Transparency, Transmission and
Refraction
By Neil Blevins
Created On: June 19th 2002
Updated On: Dec 15th 2024
Software: Any
As well as the standard material properties of Diffuse and Specular, we also
need to consider how see-through your object is. This short tutorial
will discuss in general terms the various attributes of Transparency,
Transmission and Refraction.
Transparency / Opacity /
Alpha
Before raytracers become more commonplace in production, most
materials in 3d applications had a Transparency or Opacity feature. One
is the opposite of the other, a fully Opaque object has no transparency
(fully solid), and a fully transparent object has not Opacity (fully
see-thru). Transparency and Opacity aren't used as much these days
since the equation has no real world equivalent (except perhaps the
thinnest of pieces of see through plastic.
The images below are from the Blender manual showing Opacity from 0
to 1 as you go right.
Refraction
Now that most people are using raytracers, we have true raytraced
refraction. In the real world, when a light beam goes through a
transmissive object (a transmissive material is any material that lets
light through), the surface bends the light slightly, causing what's
behind the surface to appear warped. Different materials and the
thickness of the surface modify how much bending occurs. You might
remember these equations from high school physics classes.
American
Museum Of Natural History
The most common example of a refractive surface in everyday life is
Glass.
Notice how distorted the counter surface is when viewed through the
glass surface of this casserole dish.
A refractive surface has a number of different properties, most of
which can be controlled in most cg render engines.
IOR
<>How much a particular medium distorts the light rays passing through
is controlled with an IOR value (Index of Refraction). Depending on
what sort of material you're trying to create, you should change the
IOR of your material. Some common examples:
- Air: 1.0 (no light bending)
- Glass: 1.5
- Diamond: 2.5
- Plastic: 1.4
- Water 1.3
Tinted
As in the real world reference above and below of the
casserole dish, your refractive material can be tinted with a color.
Frosted
If the surface of interior of your refractive
material has small bumps, this can create a frosted effect, which
further distorts the background behind the surface, making it blurry.
This is commonly controlled with a roughness parameters in CG, and in
the real world is commonly seen in things like shower doors.
See below these images of a frosted plastic container.
Prismatic Effects
Some refractive surfaces are manufactured in such a
way that they reveal prismatic effects. Basically, a light ray doesn't
evenly bend, but the different wave lengths of light bend at slightly
different angles. This can also be simulated in CG in some renderers.
Chromatic
Prism Glass
Caustics
While considered more of a lighting effects in CG,
it's also important to know that refractive materials also produce very
different shadows than an opaque material, especially in glass or
crystal, you get caustics, which is where light enters your surface,
and refracts out in a way that focuses the light into spectacular
patterns.
Conclusion
Hopefully this covers most of what you need to know
about see-through objects. Check your favorite renderers for details
about their specific implementation of these features.
