#68 pngcrush fails to remove gAMA, bKGD, tEXt chunks

[wintakeall]

I can't get pngcrush 1.8.0 to remove gAMA, bKGD, tEXt chunks from some files such as the attached test.png It works with other files though - I don't know what makes the difference.I have with no success tried Windows 32-bit and 64-bit builds with commands such as:

pngcrush -rem alla -ow test.png
pngcrush -rem allb -ow test.png
pngcrush -rem gAMA -ow test.png
...and so on wih bKGD and tEXt, including the lowercase versions.

I can successfully remove these chunks with TweakPNG at: http://entropymine.com/jason/tweakpng/

[Glenn Randers-Pehrson]

Your test.png doesn't seem to have any of those chunks:

glenn.rp> pngcheck -v test.png
File: test.png (1599 bytes)
chunk IHDR at offset 0x0000c, length 13
174 x 200 image, 1-bit palette, non-interlaced
chunk PLTE at offset 0x00025, length 6: 2 palette entries
chunk IDAT at offset 0x00037, length 1524
zlib: deflated, 32K window, default compression
chunk IEND at offset 0x00637, length 0
No errors detected in test.png (4 chunks, 63.7% compression).

[wintakeall]

Indeed when I re-download the attachment I posted it doesn't have those chunks. Turns out SourceForge put it on a diet and it's down from 1741 to 1599 bytes.

Lesson learned: SourceForge strips metadata from picture uploads (and apparently not using pngcrush!) So you can't just upload a straight picture file when metadata matters. Probably very common for sites accepting user content. Wouln't have been a problem for a pic of a GUI bug - normally only pic content matters.

Re-uploading my original file renamed test2.png (1741 bytes long) and now zipped hoping that can shield it from tampering.

[wintakeall]

I have now re-downloaded to verify and as I suspected I could get around SourceForge's tampering with picture metadata by zipping. So download test2.zip, unzip, and you should be able to reproduce this problem.

[Glenn Randers-Pehrson]

Using the "-force" option works.

pngcrush -force -rem alla -ow test2.png
pngcrush -force -rem alla test2.png test2-stripped.png

But pngcrush-1.8.0 and later are supposed to use force by default; I'll have a look at that.

[Glenn Randers-Pehrson]

Thanks. I've found the problem and will fix it in pngcrush-1.8.1.

[wintakeall]

Using the "-force" option works.

I came across another file where even with the -force option, pngcrush 1.8.0 is unable to remove a pHYs chunk, see attached.

[Glenn Randers-Pehrson]

Yesterday's fix also causes pngcrush-1.8.1 to remove the pHYs chunk properly.