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A Look At Caustics
By Jeremy Birn

Ordinary scanline rendering and raytracing works "backwards" - the rays start at the camera.  In an ordinary raytracing, the whole scene is scanned from the camera's point of view.  Where the camera sees a reflective or refractive object, the ray that started at the camera may be bounced or bent and projected off that object into another direction, such as to render what is seen in a reflection.  Only the rays from the camera are bent or bounced by raytracing.

Caustics are an addition to your rendering that calculates photons of light starting at a light source (like real light), which can be reflected, refracted, bounced off mirrors, or concentrated by a lens, accurately simulating more of the ways real light can move through a scene.  The rendering below combines raytracing (for the mirrors, shadows, refractive vase) and photon mapped caustics (for the light reflected off the mirror onto the side wall, the light focused by refraction through the vase, and the light reflected onto the top of the dresser by the vase.)

Caustic Dresser Scene
Rendered by Jeremy Birn in Mental Ray, Image Copyright © 1999 by Jeremy Birn.

This rendering was made in Mental Ray v2.1  Mental Ray now supports Caustics, as well as full Global Illumination that calculates diffuse-to-diffuse energy transfer (which is similar in output to a progressive radiosity calculation.)

Casutics are similar to raytracing effects in that they can look cheesy and superfluous when overused, but can add greatly to a rendering when used in a more subtle, reserved way.  They also seem similar to raytracing in that some people will try to simulate them and work without them to save rendering time.  However, the value of a Caustics option becomes clear if you compare side-by-side the results of conventional raytracing (below, left) to raytracing plus caustics (below, right).  You can see important extra touch made possible by rendering with Caustics.

Compare Raytracing vs. Caustics
Raytracing (left) is enhanced by the addition of Caustics (right).

Notice how the light reflected off of the mirror actually creates a new shadow of the vase on the side wall - the shadow within the bounced light is created entirely from blocking bounced caustic photons, independent of any raytraced or depth mapped shadow from the light.  Another effect of the Caustics is that the vase looks better "attached" to the top of the dresser in the version on the right, because of the realistic reflected light bouncing back from the glass to the wood.

Jeremy Birn's new book Digital Lighting and Rendering explains much more about lighting, global illumination, and other topics related to the art of rendering. The image above was adapted for the cover.

Web Links:

See Rendering Caustics on Non-Lambertian Surfaces by Henrik Wann Jensen, for more color pictures and a .pdf publication about Caustics.  Also see Henrik Wann Jensen's Global Illumination using Photon Maps for a broader overview of Photon Mapped Global Illumination.  Henrik Wann Jensen now works at Stanford University, where you can visit his latest homepage.

The choice of this dresser-mirror scene to demonstrate Caustics was an homage to an older Dresser image by Larry Gritz, the creator of BMRT, which supports Specular-to-Diffuse Illumination.

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All text & images shown here are Copyright © 1999-2000 by Jeremy Birn.
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