It hooks into the DirectX fixed-function pipeline used back in the day (i.e. where you set up model/view/projection matrices and lightsource coordinates and sur

It hooks into the DirectX fixed-function pipeline used back in the day (i.e. where you set up model/view/projection matrices and lightsource coordinates and surface materials and sent to OpenGL/Direct3D), so it knows what the programmer wanted to do, and from there it's "easy" to replace everything. Excerpt: "Scene manager, which uses information coming through the D3D9 fixed function API to create a representation of the original scene, track game objects frame to frame, and set up the scene to be path traced."