Understanding the Principles of Light Behavior - Photographic Rendering with V-Ray for SketchUp - page 171

This of course is not especially helpful given that the term linear is generally applied
to something that happens in a straight or constant manner. This would naturally lead
us to expect a constant or straight falloff when using this decay type, rather than the
option referring to the constant or unchanging nature of the direct light itself.
Be that as it may, having a no (or linear) decay option can prove to be extremely use-
ful if we need to add a constant or unchanging light source to our scene. Here, we
see a render of our 30 x 30 feet square room with the light source set to use Linear
decay. The light never loses energy now, no matter how far it is required to travel.
Understanding Inverse decay
The next option we can look at is Inverse decay. This particular decay type can be
thought of as a half-and-half solution that allows us to add some decay to our scene
lights, but without having to use the kind of rapid falloff that we would normally asso-
ciate with physically accurate light sources. This particular option can only be found
on the Omni and Spot light types. This means that if we find ourselves working with
a scene that has some very specific lighting requirements, one or both of these light
types could become our new best friends.
Again, there is plenty of potential for confusion here, especially among users coming
from other rendering packages, as the functionality provided by the Inverse decay
option is what some people would correctly describe as Linear Rate falloff. Again,
the decay is described as linear because it is constant or straight.
We could illustrate the way this works by considering a 100-pixel greyscale gradient
that makes a strictly linear transition from black to white. Only the first pixel in that
scale will be 100 percent black and only the last pixel will be 100 percent white. All
the other pixels in the gradient will be precisely 1/100th brighter than the previous as
we travel from left to right, with pixel number 50 sitting at exactly 50 percent grey.