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Producing Caustic Effects with Glass in Softimage XSI
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In real life glass casts caustic effects on nearby surfaces when illuminated. Caustics are specular light patterns created by the reflection or refraction of light which then hits a diffuse surface. Such effects can be seen in the movie clip opposite which shows caustics illuminating the surfaces around the window whilst the light source is animated. Examples in this website simulate a scene in which a light shining through a specular glass window is casting caustic effects onto the diffuse surfaces in the room.

Lighting effects in a scene usually only take into account direct illumination. In XSI reflection and refraction of glass surfaces are calculated with raytracing a method which traces the paths taken of rays of light color from the camera around the scene back to the light source. Caustic effects can be acheived using a form of raytracing where lots of rays are sent from the light source and their paths followed and the indirect lighting calulated.

In XSI and Mental Ray a faster technique of calculating caustics called Photon Mapping is used. The simulation of all light intereflection in the scene which includes the indirect illumination is described as Global Illumination. Caustics are a localised and often more visible form of Global Illumination. Photon mapping stores information on the paths of small packets of light energy (rather than color) as they are emitted from the light source, bounce around the scene and then hit the camera.

Simulation of caustic effects using photon mapping can be used to create physically accurate illumination in a scene and produce realistic effects for applications such as film and architecture. This study concerns how the appearance of caustic effects through glass can be adjusted during scene creation and also at the rendering stage. This website describes the controls available at each point in the process and their relative effect. The links to the left cover each area in which adjustments can be made and expand on each area of controls in more detail.

Producing Caustic Effects with Glass in Softimage XSI
Further Information
Susan Windeatt