns true for IsDefault(), that
+// frame should not be part of the a.
+// The type of Image returned depends on the PNG contents.
+func DecodeAll(r io.Reader) (APNG, error) {
+	d := &decoder{
+		r:           r,
+		crc:         crc32.NewIEEE(),
+		frame_index: 0,
+		a:           APNG{Frames: make([]Frame, 1)},
+	}
+	d.a.Frames[0].IsDefault = true
+	if err := d.checkHeader(); err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		return d.a, err
+	}
+	for d.stage != dsSeenIEND {
+		if err := d.parseChunk(); err != nil {
+			if err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return d.a, err
+		}
+	}
+	return d.a, nil
+}
+
+// Decode reads an APNG file from r and returns the default image.
+func Decode(r io.Reader) (image.Image, error) {
+	a, err := DecodeAll(r)
+	if err != nil {
+		return nil, err
+	}
+	return a.Frames[0].Image, nil
+}
+
+// DecodeConfig returns the color model and dimensions of a PNG image without
+// decoding the entire image.
+func DecodeConfig(r io.Reader) (image.Config, error) {
+	d := &decoder{
+		r:   r,
+		crc: crc32.NewIEEE(),
+	}
+	if err := d.checkHeader(); err != nil {
+		if err == io.EOF {
+			err = io.ErrUnexpectedEOF
+		}
+		return image.Config{}, err
+	}
+	for {
+		if err := d.parseChunk(); err != nil {
+			if err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return image.Config{}, err
+		}
+		paletted := cbPaletted(d.cb)
+		if d.stage == dsSeenIHDR && !paletted {
+			break
+		}
+		if d.stage == dsSeenPLTE && paletted {
+			break
+		}
+	}
+	var cm color.Model
+	switch d.cb {
+	case cbG1, cbG2, cbG4, cbG8:
+		cm = color.GrayModel
+	case cbGA8:
+		cm = color.NRGBAModel
+	case cbTC8:
+		cm = color.RGBAModel
+	case cbP1, cbP2, cbP4, cbP8:
+		cm = d.palette
+	case cbTCA8:
+		cm = color.NRGBAModel
+	case cbG16:
+		cm = color.Gray16Model
+	case cbGA16:
+		cm = color.NRGBA64Model
+	case cbTC16:
+		cm = color.RGBA64Model
+	case cbTCA16:
+		cm = color.NRGBA64Model
+	}
+	return image.Config{
+		ColorModel: cm,
+		Width:      int(d.a.Frames[0].width),
+		Height:     int(d.a.Frames[0].height),
+	}, nil
+}
+
+func init() {
+	image.RegisterFormat("apng", pngHeader, Decode, DecodeConfig)
+}