Integrator Settings 21
What is an integrator?
In simple terms, the integrator is responsible for tracing the way light travels through the scene, which is the first stage in turning a virtual 3D scene into a 2D image. Because light is capable of many complex effects, most rendering integrators only look for a subset of these effects in order to keep rendering times reasonable. RenderMan RIS has several available raytracing integrators, and each one is tuned for different uses and lighting situations.
A brief summary of each integrator is listed below. If you are looking for detailed information on the settings for each integrator, links into the main Renderman documentation are also provided.
Helpful Tip: PxrPathTracer, PxrVCM and PxrUPBP are the only three options designed to create realistic images. All of the other integrators are designed for troubleshooting or debugging.
Helpful Tip 2: While this is not a hard and fast rule, in many cases the Path Tracer excels at outdoor scenes or scenes lit with Image Based Lighting (IBL). The VCM and UPBP integrators are more useful for interior scenes or other situations with complex indirect lighting, including caustics and volumetric caustics.
Path Tracing: Path Tracing is the main method PRMan's RIS mode uses to render a scene. A virtual light ray is sent into the scene from the camera. That light ray then strikes an object in the scene. Depending on the BXDF assigned to the object, the light ray will either be reflected, absorbed, or transmitted. The ray will continue to bounce around until it either runs out of energy or reaches a 'bounce' limit set by the user. To aid in rendering the direct illumination, at each bounce point the renderer will also attempt to fire a ray from that point directly to one of the light sources in the scene. This source is picked through a sophisticated procedure that looks at a lights intensity, its distance to the object, and its distance to the camera.
Bi-Directional Path Tracing: BDPT is a more complex version of Path Tracing. In addition to the light rays traced from the camera, rays are also sent into the scene from the light sources. Once both of these rays have reached their maximum bounces, the renderer then attempts to connect the bounce points of each path together. Renderman’s implementation of Bi-Directional tracing also includes Vertex Merging, a type of photon tracing. This improves the convergence of caustic light effects.
Unified Point, Beam and Path Tracing: UPBP is an advanced variation on Bi-Directional tracing that includes Beam Tracing in addition to Vertex Merging. This improves the convergence of light caustics in a volume (such as fog). However, this is by far the slowest integrator currently used in Renderman. It is also experimental in Renderman 21, so expect potential glitches and odd behavior in some instances.