"Surface Portals" to reduce noise with indirect lighting?

[linvanchene]

edited and removed by user

[linvanchene]

edited and removed by user

[dsyee]

In your case I'm 99% certain the fireflies are being caused by something besides your backplane. It looks like the character is standing on a reflective surface (water?), and the light reflecting back on her from underneath is causing fireflies. Did you try turning off hotpixel removal?

Here is an example of the same scene with and without reflective planes behind the camera (Blender -> Octane Standalone).

With fully reflective plane (perfect mirror):

Reflective backplane

With white diffuse plane (no specularity):

Diffuse backplane

With no plane:

Reverse angle scene view (diffuse version):

You can see there are no fireflies at all (250 s/pixel, path tracing). [Edit] FYI, yes this has an HDRI environment in addition to Daylight, but I get the same result even with the environment set to all black.

[linvanchene]

edited and removed by user

[dsyee]

Hmm, I'm not sure how feasible A is, because if I understand correctly, you're basically asking the renderer to produce a biased result. In the case of your scene, since the water is not visible in the scene, there should be no paths being sent directly to it. Instead, the paths being sent to the water are the ones that start off intersecting the character.

Some of them will reach the sun, which seems to be what's producing the highlights and fireflies, and others will reach the "sky." In an outdoor Daylight scene, "infinity" is also contributing light, since the sky is lit. I think what you're saying is "send more paths toward the sun," but this would skew the ratio between sun-terminated and sky-terminated paths, so the result will no longer be physically accurate.

Really, the only "wasted" paths are ones that do not terminate at a light source, but this is typically only a problem with indoor scenes. But that's what portal materials are for.

For B, you can already basically do this using area render. I've found it to be very useful for this, actually!

Basically, if you tell it to send "extra" paths at any specific mesh, it may produce a biased result, because in real life the object is not going to receive extra light. You can tell it to send extra paths from the camera to specific pixels, which is what importance sampling does. Portals seem to direct paths at specific meshes under certain circumstances, but I believe they will also produce incorrect results if e.g. you have two windows and only one of them is covered by a portal.