Wednesday, April 8, 2020

AMD GPU BC1 decoding lookup tables

Here are the lookup tables you can use to determine how AMD GPU's decode BC1 textures:

These tables were gathered straight from a Radeon RX 580 by using a small D3D9 app that rendered a textured BC1 quad with point sampling and did a CPU readback. I used this same D3D9 app on an NVidia 1080 and the pixels I read back exactly matched what the NV BC1 formulas on the web predicted, so I'm confident in the approach.

For selectors 0 and 1, the 5->8 and 6->8 endpoint conversion just uses bitshifts/OR's (same as ideal BC1). For 4-color selector 2, use the tables. For selector 3, just invert the low/high endpoints. (I've verified you can do this.) For 3-color selector 2, use the tables.

To access the tables, use [color0_component*32+color1_component], or *64 for 6-bits:
Block Compression (Direct3D 10) - Win32

Converting the tables to formulas sounds like an interesting puzzle.

Example showing exactly how to use the tables to decode AMD BC1:

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