Saturday, June 14, 2008


A drinking glass or a water-filled vase can act like a lens to focus light rays into curving projected lines or spots of light. This field of optics is called “caustics.” The word connotes the sense of burning, reminiscent of the way that a light from a lens can burn.

In this arrangement, set up in the morning sunlight, note the difference in the shapes of the caustic projections. Essentially, the objects are imperfect lenses. Also, check out the rich assortment of internal highlights in the glass, and the cast shadows of both forms. The cast shadow of the surface of water itself is visible as the dark shape within the cast shadow at lower left.

I’ve mentioned the digital imaging pioneer Henrik Jensen before. He has produced some striking images of caustic effects, like this cognac glass…

…and this one of a transparent ball. Artists, scientists, and mathematicians in the field of digital imaging have been breaking new ground in understanding this realm of optics, and their work inspires all of us traditional painters.

The hard thing to capture with any digital simulation is the quality found in Nature of infinite surface variation and complex interactivity of light, which causes unexpected nuances like that array of internal highlights.

Caustics are also at play when sunlight is refracted by a gently undulating water surface. The waves act like lenses. They focus a network of dancing lines on objects below the surface—like this Dunkleosteus from Dinotopia: The World Beneath.

Note the chromatic effects on the borders of the caustics above. Keep in mind that underwater caustic effects don’t occur much deeper than 20 or 30 feet. It would look wrong to include them in a deep-sea picture.

Dr. Jensen’s website, with explanations, link.
A 3D digital scene before and after caustics, link.


Unknown said...

This has caused me so much trouble over the years, like hands the complexity and risk of portraying is incredibly off putting. I'm always so blown away by how eagerly you embrace the problems of rendering the world, even looking to science for the answers.

Mr Atrocity said...

As well as the caustic refraction you describe here you can also get caustic reflections. Think of the patterns of dancing light you get on the ceiling of a swimming pool. Another example would be the metal ring in this render. It's the same priciple at work of course but I just wanted to point out that it isn't just lens like materials that do this.

hj said...

refraction, very difficult and very interesting!

Anonymous said...

Hmm, the raytracing topic is taking shape in my mind again. This guy here has been working on such hardware. Funny that I must have met him on campus way back, but of course I didn´t know how important all of this would become, or I would have talked to him.

I guess that in a few years in the future we will have fully raytracing capable GPUs for a reasonable prize in our computers. This is really amazing.

Planet Frugal said...

I came home once and the house smelled faintly like smoke. I discovered that my wooden dining table had a large teardrop shape burn in it. Near that burn was my big roundish vase of flowers. I thought it possible that aliens had showed up while I was gone and sampled a piece of the table but I stopped leaving the vase in the sun just in case:)