Bizarre render element request

[tricky]

Not sure if this is possible, or even if it exists already:-

Example situation:
Exterior sunny scene. A few windows on a building facade. I want to 'fake' reflected light from the sun, off the glass in the windows, onto the road surface.

Could an element be created where the light 'rays' from a selected light (the vraysun for example) can be traced onto another object?

Example image:

[rivoli]

those are caustics reflected by the windows' glass, so you can already isolate them in the caustics element. you'd have to calculate them first though.

[Neilg]

You cant really fake caustics quickly.

Theres a method vlado posted a while ago to get quick caustics which i've used a few times for this, layering in post.

edit: still got it in my sentbox

So some tips on how to enable these in RC3:
(*) Go to the maxscript listener and type "renderers.current.caustics_showCalcPhase=true " without the quotes. This will cause the photon tracing progress to be directly mapped on the image and re-used for the final rendering; (This is the little white box in the bottom left of max itself. Just copy, paste & press enter - you're then good to go)

Notes:
(*) Only directly visible caustics can be visualized in this way. Caustics visible through reflections/refractions will appear in the normal (interpolated) way.
(*) If you are tracing lots of photons, set the Max. density parameter for the caustics to something other than 0.0 as otherwise you'll get memory overflow.
(*) There are limitations on how this works - e.g. it will not work well with DOF and/or motion blur.
(*) This option is experimental. There is no guarantee that it will be present in future versions in the same form.
(*) This option requires additional memory. It may be quite a lot, depending on the image resolution.



After this, go to your caustics rollout (under all the GI settings) and check them on. If you select a light, right click and go to vray properties you can change the number of samples each light has. 5000 tends to be pretty clean, but it also starts to get slow.

Also uses an obscene amount of memory by the time you get to print resolutions, so if you were to use them i'd suggest doing them half res, turning on dont render final image, set the path to save the image as normal and go. This will give you a completley black image apart from where the caustics lie, which you can then take into photoshop and apply over your render with a blending mode.

Might start getting impractical when you do entire scenes with it, but for certain things it'll add a lot. Going to be a godsend for swimming pool water, shiny objects in kitchens, and even the glass of a building reflecting the sun onto the road.

Here's what I got out in little over a minute with it:

You can render it at a much lower resolution if you dont need the detail.

[tricky]

Thanks for this. Will look through it.

However, I really don't want anything as complex as real caustics. I just want to know where the reflected ligt from a rectangular window will hit a surface without taking into account diffraction etc. It should be a matter of tracing 4 lines from the selected light source through each corner of the square window and then bouncing those same 4 lines to the floor surface. Simply filling in the area between them on this surface would suffice.

Hopefully my sketch will explain better:-

[rivoli]

it's perfectly clear, those are exactly reflected caustics as cubiclegangster suggests, you can render the direct caustics with no need to actually render the final image. then use it as a sort of render element.
something along these lines (the abstract shapes in the vray fb are caustic reflected from the glass panels):