Hi Jenny,
could you elaborate a little more on why 1-bit alpha can’t be use for a metalness mask? Most metallic masks only need pure black and pure white values, so why wouldn’t a mask stored in the alpha channel with only those 2 values wouldn’t work for metalness? We didn’t have PBR in our old game engine, so we used 1-bit alpha for masking color modulation areas, or for foliage probability masks. Now I understand that a 1-bit alpha wouldn’t work for a roughness mask for example, since it needs more then 2 grayscale values, but I would think a metallic mask would be the perfect candidate for that (only if you want either pure metal and pure non-metal).
Thanks!