Sampling Worklfows

[Ondra]

I am really sure that the fireflies are not from hitting lights directly, because Multiple Importance Sampling automatically accounts for that (unless you have lights sampling set to something other than MIS).

Also, you cannot exclude lights from GI calculation, GI is induced solely by direct lighting, you wouldnt have any GI without the lights.

So, the possible reasons for fireflies are either
- caustics (light behind some solid glass/mirror)
- bright spots (light very close to some geometry)
- general inefficiency of path tracing in interior scenes

First two causes can be easily fixed at the cost of some small bias by setting "max sample intensity" to about 5-10. The last is intristic deficiency of path tracing, I would recommend switching to PT+HD in that case.

Anyways, it would be much better, if you could post renders and screenshot of your render settings ;)

[racoonart]

Ok, I can't show you the scene I'm talking about but i made an example which is pretty similar. So, it will likely be the second point of your list. I didn't think about this... but it's pretty obvious now

My question about excluding light was meant in that way: "Do GI rays directly hit the lightspheres?" I thought direct lighting would be the light emitted by the spheres and GI would be bounced light from the surface the direct light illuminates.

Anyways, It seems the fireflies have their origin at the surface beneath the lightsources.


P.S. Is max sample intensity for GI only or for both directlight and Gi samples?

[Ondra]

Its only for GI; its not needed for direct light. Just set it to some reasonable value to balance the bias/variance ratio ;).

About the direct/indirect: GI rays can randomly hit lights. But because Corona uses MIS, such cases are properly weighted, and do not cause any fireflies or bias. They, in fact, increase the robustness when using large area light sources and when sampling very close lights. When using MIS the distinction between direct lighting/GI partially vanishes.

[racoonart]

whoaaa... that's fixing several problems at once! Awesome! :) You should consider moving the spinner to the Global illumination group - that would make those things clear. I didn't use the max sample intensity spinner a lot since i didn't know exactly which values it will clamp (like direct light for example).
MIS is always active here :)

[Ludvik Koutny]

You should always press production defaults button located in "About corona renderer" rollout. It sets render settings to correct defaults (initial defaults are for debug purposes only). And it, among other settings, sets also max sample intensity to 10, which is recommended value ;)

Press this button everytime you create a new scene ;)

[racoonart]

I do - everytime. But 10 was simply to high for this scene.

[Ludvik Koutny]

Value of 10 should not be very noticeable... ;) It could introduce some weaker secondary reflections and things like that, but it should not affect scene such as this one that much :)

[racoonart]

I can only say that lowering this value helped a lot...
Why does GI sample intensity have an influence on reflection (not considering the intensity of reflected surfaces of course) ?

[Ondra]

thats great suggestion (moving it to GI rollout), I've already done it ;)

I think Rawalanche means diffuse reflections