Objects in the scene
must be defined as a either a transmitter or receiver of photons in
order to cast caustic effects. To define an object as such requires
the caustics transmitter or receiver box in the object visiblity
options to be ticked on. An object, in this case such as the glass,
can be both a transmitter and a receiver. Transmitters and receivers
must be geometric objects and cannot be lights.
transmitting and receiving objects must have a material that
contains a photon shader statement. Photon shaders can store absorb,
reflect or transmit photons. If the photon shader is missing on the
transmitting object photons are not reflected or transmitted by it
and if it is missing on the receiving object no photons are
The transmitter in this scene is the glass window.
Transmittance is the fraction of light which passes through a
material. Caustic casting materials should not be diffuse as diffuse
objects scatter the light rather than focusing it which is what
causes caustic effects.
Caustic receiving materials in the
scene must be diffuse because otherwise no photons are stored. If
many objects in this case such as the walls and floor are to be
receivers all objects can be multiselcted and assigned as receivers.