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image/png: degrade gracefully for palette index values that aren't

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defined by the PLTE chunk. Such pixels decode to opaque black,
which matches what libpng does.

Fixes #4319.

On my reading, the PNG spec isn't clear whether palette index values
outside of those defined by the PLTE chunk is an error, and if not,
what to do.

Libpng 1.5.3 falls back to opaque black. png_set_PLTE says:

/* Changed in libpng-1.2.1 to allocate PNG_MAX_PALETTE_LENGTH instead
 * of num_palette entries, in case of an invalid PNG file that has
 * too-large sample values.
 */
png_ptr->palette = (png_colorp)png_calloc(png_ptr,
        PNG_MAX_PALETTE_LENGTH * png_sizeof(png_color));

ImageMagick 6.5.7 returns an error:

$ convert -version
Version: ImageMagick 6.5.7-8 2012-08-17 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2009 ImageMagick Studio LLC
Features: OpenMP
$ convert packetloss.png x.bmp
convert: Invalid colormap index `packetloss.png' @ image.c/SyncImage/3849.

R=r
CC=golang-dev
https://golang.org/cl/6822065
This commit is contained in:
Nigel Tao 2012-11-01 11:46:06 +11:00
parent 15e50d7c7a
commit de6bf20496
1 changed files with 20 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

34
src/pkg/image/png/reader.go
View File

@ -193,10 +193,18 @@ func (d *decoder) parsePLTE(length uint32) error {
d.crc.Write(d.tmp[:n])
switch d.cb {
case cbP1, cbP2, cbP4, cbP8:
d.palette = color.Palette(make([]color.Color, np))
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 isn't
// clear whether palette index values outside of those defined by the PLTE
// chunk is an error: libpng 1.5.13 falls back to opaque black, the
// same as we do here, 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).
@ -221,8 +229,8 @@ func (d *decoder) parsetRNS(length uint32) error {
case cbTC8, cbTC16:
return UnsupportedError("truecolor transparency")
case cbP1, cbP2, cbP4, cbP8:
if n > len(d.palette) {
return FormatError("bad tRNS length")
if len(d.palette) < n {
d.palette = d.palette[:n]
}
for i := 0; i < n; i++ {
rgba := d.palette[i].(color.RGBA)
@ -279,7 +287,6 @@ func (d *decoder) decode() (image.Image, error) {
}
defer r.Close()
bitsPerPixel := 0
maxPalette := uint8(0)
pixOffset := 0
var (
gray *image.Gray
@ -308,7 +315,6 @@ func (d *decoder) decode() (image.Image, error) {
bitsPerPixel = d.depth
paletted = image.NewPaletted(image.Rect(0, 0, d.width, d.height), d.palette)
img = paletted
maxPalette = uint8(len(d.palette) - 1)
case cbTCA8:
bitsPerPixel = 32
nrgba = image.NewNRGBA(image.Rect(0, 0, d.width, d.height))
@ -421,8 +427,8 @@ func (d *decoder) decode() (image.Image, error) {
b := cdat[x/8]
for x2 := 0; x2 < 8 && x+x2 < d.width; x2++ {
idx := b >> 7
if idx > maxPalette {
return nil, FormatError("palette index out of range")
if len(paletted.Palette) <= int(idx) {
paletted.Palette = paletted.Palette[:int(idx)+1]
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 1
@ -433,8 +439,8 @@ func (d *decoder) decode() (image.Image, error) {
b := cdat[x/4]
for x2 := 0; x2 < 4 && x+x2 < d.width; x2++ {
idx := b >> 6
if idx > maxPalette {
return nil, FormatError("palette index out of range")
if len(paletted.Palette) <= int(idx) {
paletted.Palette = paletted.Palette[:int(idx)+1]
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 2
@ -445,18 +451,18 @@ func (d *decoder) decode() (image.Image, error) {
b := cdat[x/2]
for x2 := 0; x2 < 2 && x+x2 < d.width; x2++ {
idx := b >> 4
if idx > maxPalette {
return nil, FormatError("palette index out of range")
if len(paletted.Palette) <= int(idx) {
paletted.Palette = paletted.Palette[:int(idx)+1]
}
paletted.SetColorIndex(x+x2, y, idx)
b <<= 4
}
}
case cbP8:
if maxPalette != 255 {
if len(paletted.Palette) != 255 {
for x := 0; x < d.width; x++ {
if cdat[x] > maxPalette {
return nil, FormatError("palette index out of range")
if len(paletted.Palette) <= int(cdat[x]) {
paletted.Palette = paletted.Palette[:int(cdat[x])+1]
}
}
}