PNG decoder seems to get pixel format wrong about wuffs HOT 4 CLOSED

There are more incorrect readings but i'll leave it at that for now.

from wuffs.

basn0g16.png (16 bit (64k level) grayscale):
bits_per_pixel is incorrectly reported as 8. (should be 16)

Yeah, that's a known bug / TODO.

basn2c16.png (3x16 bits rgb color):
bpp incorrectly reported as 64, should be 48.
channel_3_bits incorrectly reported as 16, should be 0.

Or am I misunderstanding wuffs, and it's doing some automatic conversion in some weird way?

It's sort of doing automatic conversion. Wuffs' pixel swizzler supports converting from pixel format S to pixel format D for a variety of S and D pixel formats, but it's not the full set of all possible pairs of pixel formats. The pixel format set by decode_image_config is conventionally one of the supported S pixel formats. Whilst the PNG file format has 3x16 RGB separate from 4x16 RGBA, the former seems rare enough in practice that it wasn't worth dedicated code paths converting from 3x16 RGB to all of the supported D pixel formats. Instead, for Wuffs' PNG decoder, 3x16 RGB is 'rounded up' to a superset S format: 4x16 RGBA (and the Alpha values are always 0xFFFF).

There are more incorrect readings but i'll leave it at that for now.

I'm curious what the others are, although I suspect they're expanded for the same reason. For example, YA (Gray + Alpha) is expanded to RGBA (with R = G = B = Y).

from wuffs.

Yeah, basn4a08.png, which is 8 bit grayscale + 8 bit alpha-channel, is reported as 32bpp with 4 channels.

from wuffs.

Closing as Working As Intended.

from wuffs.