[Mesa-dev] [PATCH 04/12] mesa/format_info: Add support for the BPTC layout

[Neil Roberts]

Jason Ekstrand <jason at jlekstrand.net> writes:

> Sorry, said that just a little early.  Do we really want 4 bits for a
> floating-point format?  How many bits does nvidia report?

NVidia reports the RGB components as 8/8/8 for the two normalized
formats and 32/32/32 for the two half-float formats. I think the 8 makes
some sense because the interpolation phase of the decompression is done
on an 8-bit value so it you wanted to accurately store all of the
potential values of the decompressed image you would need 8 bits. The 32
bits is a bit more weird because the decompressor can only generate
half-float values so at most 16 bits should be needed. I don't know what
it says for the alpha components because I just now realised that I made
a mistake in the quick test code that I wrote late last night when I had
access to an NVidia card.

It also says 8/8/8 for the DXT3 and DXT5 formats of S3TC so I guess if
we wanted to copy what NVidia does we could also change the bits we
report for those formats.

Regards,
- Neil