Putting It Together

Ray tracers are best known for mirrors and glass. Take some time and experiment, to see why. Here are a few tips for figuring out how to employ reflection and refraction effectively in your scenes.

1. We tend to think of glass as exclusively transparent, but no one is surprised to look in a window and see their own ghostly reflection superimposed over the scene. When rendering glass or any similar material, set both transparency and reflectivity to high values, 0.9 or even 1. This allows the Fresnel effect to kick in, and gives your material an added touch of realism!
2. Because the reflected and refracted colors are added to the surface color, they’ll tend to make such objects brighter. You can tone down the material’s diffuse and ambient properties to compensate. The more transparent or reflective the surface, the smaller the diffuse property should be. This way, more of the color comes from the secondary rays, and less from the object’s surface.
3. If you’d like a subtly colored mirror, or slightly tinted glass, use a very dark color, instead of a very light one. Red glass, for instance, should use a very dark red, almost black, instead of a very bright red. In general, the more reflective or transparent the surface, the darker its surface color should be. Note that if you add color, make sure that you have some diffuse and possibly ambient contribution, too; otherwise, your surface will render as black regardless of what color you give to it.
4. Reflective and transparent surfaces pair nicely with tight specular highlights. Set specular to 1 and bump shininess to 300 or more to get a highlight that really shines.

Also, here’s a closing challenge for you: suppose you wanted to render a scene where you were looking through the surface of a pond at some rocks beneath it. In terms of implementation, that would be a transparent plane, with some spheres scattered below it. As your ray tracer is currently implemented, the plane is going to cast a shadow on anything beneath it, which leaves everything under the water in darkness, ruining the effect. You could add a light source beneath the plane, but that will introduce odd shadows and highlights—not a good solution either.

What you really want is for some objects to “opt out” of the shadow calculation. The surface of the pond, for instance, should be ignored when calculating shadows.

How would you go about changing your ray tracer to support that? What would you need to do to allow objects to individually declare that they cast no shadow?

Chew on that one for a bit. When you’re ready to move on, turn the page! Next up, you’ll add another primitive shape to your ray tracer: the humble cube.