Hey Ninjin,
I made a blueprint function that should be in 4.13 that make it easier to control brush size and optimize RTs by only drawing specified area. It is called “Set Canvas Material Scale and Position”. It will scale the RT material to fit the desired size. here is how to use it:
CanvasSize: hook the “Size” pin from “Begin Draw Canvas to Render Target”.
Position: This is the 0-1 position for the center of the material. 0.5 would be at the center of the RT.
Scale: If its set to 1, it will fit the material to the whole render target. If its 0.1, it will take up 10%.
If you set it up this way, you don’t need a position at all inside of your brush material and then you can use a simple texture. The postion would just be decided in the BP and used to position the material itself.
You can also do all of this in the material as well, but you will be paying to render all pixels even if your mask ends up being small. And you will have to use UV math to control the positioning of the textures. I also have some examples of how to do that but suggest people try using the BP macro mentioned above first since its a bit easier.
(edit: just looked at the function and 4.13 and for some reason it was marked as Non Pure. Shouldn’t affect anything but just know that it was meant to be a Pure function without exeuction pins. This is a known engine bug).