image/png: DecodeConfig sometimes returns a palette, sometimes not

[gopherbot]

by jsummers3456:

For 8-bit paletted PNG images, DecodeConfig() returns a populated palette in the
ColorModel field, though it lacks correct alpha values. For 1, 2, and 4-bit paletted PNG
images, and GIF images, it returns a nil palette.

Test case http://play.golang.org/p/3zMu7XFlQX

Actual output:
 pal4:
 is paletted
 palette is nil

 pal8:
 is paletted
 palette size = 2
 pal[  0] = 65535,    0,    0,65535
 pal[  1] =     0,65535,65535,65535

Expected results: Whether DecodeConfig returns a populated palette should be consistent
throughout the standard library. (I suggest that it never return a populated palette.)
If a palette is returned, it should have correct alpha values.

Compiler: 6g
OS: Linux
version: devel +8d919bfe75d3 Tue Oct 23 02:33:51 2012 +0800

[griesemer]

Comment 1:

Owner changed to @nigeltao.

[nigeltao]

Comment 2:

That play.golang.org program gives the raw bytes; can you describe how that test image
was generated?

[rsc]

Comment 3:

Labels changed: added size-m.

[rsc]

Comment 4:

Labels changed: added priority-later, removed priority-triage.

Status changed to Accepted.

[vdobler]

Comment 5:

I admit I have some problems understanding this issue:
What is wrong with the alpha values? Why is 0xFFFF as 
returned by RGBA not the right value?

[vdobler]

Comment 6:

But it is true, that reading the PLTE chunk is done for
8bit images only:
http://tip.golang.org/src/pkg/image/png/reader.go?s=16273:16287#L634
This is inconsistent and I believe not backed by the spec.

[nigeltao]

Comment 7:

This issue was closed by revision 9d6e027.

Status changed to Fixed.