In the previous chapter, we mentioned that forward lighting has performance limitations. If you recall the basic algorithm for forward lighting, we potentially need to draw every mesh once for each light source. For a scene with N light sources and M meshes, we may need N times M draw calls. This means that every additional mesh is going to add up to N new draw calls and each new light source will add up to M new draw calls. Those numbers add up very quickly and pose a major limitation on the amount of elements you can render in an interactive frame rate.

Over the years, as graphic cards' memory size increased and GPUs commonly supported rendering to multiple render targets (MRT), a couple of solutions to the performance problems were developed. At their core, all the solutions work on the same principle – separate the scene rendering from the lighting. This means that when using those new methods, a scene with M meshes and N light sources is going to only take M + N draw calls...