QUOTE
a volume caustic is a caustic in a volume space,for example,
in water.i just use the plugin from www.aliaswavefront.com
but i cant create the effect in volume(only on a surface).could anyone help me?
You have to wait until someone develops a Mel that uses the caustics rays for the volume too.. Or use Mental ray for Maya. It has REAL volumetric caustics (very expensive).
IMHO, there is always a way to fake volumetric caustics especially in water by taking a animated caustics video loop, and apply it to fog. Then use the fog parameters to blur/focus/distance/etc.. the effect. On the Ground of the water you can apply the caustics plugin OR use the same sequence (which is better especially if the water atmosphere should be sincronised in motion with the ground) of caustics as an animated texture. The setup is much more difficult than simply pressing a volumetric caustics plugin button, but it works even on noncaustic-renderers like Prman, and it's a perfekt faker.
Note, that you need precalculated animated caustic textures for that, which should be hires. And there it's VERY important that the refractive surface matches very close the caustics effect of the projected video loop. A glass water wave has different caustics compared to an ocean wave.
It is applicable on ALL solutions (Glass,Water,etc.) but needs a lot of manual tweaking.
ENILNACS
Faking, faking, faking... fak... fa.... f... Why?
'Cause there's no choice :-(...