Error decoding a valid PNG image

[matt-42]

Hi @bitbank2,

Used PNGdec with success until I tried it on image created by my Iphone 7: It fails to decode this PNG created on IOS 15.7.1 (and all the other png I generated with this phone):

Upgrading my iphone and PNGdec did not help. Also tried to update zlib, did not help either.

The code is failing here: https://github.com/bitbank2/PNGdec/blob/faf03e7597aff70fdd09e1b66c0af306adb9634a/src/inflate.c#L899

 if ((hold & 0xffff) != ((hold >> 16) ^ 0xffff)) {
                strm->msg = (char *)"invalid stored block lengths";
                state->mode = BAD;
                break;
            }

The PNG seems to contains 2 iDat chunks, could the bug related to that ?

the same image in base64 just in case: 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

[bitbank2]

iOS PNGs use a different initialization parameter. I didn't add support for them to this library (yet), but have supported them in the past. Search png.inl for "PIL_COMP_IPHONE_FLATE" and you'll see it. If you want to support this option, make the change to the code and submit a PR.

[matt-42]

Hi @bitbank2, It was not an ios specific problem but some bugs that prevent PNGdec to read png that have multiple IDAT chunks. I fixed it and will send a pull request later when I'll have time.

[bitbank2]

oh - thanks for that. I look forward to seeing your fix.

[urjaman]

oh - thanks for that. I look forward to seeing your fix.

Still looking forward I suppose, but in case you're curious, i think this is https://github.com/bitbank2/PNGdec/blob/master/src/png.inl#L768 which feeds the content following the IDAT chunk (CRC and length of next one etc) into inflate, instead of capping it to the current iLen somehow.

[Tenkir]

I'm encountering this same issue with PNGs from photoshop. Attached is the png I'm attempting to load with the SD slideshow example code. It's a very small (3x11) test pattern I'm attempting to map to an array of WS2811 LEDs.

Looking at the file with a chunk inspector, I only see a single IDAT chunk. pngcheck also gives it an okay with OK: 01.png (3x11, 24-bit RGB, non-interlaced, -9.1%).

I admittedly don't know a ton about the inner workings of PNG so may there's something else I'm missing here.

[matt-42]

This fixes decoding of PNGs created on IOS: https://github.com/bitbank2/PNGdec/pull/19 Hope this fix your issue too.

[Tenkir]

This fixes decoding of PNGs created on IOS: #19 Hope this fix your issue too.

Yes this also solves the issue for photoshop PNGs! Thank you!