Unreal Engine and Unity may be the most popular engines for game developers today, but there are still some who try to go against the grain, such as Wax Heads using Godot. But Skin Deep is even more radical, as it's a new game made using Id Tech 4 – more popularly referred to as the Doom 3 engine – which is now more than 20 years old.
Having traditionally made its engines open-source, this was the last open-source engine from id Software before it was bought by Bethesda, which still has a passionate community, although Skin Deep is a rare example of the old tech being used for a commercial release, the other being developer Blendo Games' 2016 cyberpunk adventure game Quadilateral Cowboy.
