9

82 Chapter 17: Rendering
flattop.Now,yougotorendertheglass. After
rendering the scene, however, there’s something
w rong: the inner surfaces of the g lass don’t seem
reflective enough, and the wine isn’t refracting
properly. What’s wrong?
What’swrongisthatyouprobablyhavethe
number of reflections and refractions set too low
for the number of surfaces you have. To check this,
go to the Renderer panel > Rendering Algorithms
rollout (page 3–116) and look at the Maximum
Trace Depth spinners. If you haven’t changed the
parameters, then you should see Max. Reflections
and Max. Refractions set to the default of 6, and
Max. Depth set to 6.
There’s the problem: you actually have six surfaces
that need to be traced by the lig ht rays for both
reflections and refractions. The way to always
calculate the number of rays needed for a scene is
to take the ray-traced objects in your scene and
draw an imaginary line through them, originating
at the point of view. Then, count the number of
surfaces the line intersects.
For the wineglass and wine, you need at least six
reflections and refractions that correspond to the
following surfaces:
• Near outer glass surface (“near” relative to your
Camera v iew point)
• Near inner g lass surface
•Nearwinesurface
• Far wine surface
• Far inner glass surface
•Farouterglasssurface
Therefore, increase the value of Max. Depth to 12.
Ca ustics and Global Illumination
Before rendering with caustics, there are several
things you need to set up in your scene:
• For caustics to work properly, the generating
object must use a material that contains some
degree of shininess, reflectivity, or refraction.
AssignaRaytraceorothermapaseithera
Reflection map or Refraction map before you
render caustics.
• Most often, you’ll be using ver y shiny, highly
reflective materials (such as chrome and other
metals), or transparent or translucent materials
(such as glass goblets or water), to generate
caustics in your scene. If you’re using a glassy
material,makesureit’sdouble-sidedtocreate
the proper results.
•Makesureyouhaveobject properties (page
1–126) set to Receive Caustics or Generate
Caustics (or both). To set up these properties,
right-click an object and choose Properties.
For example, if you’re rendering a wineglass on
a tabletop, you probably want the wineglass
both to generate and receive caustics (so that
caustics are scattered within the g lass itself),
and the tabletop only to receive caustics (unless
it’schrome,say,insteadofwood).
• If the rendering of your scene is washed out by
light, double-check the Multiplier settings: one
in the Basic group of the Final Gather rollout
(page 3–111), and one e ach in the Caustics
andGlobalIllumination(GI)groupsofthe
Indirect Illumination pa nel > Caustics And
Global Illumination rollout (page 3–106).These
apply to all lights in the scene. Reducing the
Multiplier values can eliminate washout.
If a sing le light object is causing the problem,
you can reduce the Energy multiplier’s value
in that light object’s mental ray Indirect
Illumination rollout (page 2–1343),availableon
the Modifier pa nel.
• Toimprovethequalityofcaustics,gotothe
Caustics group of the Caustics And Global
Illumination rollout (page 3–106) and increase
the Max Num. Photons Per Sample setting.