[Mesa-dev] [PATCH 0/6] Add support for BPTC texture compression

[Neil Roberts]

Matt Turner <mattst88 at gmail.com> writes:

> I was thinking one of the cool things we might be able to do a ETC2 ->
> BC7 transcode on platforms without ETC2 hardware decompression. We
> won't be able to do that without a good compressor.

That sounds like a fun project. It would be a shame to have to
completely decompress it and then recompress it though. It looks like
for ETC1 it wouldn't be too tricky to losslessly map it to mode 3 of
BPTC. You could emulate the 2x4 or 4x2 selection by using either
partition 0 or 13 resepectively and then you could pick 4 endpoint
colours that match the range allowed by the difference values.

The 'T' and 'H' modes of ETC2 would probably be a lot trickier to map
though because the indices effectively let you pick between either of
the two subsets on a per-texel basis. Maybe you could do a
decompress-recompress per-block only if it is using one of these two
modes.

I'm not sure about the planar mode. There might be some complicated way
to represent it with the 3-subsets of mode 0.

- Neil