Caustics

Caustics are bright patterns created when a curved or transparent surface bends or reflects light and concentrates it in one place. The rippling web of light on the floor of a pool, the bright spot a glass of water throws onto a table, the curve of light inside a metal ring are all caustics.

Why caustics are hard to render

A caustic forms when light from a small source travels along a long, narrow path before reaching a surface. Path tracing that starts at the camera struggles to find those paths by chance, and the result is often noise or bright specks known as fireflies. That is why many engines keep caustics off by default or use special techniques such as photon mapping or tracing from the lights.

Caustics in architectural visualization

• In pool and water feature scenes, the underwater light pattern is striking.
• On glass tables, crystal chandeliers and vases, they add a subtle layer of realism.
• In most interiors the effect is small and may not justify longer render times.

In practice, strong caustics such as pool light are sometimes faked with a texture or a projector light. Whether physical accuracy or render time comes first depends on what the project needs. If a pool is the hero of a sales image, caustics are worth the effort; for a carafe in the background, usually not.