Simon Frankau's blog
Ray-tracing fun: Transparency and refraction
So, I've been doing all the distributed ray-tracing effects, and this isn't one of them. It is a traditional ray-tracing effect, though. Ray-tracing's best claim to fame is fancy reflection and refraction. Turns out I don't like this one particularly. Refraction is actually a fairly unsubtle effect, and I can see why crystal balls were used for gazing into - it's almost impossible to see what's going on. If you render a sphere, it's basically a big lens, and setting a scene up to render so that you can properly see what's going on with the effect is a nightmare.
Nonetheless, I've had a go. I think it more-or-less works, but basically I don't care if there are bugs in it, as I've got an image out of it and I'm done with it.
(The image shows various different refractive indices - I quite like the one on the right, actually.)
One thing I didn't expect to be thinking about was the attenuation of light. I'm demonstrating transparent, coloured materials. The logical basic model is exponential attenuation. However, this reveals the problems in a basic RGB light model. Let's say I create a material that almost immediately attenuates blue, doesn't touch red, and mildly attenuates green. In this case, a thin piece of material looks yellow, and a thick piece looks red. In other words, the colour qualities change with the thickness of the material. So far, so nice - real materials can behave like this.
However, under this model, asymptotically the material can only become red, green or blue (or cyan, magenta, yellow or white, if the components are precisely balanced). The separation of selectively attenuating frequencies from a spectrum and then projecting into RGB space really does not work the same as applying the function component-wise. I vaguely remember reading something about this kind of stuff in Foley and van Dam, but this is actually the first time I've actually really dealt with it...
In any case, I think I'm done with all the basic ray-tracing effects now. Hurrah.
Posted 2015-01-01.