Direct lighting in general and in SPPM

[guibou]

Hello,

It appears (core/transport.cpp in ::Connect function) that direct lighting can get through some surfaces (in fact every surface which are BSDF_SPECULAR | BSDF_TRANSMISSON) and for which F(-ray.d, ray.d) != 0. This allows direct light sampling to connect a diffuse vertex with a light through architectural glass / null material / ...

This is an issue for SPPM. Currently photons with pathLength of 0 are not taken into account in the density estimation because they are already taken into account by the direct lighting. But how to remove photons of pathLength > 0 which may be taken into account through architectural glass ? (See the attached image for an idea of the issue. It is a null cube split by a matte plane)

Also, I started to wonder after I saw this picture from the SPPM thread:



(generated with direct lighting integrator, but you can also get the same with path tracing and depth of 1, or SPPM with 0 photon / pass and direct lighting on)

Is this really direct lighting ? I mean, I can see no direct path from the roof to the light. The only way for light to come inside is through CURTAIN of the kitchen... Which is a matte translucent. How the hell do light get in the kitchen ?

Now, the kitchen scene is litted by three ligts, the kettle, sky and sun. If you disable kettle and sky, the sun can *normally* lit only the floor, but the roof is also lit. I tried to put thickness to the roof, but it does not help... Direct lighting of the sun is able to lit the roof !

[jeanphi]

Hi,

What I do in bidir is that I have 2 counters: one for real surface intersections which is used for sample data indexing, and one for path vertex indexing which is not incremented when hitting a pass through surface.

Jeanphi

[guibou]

What I do in bidir is that I have 2 counters: one for real surface intersections which is used for sample data indexing, and one for path vertex indexing which is not incremented when hitting a pass through surface.

I just saw that (I was looking in the path integrator code for something similar, but was unable to find it...). So in your definition passthrough materials are specular transmission with a pdf of 1.0 (I think one day we'll have to list all the subtle conventions that lux use

Ok, I'll do the same for SPPM. Thank you.

By the way, why matte translucent is not a passthrough material ? And also, why the hell does the direct lighting is able to lit the roof of the kitchen scene ?

[jeanphi]

I just saw that (I was looking in the path integrator code for something similar, but was unable to find it...). So in your definition passthrough materials are specular transmission with a pdf of 1.0 (I think one day we'll have to list all the subtle conventions that lux use

That's slightly more complex: a passthrough material is a specular transmission with wi=-wo, the pdf is 1 because that's the case for all specular materials (actually it is infinite), and a call to pdf(wo, wi) returns a non null value.

By the way, why matte translucent is not a passthrough material ? And also, why the hell does the direct lighting is able to lit the roof of the kitchen scene ?

Matte translucent is not passthrough because it scatters light in every directions, a passthrough material must not change the ray direction. And I don't know why the roof is lit, maybe an issue with SPPM tracing code? Is it also lit if you add another mesh to prevent light from hitting the roof from outside?

Jeanphi

[SATtva]

guibou, i think what you're observing with directlight is to be expected -- the light on the room ceiling is not reflected off the floor, but is indeed a direct light from the sky infinite light, its "bottom" part specifically (light from below the horizon). Here's a really simple scene, just a hollow cube with thick walls and an opening:

Reference: bidir, sunsky

Directlight, sunsky

Directlight, sky only

Directlight, sun only

dl.sun.jpg (9.96 KiB) Viewed 616 times

See the front wall and the refrigerator on your image -- they are pitch black too as no light is hitting them.

[Chawl]

Kitchen scene has two major lights. One is named Sky which is "Sky only", other is named Sun which is "Sun+Sky". Perhaps guibou only disabled rendebality of the Sky and forgot to change Sun from"Sun+Sky" to "Sun only". I rendered the kitchen with Direct Light with proper changes and got this result, but I'm not sure if glitter on some metal surfaces should be there:

Direct Light, Sun only illumination

[guibou]

Kitchen scene has two major lights. One is named Sky which is "Sky only", other is named Sun which is "Sun+Sky". Perhaps guibou only disabled rendebality of the Sky and forgot to change Sun from"Sun+Sky" to "Sun only". I rendered the kitchen with Direct Light with proper changes and got this result, but I'm not sure if glitter on some metal surfaces should be there:

kuhnya.Scene.sun-only.dl.png

Yes, you are right, I forgot to disable the Sky lamp

Now I'm wondering how the floor can be lit by the sky in direct lighting through the curtain, which is not a passthrough material ?

[jeanphi]

Hi,

Isn't the curtain a mix with the null material?

Jeanphi

[guibou]

Isn't the curtain a mix with the null material?

Yes, indeed, you are right. It does not appears easilly as such is luxblend because it is hidden inside the "Luxrender alpha transparency panel".

Today I learn a new trick, don't trust luxblend and see the .lxs by myself.

Thank you.