png support for 1, 2 or 4 bit depths?

[webisu]

Does g2c support png-encoded grib files with bit depths of 1, 2 or 4?

[DusanJovic-NOAA]

As far as I can tell, no. The reason is that in png decoding routine, if the image is encoded with less than 8 bits per pixel, this line:

174     bytes = bit_depth / 8;
175     clen = (*width) * bytes;
176     for (j = 0; j < *height; j++)

for bit_depth less than 8 (ie. 1, 2 or 4) will incorrectly set bytes to 0, and then clen will be 0, which is incorrect.

Instead, to support bit depths < 8, I think this should be changed to either bytes = (width * nbits) / 8 or to  bytes = (width * nbits) / 8 + 1 depending on whether width * bits is divisible by 8 or not.
And probably the loop that copies data from png arrays to g2c arrays needs to be changed as well.

I can try to update the code if we need to support this feature.

[DusanJovic-NOAA]

Are there sample files encoded with 1, 2 and 4 bit depths? Is this needed for production?

[webisu]

I have a sample file for 4 bits. This problem was reported to wgrib2 support and fixed 2021 by someone working for the USAF. This is part of an effort to get wgrib2 to use the g2c code. In order for this to happen, some g2c code need to upgraded. There will be another issue submitted to support larger grids with png. OTOH wgrib2 will have to upgraded to support Jasper v2+.

…

On Wed, Oct 2, 2024 at 3:42 PM Dusan Jovic ***@***.***> wrote: Are there sample files encoded with 1, 2 and 4 bit depths? Is this needed for production? — Reply to this email directly, view it on GitHub <#508 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEIB7ZUSA4SZX4DNYKL547LZZRECVAVCNFSM6AAAAABOOMOM52VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDGOBZGU2DCNJWHA> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[DusanJovic-NOAA]

See changes in #526