So you want to do caustics? But either haven't got a software which can do them for you or don't need them ultra-realistic but fast?
Then here you go. This tutorial is done with 3DStudio Max 3.0 but can be done with any rendering-software which got the features you need for fake-caustics (that's: colored shadows and falloff transparency).
Ok. We start up Max and generate a simple scene: a nice cylinder on an even nicer plane with an even more nicer spotlight. I guess you know how to do this, otherwise get yourself a "Max for beginners" tutorial and come back later.
So here's our scene:
Next we are going to assign some material. Let the floor for example be concrete, the cylinder gets a standard blue glass material (you don't need to use raytrace material unless you want to; we will assign a raytraced refraction to the glass, but that has nothing to do with the caustics, it's just for a better look). Notice that we turn down the refraction to about 65 or you won't see much of the "blue" in your blue glass.
Turning on raytrace-shadows on the spotlight gives us something like this (note: it is necessary to use raytrace shadows; shadow maps ignore transparency and that's definitly not what we want):
Now for the first "trick": in the material editor for the glass material go to "Extended Parameters" and switch the Falloff to "out" with an amount of 100.
We end up with:
Hmmm... Seems the way to go, but not exactly what we want. So here's the next trick: we are going to clone our spotlight twice and set the shadow color to blue and yellow. Note that these colors add up to a bright white if combined. We choosed blue (and yellow respectively) because our cylinder is blue. Those colors need to be adjusted if your object has any other color. So set one of the clone shadow colors to the object color and the other to its complement color. Additionaly we set the light color of the two clones to black, otherwise we'll get a very, very white scene. If you don't see much of an effect after rendering, try increasing the density of the shadows.
After rendering it looks like:
Getting nearer, aren't we? But where's this famous blue I talked about? The answer is simple: Max can't do it. (not automatically) But that's why we set up two clones. All we have to do now is adjust the density of them, so the blue one's a little brighter than the yellow one:
So nice "thing" there... Does not look like a shadow, doesn't it? But (remember! ;) ) that's not what we want. What we want are caustics. But caustics are not scattered over the shadow surface, they are centralized (somehow...). Easy. We adjust the filter color in our glas material ("Extended parameters"-rollout) to simple white and end up with:
So here are the caustics. To get the a shadow additionaly, we just switch the shadow-type of our first spot to "shadow-map", change the shadow color to something blue and voil:
You can also play with the attenuation of your blue and yellow spotlights. Note that everything you do now must be done in both your shadow-color-lights, otherwise it'll look weird. But (as always) experiment...