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Caustics are an addition to your rendering that calculates photons of light starting at a light source (like real light), which can be reflected, refracted, bounced off mirrors, or concentrated by a lens, accurately simulating more of the ways real light can move through a scene.It's the term used that describes the patterns of light and color that occur when light rays are reflected or refracted from a surface. Caustics can be seen in everyday life, from the light distortions at the bottom of a swimming pool to the light patterns being refracted from a transparent object like a vase or wine glass. Caustics are a form of indirect illumination but very specific in how it happens and where it occurs.
Enable GI / Enable Caustics
These are global switches to enable/disable Diffuse GI and Caustics.
Mode/Filename
When "Mode" is set to "Rebuild (don't save)", new Photon Mapping data will be generated each frame. The data will not be automatically saved to disk.
When "Mode" is set to "Rebuild", new Photon Mapping data will be generated each frame. After the Photon Mapping processing stage is complete, the results are written to the user-specified file.
Setting "Mode" to "Load" will skip the processing stage and, instead, load the data from the user-specified file.
Because light-emitted photons are independent from the camera settings, fly-through animations can get away with generating the photon map once (on the first frame) and re-using it for all other frames. If, however, photon-emitting lights move or material settings are animated, a new photon map will have to be generated.
Reflection/Refraction/Combined Tracing Depth
These three settings control the number of times that photons may bounce around the geometry. The defaults should work fine for most scenes.
If you are rendering caustics and don't want reflective caustics but only want refractive caustics, you can try limiting the reflection trace depth to zero.
Max Num GI Photons / GI Search Radius / Max Num Caustics Photons / Caustics Search Radius
During rendering, the photons generated during the Photon Mapping pass are used to shade the final pixels. To get a smooth result, each final shaded pixel needs to find its closest photons and blur them together.
The "Search Radius" setting controls the distance around each shaded pixel that will be used to find photons. This number is scene-dependent and is in world units. Larger values mean smoother but blurrier results while smaller values mean sharper but, sometimes, splotchy results. This setting is very closely tied to the number of photons shot from the lights. If lights have been configured to shoot many photons, then usually the search radius can be shrunk without artifacts. If too few photons were shot from lights, the search radius has to be large in order to not be splotchy.
The "Max Num Photons" controls the upper limit of photons that can be stored in the "Search Radius". Larger numbers will allow the photon shooting algorithm store many photons per unit area which means a larger memory requirement and not necessarily an improved image quality! We recommend leaving this setting at the default value.
Fast Irradiance
Searching for photons during final rendering can be time-consuming and might slow the rendering down. "Fast Irradiance" allows blurry effects to use a faster, pre-processed approximation that needs less searching and is, thus, faster. The preprocessing step happens after the photons have been computed and might take a few seconds – but the savings during rendering are worth it, so it is recommended that you leave this option enabled.
source:redshift doc
source:redshift doc
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