Diffuse Reflections
By Neil Blevins
Created On: Dec 8th 2024
Updated On: Dec 8th 2024
Software: Any
When light hits a
surface in the real world, it reacts with that surface in some
fashion.
Reactions include reflection, absorption, refraction, diffusion,
scattering,
etc. This is simulated in raytracing in computer graphics by firing a
ray from the
light
source, hitting a surface, calculating what color that surface is at
that point (refered to as getting a shade call from the BRDF of the
surface
(Bidirectional Reflectance Distribution Function)), then the ray
bounces to the camera and shades the appropriate pixel in your final
image (in view raytracing, the
ray travels backwards from the camera to the light, but for the sake of
clarity I'll be referring to light raytracing for this tutorial). In
the
real world, this process is actually a two step process, light hits a
surface,
the surface absorbs certain wavelengths of light based on its material,
and other wavelengths are reflected back to the viewers eyes, producing
the color we see. The actual material weighs heavily in the equation,
rough
surfaces for example will reflect light back very differently than a
smooth
surface. But since most cg artists aren't interested in playing with
the
molecular structure of the material, many approximations and tricks
have
been developed to replace the actual science.
Most shaders break up your material into various properties, such as
Diffuse Reflections, Specular Reflections, Refraction and Translucency
to name some of the more common ones.
This lesson will focus on the first of these properties, Diffuse
Reflections.
Lambert
The most common Diffuse reflection type in CG is Lambertian. It is
named after Johann Heinrich Lambert, who introduced the concept of
perfect diffusion in his 1760 book Photometria. It is a good
approximation of an ideal "matte" surface. And the brightness of this
surface is the same regardless of the camera's point of view (as
opposed to Specular Reflections, which are view dependent).
Here's an example of a Lambertian Diffuse Reflection.
Many pieces of 3d software create materials that are a combination
of multiple shaders. For example, the Principled BSDF inside of Blender
is actually a combination of several shaders...
The Base Color portion at the top is in fact a simple Lambert
Shader, and drop downs such as Subsurface and Specular are other
shaders mixed overtop. So even if the material doesn't say lambert,
you're likely using a Lambert shader.
As a side note, you can also
create a pure Lambert shader inside of Blender with a different node,
which doesn't contain any other shading attributes:
Oren Nayar
The classic Lambert model makes certain assumptions, such as the
surface would be completely
smooth when in fact there are very few surfaces anywhere that are
completely
smooth. This led to the creation of other models, the most common is
the Oren-Nayar model, which allows for surfaces to be "roughened",
producing
a better visual result than making everything purely Lambertian.
Here's the difference between pure Lambert and Oren Nayar with the
roughness turned up. You can see how some of the darker diffuse color
sort of leaks into the brighter part, which simulates a rougher
surface.
In Blender, you can't add Oren Nayar roughness to the Principled
BSDF (the roughness in that material is for your specular reflection),
but the Diffuse BSDF node does have a roughness which goes from 0 to 1,
a value of 1 being full Oren Nayar. So if you need to create a really
rough
surface like unfinished wood or rubber in Blender, use this shader
instead.
Other renders like vray do add Oren Nayar roughness to its standard
material.
A Note On Ambient
As well as a Diffuse color and a Roughness value,
some older materials in some 3d apps have another attribute called
Ambient Color.
Ambient And Diffuse Reflection
What is ambient? Well, before the popular use of Global
Illumination,
ambient was a cheat to give a little boost to the brightness of your
objects in the shadow areas, basically it was trying to mimic fill
light. But sadly, it applies itself uniformly over the entire surface,
so if you have an ambient color to your material, or have an ambient
light in your scene, you will never be able to achieve pure black in
your render.
While this may be fine for some sorts of stylization, in general, this
makes your scenes look very unrealistic and flat, so my recommendation
is if you are using a shading model that has an ambient parameter,
always
turn ambient colors to black and never add an ambient light to your
scene.
Conclusion
That covers the basics of Diffuse reflections. Go to
the next few tutorials in the menu on the left to read about some other
material properties such as Specular Reflections and Translucency.
