Here are the lookup tables you can use to determine how AMD GPU's decode BC1 textures: https://pastebin.com/raw/LSgn0ent
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 appsdocs.microsoft.com
Converting the tables to formulas sounds like an interesting puzzle.
Example showing exactly how to use the tables to decode AMD BC1:
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 appsdocs.microsoft.com
Converting the tables to formulas sounds like an interesting puzzle.
Example showing exactly how to use the tables to decode AMD BC1:
