Hi, I’m trying to export a texture with a complete black alpha channel in BC1a but, in both the preview and the exported file, all the channels are also 0. I don’t know if this is a limitation of the compression or a bug of the program.
-I have both Premultiplied Alpha Blending and Export Premultiplied Alpha disabled.
-BC3 and BC7 seem to work fine.
Hi @endercapito! This is a limitation of the BC1a compression format.
The short explanation is that in BC1, every block has two RGB endpoint colors, which determine a 4-color RGBA palette. Depending on the order of the endpoints, this palette is either {color_0, color_1, lerp(color_0, color_1, 1/3), lerp(color_0, color_1, 2/3)} or {color_0, color_1, lerp(color_0, color_1, 1/2), 0}. The last element in the second palette is the only one that is transparent, and its RGB components are always 0.
Here’s where this appears in the documentation from Microsoft – the key line is color_3 = 0; in the last code block.
BC3 and BC7 don’t have this limitation, because they can encode alpha separately from RGB.
Hope this helps!