The number of photons used to estimate the caustic brightness can be changed with the global option caustic accuracy. The accuracy controls how many photons are considered during rendering. The default is 100; larger numbers make the caustic smoother. There is also an optional radius parameter. The radius controls the maximum distance at which mental ray considers photons. For example, to specify that at most 200 photons should be used to compute the caustic brightness, and that only photons within 1 scene unit away should be used, specify:
caustic accuracy 200 1.0
in the options. (Similar accuracy parameters are available for global illumination, volume caustics and global illumination, and final gathering.)
Accuracy parameters can be used to select two fundamentally different sampling policies. If R is relatively large (as is the default), the overall limiting factor becomes N and R only catches runaway situations with very few photons. Since darker areas have fewer photons than brighter areas, the effective radius within which the N photons are found is larger in dark areas. The effect is that low intensity areas will have less detail than high intensity areas. Also, increasing the number of photons in the scene will result in the effective radius becoming smaller, so if N is not adjusted to compensate, the same amount of noise will be seen, only on a smaller scale.
The other policy is to select R small, on the scale of the detail the user wishes to see. N is then kept high, or even set to 0 (unlimited). In this case, a constant radius is examined which results in the scale of the detail remaining constant between light and dark areas. Increasing the number of photons in the map will have the effect of reducing the error noise. In practice, one will often use a combination of the two, with a small radius to get detail in dark areas, and N set at a moderate value to speed up rendering. Irrespective of the chosen policy, a large effective radius gives less noise, but a more blurred result. To decrease the noise without blurring detail, it is necessary to increase the number of photons in the photon map. It is very instructive to explore the effect of setting these options with the aid of the diagnostic photon option since the false color image it generates shows the difference in estimated density more clearly. For fast previewing of caustics it can be useful to use N=20.
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