Set the default lighting mode for interactive and still frame antialiasing of materials.
CPU Rasterization - Does not compute direct reflection, refraction, or any other sophisticated visual effect.
Precomputed Illumination - Comparable to VRED OpenGL rendering mode, this uses precomputed ambient occlusion and indirect illumination for rendering and calculates specular reflections and refractions and correct shadows from light sources.
Precomputed + Shadows - Uses precomputed image-based lighting and indirect illumination, but does not use precomputed ambient occlusion values. Instead, it calculates shadows based on the active environment.
Precomputed + IBL - Uses precomputed indirect illumination and samples the environment.
Full Global Illumination - Does not use any precomputed values, however, it accurately samples the global lighting distribution in a physically-based approach. Some features like Photon Mapping require the render mode to be set to Full Global Illumination.
Calculate the global illumination in a scene. The default VRED method, full global illumination, provides high-quality results, but may require longer calculation times. Photon Tracing can reduce the time required to render a clean image, especially indoor scenes.
The most common photon tracing mode used is Indirect Only. Caustics + Indirect mode calculates indirect illumination and caustics, due to specular materials in a scene.
Trace Depth - Sets the number of reflections taken into account during raytracing, when calculating the color of a photon or ray.
Interactive and Still Frame Count - These two photon count values specify the number of photons being sent into the scene for each image sample. Specifying a photon count of 100 photons, while setting the image samples to 256, results in 25,600 photons sent into the scene for a frame. More photons result in smoother results.
Photon Radius - Specifies the radius around a scene’s hit-point used by the raytracer to find photons. A larger radius allows the raytracer to find more photons, but may result in slower lookup times.
Interactive and Still Frame Final Gather Quality - There are two ways to use the photon map. The first uses caustic photons. It gathers photons around a hitpoint to calculate the incoming illumination. This gives fast interactive performance and calculates all light paths in a scene; however, it might require a high photon count to get a clean image. The second way is to use final gathering. In final gathering, a one-bounce indirect illumination is performed before evaluating the photon maps. This function is the default photon-tracing approach in VRED, since it generates high-quality images in a short time. Setting the final gather quality to Off enables the first approach, while setting it to any other value uses the second approach.
Final Gather Radius - Sets the lookup radius used to find the nearest final gather point during raytracing. Using a smaller radius increases performance, but requires more photons to avoid dark regions.
Final Gather Refresh - If Final Gather Quality is set to 1 or higher, the update frequency of the photon map can be set. By default, photon maps are updated for each image sample, sending many photons into the scene. If Final Gather Quality is set to Off, it is often enough to update the photon map only once per frame and use it for each image sample to reduce the render times.
Use Final Gather for Glossy Reflections - Stops glossy reflections from being evaluated by path tracing and uses the final gather map, instead. This feature reduces the render time, but results in less accurate reflections.
Photon tracing is useful in interior scenes. In exterior scenes, most light illuminates the scene directly, so the VRED default mode of full global illumination is likely to offer better performance.
The number of photons emitted is not the same as the number of photons stored. If a photon misses the scene, it is not stored. A photon bouncing several times in a scene might be stored more than once. To keep the number of emitted photons as low as possible, place any light emitters in a way that most photons hit the scene.
When using final gathering, scenes may suffer from light leaks, if the photon radius is too large. Light leaks result mainly from bad geometry in architectural scenes. An example would be the interior of a room illuminated through a window, with the walls being modeled as simple planes. Any geometry near the wall would get light from outside the room, since there is no actual wall thickness. The solution would be to model the outer walls, as well. Reducing the lookup radius may also fix the problem, but may require shooting more photons into the scene.
Final gathering may also show problems in scenes with strong indirections, where the scene is primarily illuminated by light, resulting from reflections off a wall. In these situations, disabling final gathering may give you a cleaner result.