Technical - Complex Decal Interactions

Persistent decals are always a large part of a Mortal Kombat game. The goal is generally to allow blood to build up infinitely without producing any strange visual artifacts or performance issues. Because of this we ended up with a method using a handful of channels on a virtual texture and leveraging Unreal's material layer system to compartmentalize the effects and efficiently edit them. Not only does each decal "type" write into its own specific channel, it can also erase from other channels, which allows us to mix blood, water and scorch decals in very convincing ways.
Blood will always land on top of everything
Scorch decals will burn away blood and water (even taking longer to remove water)
Water decals will wash everything away and leave a reflective puddle behind
RoamyBot cleans all with incredible efficiency!

MORTAL KOMBAT TM & © Warner Bros. Entertainment Inc. (s24)