Raytrace reflection Attenuation colors

We previously had a look at Raytrace attenuation and how it can be used as a tool to make your reflections look more convincing and to make rendering more efficient.  During those discussions I mentioned the color swatches found in the Raytracer Parameters and Attenuation rollouts.

Just as a refresher, I mentioned that using the Exponential falloff type was the most efficient and controllable method that allows you to set a Start and a End point between which the reflection will attenuate.  By default the reflections just seem to get darker and darker until they disappear in most cases, but that's a coincidence and it's important to understand exactly what's happening.

So, what's happening?

As the reflections die out the pixels that would have been reflections have to be replaced with something and, by default, it's pixels from the scene Background which, again by default, are black.  This causes the reflections to slowly fade to black, which appears quite normal in most circumstances.

The Raytracer Parameters Background color swatch or bitmap replaces the pixels the on the End range where there are no more reflections.  You can also choose the Use Environment Settings option to replace the pixels with whatever is in the environmental background.

In the Attenuation rollout you can choose Background or Specify and then choose a particular color to replace pixels fading out within the reflection itself.  I've included a simple image to illustrate the results of each.  You can see the teapot reflection fading to blue away from the reflecting surface and you can see the pixels beyond 150 inches have been replaced with red.

Why would you do that?

Using these falloff attributes and attenuation can subtly or dramatically affect the viewers perception of the reflections in the scene.  For example, I almost always make sure that reflections on metals fadeout to a blue shade or to a blurred bitmap (or multiple Noise maps) that create a soft mix of pastel colors.  Replacing some of the pixels with blue makes the metal feel much "cooler" in the mind of the viewer and therefore more convincing.  The blurry pastel colors can help give the illusion that there might be a thin coating of oil or dust that's causing a slight rainbow refraction on the surface of the metal; again this can be more convincing.