Re: Speckled Caustics
First off, its much easier to get a better caustics affect from point/spot lights or very small rectangular lights. From the looks of your scene, it would seam that you have a rather large rectangular light casting the caustics. Its going to be significantly harder to get a smooth result as the nature of the light will lead to blurry caustics.
As to caustics settings, the two main players are Max Photons and Search Distance, and ultimately it comes down to choosing one over the other. V-Ray will keep looking for more photons until either it gets to the Max Photons allowed or that the photons that are being retrieved are beyond the search distance, so its really rather rare that V-Ray takes both into account. In regards to both settings, the smaller they are, the noisier the caustics will be, and the larger they are the smoother the caustics will be. However, its best to understand the words “smaller” and “larger” in relation to the density of caustic photons, which is controlled through subdivisions.
Optimally, you want to take as many photons in consideration as you can, yet limit the ones that are extraneous to that part of the solution. You can control this in a fairly absolute sense with the Search Distance parameter, and although this works well for the sharper areas of your caustics, it may not smooth out the areas where there are less caustics. However, a “good” search distance parameter does not need to scale with photon density, so you don’t really have to worry too much about it in regards to increasing the quality of your caustics.
With Max Photons, there is much more consideration that has to be made for density. Keeping them low for tests (maybe at 15-20) may work in low subdivision situations, but when the caustic subdivisions are increased, the same Max Photon values will leave a lot of the photons in your solution unable to actually contribute to the scene. Therefore, you should consider increasing Max Photons with large subdivision numbers to make sure that those extra photons are utilized. The other good thing is that Max Photons is much more adaptive for different areas of the image, so places with lower photon density will have a chance to get the photons that they need, as opposed being cut off with search distance.
So my recommendation is to choose one and adjust that while having the other at a large enough value not to get in the way. Personally I find better control in the Max Photon setting. One thing to note, and this is a very important fact, all of these settings are not tied to the calculation itself, only the interpretation/reconstruction of that solution for the end result. Therefore, if you save your solution, you can play around with these settings to see which are optimal in your situation.
One final note, these suggestions assume the you have a radius of 0 for Max Density. Changing the Max Density value changes the playing field completely and very few of the recommendations above hold true. Best to leave it as 0 most of the time.
I like your detailed in-depth explanations.