My goal is to paint areas of transparency/alpha onto a texture on a side of a part, which would reveal the part surface itself and its color and material. I want this to be done without just using a separate texture file which has transparency already on it (not a very dynamic solution).
Unfortunately, I could not find any specific feature that makes this possible, even after looking over the Roblox developer wiki and looking up solutions. If I didn’t look hard enough, or you know a workaround or particular method that makes this possible, then do let me know!
Because every individual image has to be uploaded individually and pass through moderation, I don’t think roblox has any in-engine methods for advanced image editing beyond basic whole-image properties.
But you might be able to look into the UI gradient. I’m not sure it if allows gradients to be in 2-dimension (X,Y) or whether it’s restricted to a single axis. At least the default editor for the colour/transparency gradient only allows it to be changed along one axis, but there might(?) be a method I don’t know of to apply transparency gradients in splotches rather than stripes. Maybe you could layer two UI textures over each other, with a vertical transparency gradient on one and horizontal on the other, but that would still only be able to make inaccurate squares.
But I think this isn’t likely possible with the current engine. I suppose an alternate method would be to have a thin part with the texture and just call “:NegateAsync()” to negate holes into the part.