Re: backward ray tracing

Skywise wrote:
Phil Hobbs <pcdh@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in news:47BDC86B.4050502@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:

Okay, since we've done the OP's homework for him: Since ray tracing is time reversal invariant, including scattering and thin film effects, why doesn't backward ray tracing accurately render the visual image of a caustic or a colour shift?
<snip>
In POVRay, the software I've been playing with for years, caustics
used to be simulated and were confined to being within the shadow
of the object.

Now, real caustics can be computed. The software first does a
relatively low resolution forward trace from the light source,
through the object casting the caustic, and onto any surfaces it
then encounters. This information is stored and used to modify the
results of the backward rays later on. Now, when we trace the above
example ray to the surface, there is information about the result
of the caustics. Due to the resolution of the forward ray image,
the result may be interpolated.

This technique can be utilized to simulate quite realistic scenery.
I myself once made a rotating diamond animation that was just
stunning. All the sparkles, reflections, color refraction, caustics,
etc... were calculated with amazing reality.

The price you pay is that the more realistic you want the caustics
there's an exponential cost in CPU time.


Okay, so if I understand this correctly, the problem is not so much the caustics themselves as it is the diffuse scattering surface that makes them visible. To model the caustic with the same accuracy as the main ray trace, you'd need as many rays per unit area on the diffuser as on the eyeball, which could get expensive. To do it efficiently, you'd need to trace forwards to get the caustic, and then trace backwards to see what it looks like to the eye.

Cheers,

Phil Hobbs
.

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