Description:
The
Diffraction Shader Plugin provides artists with a
specialised shader that can be used to create realistic
diffraction effects like the rainbow reflections created
by a CD.
For
installation instructions, refer to the README.txt file
included with the download.
About the Diffraction Shader
The shader computes the reflections from a
metallic surface comprised of tiny bumps coated with a
transparent layer. The amount of bending is modeled
using the Index attribute on the shader. This is the
index of refraction of the transparent layer and can
accept values from 1.0 to 2.0. When Index is set to 1.0,
no bending occurs. A typical value for plastic is
roughly an Index value of 1.5.
The distribution of the bumps is assumed to be
periodic in the U direction and random in the V
direction. The size of each bump is given by the
attributes Height, Width and Length. Width corresponds
to the U direction while Length stands for the V
direction. The sizes are in microns and are typically in
the range [0.05-5] so that they interfere with the
wavelengths of the incoming light which are in the range
[0.38-0.78] microns. The fixed separation between the
bumps in the U-direction is modeled by the attribute
Separation.
You can switch between two types of bump using
the Method attribute. The Highlight attribute controls
the size of the principal highlight on the surface (or
the principal "line" in the case of the compact disk).
This attribute varies between 0 and 1. For Highlight=0
you have no highlight, while for Highlight=1 the
highlight covers the entire surface. The shader becomes
more computationally expensive when setting values of
the Highlight parameter higher The remaining attributes
have meanings similar to the ones in the anisotropic
shader of MAYA.
The attributes are summarized in the following
table
Parameter Name |
Description |
Range |
Color |
Can be used to change the global colour of
the surface |
R, G, B |
Ambient |
multiplies the ambient light |
0.0-1.0 |
Method |
Shading method: either CD-like bumps or
diffraction grating-like wedges. |
two choices |
Index |
Index of refraction of the plastic cover on
top of the diffraction grating/CD |
1.0-2.0 |
Height |
Height of each bump |
0.0-1.0 |
Width |
Width of each bump (in the
U-direction) |
0.0-10.0 |
Length |
Length of each bump (in the
V-direction) |
0.0-10.0 |
Separation |
Separation between the bumps (in the
U-direction) |
0.0-10.0 |
Highlight |
Size of the main highlight |
0.0-1.0 |
Twist |
Rotation angle of the (U,V,N) local
coordinates |
0.0-1.0 |
Brightness |
Multiplies the intensity coming off of the
surface |
0.0-10.0 |
Glow |
Can be used to make the shader
glow |
0.0-10.0 |
Transparency |
Transparency of the surface |
R, G, B |
Reflectivity |
Multiplies the reflected color
below |
0.0-2.0 |
Reflected Color |
Light coming from the mirror
direction |
R, G, B |
What is Diffraction ?
Diffraction effects exhibit the wave-like
properties of visible light. In computer graphics the
interaction of light with surfaces and volumes has
traditionally been modeled using the "ray theory" of
light propagation. The ray theory assumes that light
travels in straight lines between surfaces and that the
radiant power flows along the geometrical rays. This
theory is fine for most computer graphics applications.
In general, whenever the size of the surface detail is
much larger than the wavelength of light (roughly 0.5
microns) the ray theory should be a very good
approximation.
Diffraction can be loosely defined as all
light-effects that cannot be modeled according to the
"ray theory" of light. Instead, the "wave theory" must
be considered. For example, when an incident planar wave
hits a "rough surface", the reflected wave is a very
complicated wave which in genral cannot be modeled by a
set of rays. Figure 1 illustrates this
point.
Figure 1: Diffraction of Waves by a rough
surface
An Example: the compact disk
Diffraction effects can easily be observed by
looking at a compact disk. By rotating the disk one can
observe many different colours appearing and
disappearing. The colours are a result of both the wave
nature of light and the micro-structure of the compact
disc's surface. The surface of a compact disk is
comprised of tiny little "bumps" of size that is roughly
1.0x0.5x0.15 microns (10-6 meters). The size
of the bumps is comparable to the wavelength of visible
light, 0.3-0.8 microns, hence diffraction effects occur.
Figure 2 shows a zoomed in picture of the micro-geomtry
of a compact disk. Notice that the separation between
the tracks is constant while the distribution of the
bumps on each track is more random.
Figure 2 : micro-geometry of the surface of a CD
The compact disk is not the only example of a
surface that causes diffraction effects. The advent of
the laser allows many different microstructures to be
created very cheaply. In fact diffraction gratings are
usd nowadays in wrapping paper, bumper stickers,
etc... Diffraction effects have also been observed
on certain insects, whose "skin" has a tiny
periodic-like micro structure. With this shader you'll
be able to create diffraction effects on many different
objects.
(click here
for QuickTime animation- 660k)
Further Examples:
These are some examples of "diffraction
gratings". The different patterns are obtained by
texture mapping the "twist" angle in the plugin shader.
Uses MAYA's "bulge"
texture. (click here
for QuickTime animation - 1.8MB) |
Uses MAYA's "cloth" texture.
 (click here
for QuickTime animation - 1.8MB)
|
Uses custom file texture.
 (click here
for QuickTime animation - 1.2MB)
|
|