It would be really beneficial to support single channel compression for ASTC. On DX11 we can use BC4 for multiple things like ao, roughness, metalness, alpha, masks and even for stationary shadowmaps(area shadows don’t mind compression artefacts that much)
This would save lot of package size, memory and bandwith. With quality settings of 3 this format compression ratio is 0.445. With quality settings of 2 compression ratio is 0.33375.
ASTC support various texture formats so there really isn’t any reasons not to support single channel format.
Shadowmap compression PR. https://github.com/EpicGames/UnrealEngine/pull/3746