They're using Unreal Engine 3 (not 4), which is cross-platform. But if they've written a lot of custom shaders for DirectX (HLSL), they would have to rewrite th

They're using Unreal Engine 3 (not 4), which is cross-platform. But if they've written a lot of custom shaders for DirectX (HLSL), they would have to rewrite them for OpenGL (GLSL). In general, it's getting easier and easier to write once and run anywhere. You can build amazing things in UE4 by staying entirely within the bounds of the engine, and never dropping down into platform-specific solutions.