[Mesa-dev] [PATCH v2] mesa/format_info: Add support for the BPTC layout

[Neil Roberts]

I'm not sure if we came to a decision about what to do with the
channel bits. I guess it's not really that important because probably
nothing uses it for compressed formats except to check for the
existence of the channels. Here is a v2 patch which picks 8/8/8/8 for
the RGBA formats and 16/16/16 for the half-float formats. Would that
be ok?

I guess we should pick a policy and make it consistent for the other
formats too but perhaps we can leave that to a later patch. It looks
like the NVidia approach might be to return a number of bits that you
would need to create a texture to represent the possible generated
values from the decompressor. However that doesn't quite add up for
the half-float formats because in that case you could accurately
represent the values with a half-float texture and only 16 bits.

If we wanted to pick that policy we would also have to change what we
return for the S3TC formats because they can also return 8 bits of
variance. The ETC2 formats already return 8.

- Neil

------- >8 --------------- (use git am --scissors to automatically chop here)

Adds the ‘bptc’ layout to get_channel_bits. The channel bits for BPTC depend
on the mode but as it only has to be an approximation this sets it to 8 for
the two UNORM formats and 16 for the two half-float formats. These represent
the minimum number of bits of variation that can be generated by the
interpolation of the two formats.

This doesn't quite match what we do for S3TC which only returns 4 even though
it can similarly generate 8 bits from the interpolation. However it does match
what we return for ETC2. For reference, NVidia seems to return 8 bits for the
UNORM formats and 32 bits for the half-float formats.

v2: Change the number of bits to 8/8/8/8 for the UNORM formats and 16/16/16
    for the half-float formats.
---
 src/mesa/main/format_info.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/mesa/main/format_info.py b/src/mesa/main/format_info.py
index a0eecd3..7424fe0 100644
--- a/src/mesa/main/format_info.py
+++ b/src/mesa/main/format_info.py
@@ -127,6 +127,9 @@ def get_channel_bits(fmat, chan_name):
 
          bits = 11 if fmat.name.endswith('11_EAC') else 8
          return bits if fmat.has_channel(chan_name) else 0
+      elif fmat.layout == 'bptc':
+         bits = 16 if fmat.name.endswith('_FLOAT') else 8
+         return bits if fmat.has_channel(chan_name) else 0
       else:
          assert False
    else:
-- 
1.9.3