I've been playing a bit with the relatively new Maya Ramp Shader. It looks promising, but (if I'm using it correctly) not entirely bug-free. (What is that white transitional color?)
It takes as its solution the idea of using light to modify the surface color of an object. Nothing wrong with that; I did it myself here.
I'd like to suggest that Maya consider adding similar functionality into the light itself, as I did here.
It's hard to light highly saturated colored objects in a sophisticated manner, even when (and one might say especially when) using saturated light.
Suppose I have a bright red flag. Further suppose I wish its shaded areas to fall into a rich dark purple.
Easier said than done.
My first attempt might entail filling the dark areas of my waving red flag with a saturated purple light. As you can see from the images above, saturated purple light has no effect on a saturated red object because saturated red (1 0 0) times dark purple (.5 0 .5) only equals dark red (.5 0 0)
What I need to do is use the lighting of the flag as a matte into another texture map - a purple one.
First a create a second texture map in a deep rich purple.
Next I use surface luminance to control a blend between the red and purple flag textures.
It's not a bad idea. The only problem is Maya continues to LIGHT the flag. See how dark the shaded area still is? I don't want that effect.
I compensate for the lighting by DIVIDING the object's color by its own luminance. This effectively turns the Lambert lighting into a surface shader.
Ah, that's more like it! See how nice and purple the shaded region is? It's not just a darker version of the red flag; it's a whole other hue. Very painterly. Not at all CG-ish.
UPDATE 5/29/2006
If this line of thinking interests you, be sure to explore Maya's Ramp Shader, which addresses the issue of hue falloff in CG lighting.
Here's a pure yellow ball sitting on a white table in Maya. Since the table is white, it takes on the color of the light shining upon it, so in this image, the light is also white.
Here's a question for you: What would happen to the yellow ball if I change the light color to pure blue?
Depending on what you think happens to colored objects under colored light you might guess that the ball would become green. You might also guess that the ball would become white.
You probably would not guess that the ball would become black, but that's exactly what happens. Maya multiplies the light color by the object's diffuse color. (1 1 0) times (0 0 1) equals (0 0 0).
If you desaturate the blue light so that there is some green in it - then you will get a nice green ball.
It's very hard to light highly saturated objects.
It's very hard to light with highly saturated light.
In the real world, colors are usually not so pure.
The samplerInfo node has a flag called flippedNormal that returns either a 0 or a 1 depending on which side of a single-sided surface you are seeing.
Many people use this to drive a Maya conditional node and so create something called a double-sided shader - a single surface with one texture on one side and another texture on the other side.
The syntax for the conditional node can be confusing to some Maya users. If you'd like an easier to remember alternative, consider using the flippedNormal flag to drive a blendColors node instead.
Here's an old school way of faking volumetric fog. I'm not sure who invented the idea, but we used to do it this way back in the Eighties.
Set up a scene.
Use a camera-based grad to fake a Z-depth render. Let gray represent distance from camera - the further, the brighter. Render in 16 bits.
Slam in a ground plane. This will represent the height of the top of your ground mist.
Render out another 16-bit Z-depth image with your elevated ground plane in place.
Subtract one 16-bit Z-depth image from the other. This will give you an image of fog ray traversal lengths - exactly as if you ray-marched through the volumteric fog.
You can tweak the contrast if you wish.
Use this volumetric ray-marched mask as a key mix between your rendered scene and an image of pure fog color. (I chose pure red for my full fog image)
Here are the results again. You can adjust fog density to taste by adjusting the density of the Z-depth difference mask.
This technique is pretty old-school. For it to work you have to stay out of the fog.
