These parameters affect the calculation phase of the light cache; they do not affect the final rendering until the light cache is used.
Preset – Offers default Light cache settings for either Still or Animation rendering.
Subdivs – Determines how many paths are traced from the camera. The actual number of paths is the square of the subdivs (the default 500 subdivs mean that 250 000 paths will be traced from the camera). For more information, see The Subdivs Parameter example below.
Sample size – Determines the spacing of the samples in the light cache. Smaller numbers mean that the samples are closer to each other, the light cache preserves sharp details in lighting, but it is more noisy and takes more memory. Larger numbers smooth out the light cache but loses detail. This value can be either in world units or relative to the image size, depending on whether or not the the World scale option is enabled or not. For more information, see The Sample Size Parameter example below.
Show calc. phase – Enabling this option will show the paths that are traced. This does not affect the calculation of the light cache and is provided only as a feedback to the user. This option is ignored when rendering to fields - in that case, the calculation phase is never displayed.
Store direct light – When this option is enabled, the light cache will also store and interpolate direct light. This can be useful for scenes with many lights and irradiance map or direct GI method for the primary diffuse bounces, since direct lighting will be computed from the light cache, instead of sampling each and every light. Note that only the diffuse illumination produced by the scene lights will be stored. If you want to use the light cache directly for approximating the GI while keeping the direct lighting sharp, uncheck this option.
Use retrace threshold– When enabled, this option and its corresponding Retrace threshold value improve the precision of global illumination in cases where the light cache will produce too large an error. This is especially obvious with near corners where light leaks might be possible because of the light cache interpolation. For glossy reflections and refractions, V-Ray dynamically decides whether to use the light cache or not based on the surface glossiness and the distance from it so that the errors due to the light cache are minimized. Note that enabling this option can increase render time.
Path guiding (Experimental) – Uses a path guiding method (Intel® Open Path Guiding Library) to optimize the sampling quality. Path guiding is useful for scenes where GI or glossy reflections are hard to sample, such as interior scenes or scenes with a lot of occlusion. It is supported by VRayMtl, Environment Fog with enabled GI and Volume Grid with enabled GI. When using this option, note that the Subdivs should be at least 1000, and for same scenes - up to 3000. This option is available with V-Ray CPU only.
Note that Path guiding produces different results, when Max ray intensity or Subpixel mapping are enabled compared to when switched off. Different results in the raw render elements are expected, but they produce the correct result when multiplied by the respective filter render element. In some scenes containing diffuse surfaces with low poly count, the results may differ compared to rendering with this option switched off.
