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Spotlight Decay Regions, Alex Alvarez
All lights in Maya (area light excluded) emanate light from a finite point in space. This is completely unnatural as all lights in nature have a measurable size. The effect that light size has in nature is found in the shadow and specular qualities present. Simulating this issue in a render is for a separate discussion, as we are now going to focus on a particular situation where the finite origin of a spotlight becomes problematic... not in how objects receive light, but how one controls the origin of light.
In the above image we have a typical location flood light which has a broad area of illumination within a metal enclosure. The best way to simulate the illumination from this lightsource is the focus of our discussion.
The first step is to create a spotlight. While you may be inclined to use an area light, I prefer spotlights for reasons of efficiency and simplicity of designing how the light will illuminate the surrounding air. So first, a spotlight is created and positioned so that the cone of light coming from the flood is spreading the way I want.
The above image shows the main problem we are going to encounter. In order to get the proper light emission, the spotlight needs to be placed way behind the actual light housing. This cause three problems: 1) the light will illuminate the back of the housing. 2) if shadows are on, the light will not illuminate past the housing. 3) if fog is used, which we want, the fog will appear behind the housing.
The above image shows the results of enabling light fog. Remember to scale the spotlight so that the edge of the fog cone shape is out of view, and to enable light decay for this type of work... light fog is discussed more in another tutorial.
So what we now want to do is take advantage of a nice spotlight feature which is in the light effects folder: Decay Regions. This is an often misunderstood feature as it is poorly documented and has nothing to do with light decay. Decay regions, when enabled, control where light exists, not where it decays; they are 'intensity' regions. IPR supports these, so it is best to experiment with an IPR render active, in order to best understand them. Maya allows for three distinct regions of light, although the way that the light decays is still based on the standard decay rate attribute. The benefit of having three regions is dubious, as all I have ever found useful is controlling the start point of a spotlight's illumination.
The above image shows what decay regions can do, but what we will now do is set all the regions to butt up against each other. I set regions two and three to be far away so that we have something like this: Region One: 0-100 units. Region Two: 100-200 units. Region Three: 200-300 units. This way there is no gap between them. The numbers are arbitrary, however... enter whatever you wish as long as the light is moving as far into the scene as required. The only thing we are concerned with is the starting value for region one. We want the light illumination to begin within the floodlight housing, so I use IPR to interactively find the right value. You can also enable the visual display of decay regions by selecting a spotlight and going to Display/ Cameras&Lights/ Light Manipulators.
At this point we've accomplished the main point of this tutorial. The back of the housing is no longer illuminated by the spotlight and the fog is beginning where it should. What is left to deal with is the shadows cast by this light. Decay regions are not considered in Depth Map calculations, so once we turn shadows on, our illumination will vanish, as the light housing will block the light. To deal with this, we can simply turn off the 'casts shadows' attribute for the geometry which represents the back of the flood light. If there are other lights which should create a shadow from this object, create a copy of the geometry which casts shadows, but is not linked to our main spotlight. Light linking and working with the 'casts shadows' & 'primary visibility' attributes are the way around these types of situations. Also remember that the light fog should itself become a lightsource, so I always create fill lights to represent the light coming from fog.
As a last step, I added a second spotlight with light fog so that I could get the fog to bleed a little over the edges of the main fog cone. This spotlight is unlinked from all scene geometry and is therefore only used to create a fog effect. I also wanted the appearance of dust particles floating in the air, so I mapped the second spotlight fog's density attribute with a 3D granite texture. By animating the granite's placement node moving through the scene, I can get the dust to travel.
Hope this all made sense... :) It is extremely handy in various situations.
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