Reflected sunlight from planar glass - how to?

Support Lighting and Rendering
1

I’m probably missing something, but I’m wondering about the following:

I have a building with some pavement (texture) in front of it.

In real life, if the sun hits the flat (reflective) glass of the building, you’d see the windows reflected on the ground as distorted rectangles.

No matter what I try in Cycles (tiny emission surface, sun lamp, point, etc), I cannot get this effect to show in blender…
If I make the pavement slightly reflective, I see some glow, but not the sharp outlines of the reflected windows.
Is this possible at all?

I’ve tried with / without Reflected / Refracted caustics, but that makes no difference, maybe that only works with curved surfaces and not with flat?

2

Cycles can not do this. Theoretically - if you render several years this scene there could be something…
This is due to how raytracing here works: rays are shot from Camera, not the light sources. Since first would be pavement, diffuse rays get spread all directions, some hit window glass where in turn some are reflected different directions depending how they came from pavement. Only tiny amount would reach Sunlamp (with size quite small, set by you to get sharper shadows). Hence - no reflected Sunlight on pavement.
Use shaped Emission Planes close to the pavement with visibility for Camera off to fake reflected light. Hook plane vertices and animate shape of planes if this is an animation.

3

Thanks eppo,

That is a very clear explanation, now I do not have to search any further…

So to do this without faking, this would require a bi-directional path tracer which is not in Cycles.

I see that when using the Mitsuba bidirectional path tracer, I do get these reflections (but takes ages…)

4

You might be able to get something if you turn up the ‘filter glossy’ value to something like 0.20 and remember to turn on MIS in the lamp settings, but it will still require many thousands of samples and even then there’s no guarantee that you’ll get a nice sharp caustic.

Cycles can do caustics given enough time, but it’s not the engine to use if you need them fast.

5

Thanks Ace Dragon,
Given my ‘hardware status’ I’m pretty much bound to CPU rendering, so I’ll just give up or fake it like eppo suggested.
Since this is no critical render, it is not a big problem.
I just wondered “why doesn’t this work”, but it is all clear now (uni directional path sampling)