From [pov:6.10.2]:
When designing a scene with photons, it helps to think of the scene objects in two categories. Objects in the first category will show photon caustics when hit by photons. Objects in the second category cause photon caustics by reflecting or refracting photons. Some objects may be in both categories, and some objects may be in neither category.
Category 1 - Objects that show photon caustics
By default, all objects are in the first category. Whenever a photon hits an
object, the photon is stored and will later be used to render caustics on that
object. This means that, by default, caustics from photons can appear on any
surface. To speed up rendering, you can take objects out of this category. You
do this with the line: photons { collect off }
. If you use this syntax, caustics
from photons will not appear on the object. This will save both memory and
computational time during rendering.
Category 2 - Objects that cause photon caustics
By default, there are no objects in the second category. If you want your object to cause caustics, you need to do two things. First, make your object into a "target." You do this with thetarget
keyword. This enables light sources to shoot photons at your object. Second, you need to specify if your object reflects photons, refracts photons, or both. This is done with thereflection on
andrefraction on
keywords. To allow an object to reflect and refract photons, you would use the following lines of code inside the object:
photons{ target reflection on refraction on }
Generally speaking, you don't want an object to be in both categories. Most objects that cause photon caustics do not themselves have much color or brightness. Usually they simply refract or reflect their surroundings. For this reason, it is usually a waste of time to display photon caustics on such surfaces. Even if computed, the effects from the caustics would be so dim that they would go unnoticed.
Sometimes, you may also wish to add photons{collect off} to other clear or reflective objects, even if they are not photon targets. Again, this is done to prevent unnecessary computation of caustic lighting.
Finally, you may wish to enable photon reflection and refraction for a surface, even if it is not a target. This allows indirect photons (photons that have already hit a target and been reflected or refracted) to continue their journey after hitting this object.
Andreas Kriegl 2003-07-23