from the author:
This article was written with 3ds max 5 in mind. It should be
noted that later versions of 3ds max contain the ability render
caustics due to the inclusion of the Mental Ray renderer.
few issues back we covered how to create reflective caustics using
3ds max 5’s base kit. However, the method described did not
cover how to produce refractive caustics. As there is no automatic
method to create these by default in 3ds max 5, we shall cover how
to convincingly fake them.
foremost, why would we want to fake something that could be easily
produced with a third party renderer? Two things – time and
money. Rendering photon-based lighting can take an age if it’s
not set up properly and to produce effective results you will have
to have a lot of photons flying around to create a nice caustic
effect. And the only way to produce such an effect “properly”
is by purchasing a third party renderer for max and these can cost
a fair amount. Therefore using a faked caustic effect can be more
caustics are displayed around the base of a wine glass (for
example) where the main key light has been refracted and focused
through the change in material density, internal reflection
and emitted onto a surface such as a table. (** insert image
: refractive_caustics01.jpg **) This can be simulated by using
a projection map emitted from a clone of the original key light,
set up to just illuminate the surface(s) that receives the caustic
a relatively simple solution, but how can we create our projection
map? With our scene already set up, we can utilise the reflective
properties of the glass object, in this case a bottle, to
generate our caustic projection map. The key light is focused
directly onto the bottle and rendered off with a square 1:1
aspect; if a non-square aspect is used, the projection map
will be distorted and will not line up correctly when re-used.
Note that there are imperfections in the glass; these are
generated using bump mapping to simulate a mass-produced product
that has been sealed and contains some heat distortion. These
imperfections can add to the effect. The resulting render
is of a black background with white highlights generated by
the glass material.
key light Viewport rendered off, this image can be re-imported
into max and used in a clone of the original key light. However,
in the original photograph the caustic effect becomes dispersed
and more diffused the further away it gets from the glass.
To simulate this we can combine two (or more) copies of the
rendered images with different blurring intensities. To combine
these we can use a Mix map with an intense blur assigned to
one copy and a lesser (or none) blur in the other. To control
the mixing, a Gradient Ramp map is used to set how far away
from the glass (or how far up in the Viewport if viewed from
the key light’s Viewport) the maps should change. Due
to this blurring, some intensity is lost, therefore the Output
amount in the render’s Bitmap map should be significantly
increased to generate that high intensity glow around the
base of the glass where the light is immediately focussed.
we can project this map using a copy of the main key light (the
light from which the highlight pass was rendered). Copy the
light and turn on raytraced shadows in the original light. Increase
the original light’s Falloff/Field to illuminate the environment
a little more. In the key light copy (the caustic emitter),
exclude the bottle from receiving light or casting shadows.
Ensure that shadows are turned off for this light. The mixed
map setup should be dropped onto the Projector Map slot of the
caustic emitter light (using instanced copying should you require
to easily tweak settings later on). The light is excluded from
illuminating (etc) the bottle as it is already being illuminated
from the original key light; lighting the bottle with the caustic
emitter would generate an undesired effect.
we have our final result .
It’s not technically accurate, but it gives a visual interpretation
of a caustic effect which can add realism to a scene, even if
it’s a subtle effect. To add further detail to the effect,
try blending an overhead rendered position of just the bottle
on a black background (as before) with the original render and
project this through the key light copy to create some nice
concentric circle caustic effects. This is especially effective
if an imperfect wine glass is used as our model. Furthermore,
if glass or other refractive material is used that has a colour
tint, the caustic will also be tinted as the glass (etc) acts
as a gel (a coloured filter). This can be simulated by tinting
the caustic map; the bottle’s shadow colour should also
be tinted to simulate this gel effect.
Arts magazine, Issue 86, August 2003.
Draper, August 2003. Reproduction without permission