What? It's the indirect reflection of the sun, specularly reflected (and focused) off the building.
You need the light source (sun) reflected onto the ground (or whereever it's reflected) from off the building.
You can't use standard path-tracing / ray tracing for that, as that traces rays from the camera outwards, and wouldn't show the caustics (well, you might be lucky and get one or two fireflies, but that wouldn't be the result needed for analysis).
At the very least, you'd need BDPT. Photon mapping would be a much faster (but cruder) solution.
Basically what you're saying is you need classic ray-tracing from light sources, such as one can get with Pov-Ray and similar old-school ray-tracing programs.
Well, I'd define classic as standard "from camera" outwards.
POV Ray does classic (which won't give you what you need as it doesn't handle caustics) or photon mapping which will.
Bi-directional path tracing (tracing rays from both the camera and lights in the scene and joining the respective light paths in the middle), and Metropolis Light Transport (a progressive variation of BDPT) are the most accurate, but are often slow to converge to a noise-free result, mainly due to the fact they do handle caustics (reflections of bright light focal patterns) accurately, which introduces noise (over the standard GI noise).
But photon mapping would give reasonable results in a few seconds.
> Well, I'd define classic as standard "from camera" outwards.
Fair enough. I was thinking of optical ray tracing, not graphic ray tracing. In optical ray tracing the point is to analyze the paths of light beams through lenses and from mirrors, not the POV of a camera necessarily.
I've been able to get pretty useful optical ray tracing out of Pov-Ray:
Yeah, I work in the VFX industry, so I'm a bit biased :)
Cool - actually, that would be a lot less computationally expensive (assuming you just modelled the sun as a disc) tracing it backwards without any GI at all. Photon mapping's pretty similar though, you've just got a final gather process at the end of sending out the light rays. The difference is photon mapping records all the hits to all surfaces. If you limited it to specular ones (at least for the first ray bounce) you'd probably have something very similar.
> What? It's the indirect reflection of the sun, specularly reflected
I am pretty sure the standard use of "indirect lighting" in normal computer graphics jargon means light that has had at least one diffuse reflection on its path. Even Merriam-Webster lists this meaning [1].
You'd just need to run the ray-tracing from light sources algorithm, to map the intensity of the specular reflection of the building. This would be equivalent to just creating the photon map. Then you could just view the photon map directly, to see the intensity of the death ray. There would be no need to render the scene (although you could, if you wanted a photorealistic view).
You need the light source (sun) reflected onto the ground (or whereever it's reflected) from off the building.
You can't use standard path-tracing / ray tracing for that, as that traces rays from the camera outwards, and wouldn't show the caustics (well, you might be lucky and get one or two fireflies, but that wouldn't be the result needed for analysis).
At the very least, you'd need BDPT. Photon mapping would be a much faster (but cruder) solution.