My little path tracer

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dawelter
Posts: 21
Joined: Sun Oct 29, 2017 3:15 pm

Re: My little path tracer

Postby dawelter » Fri Mar 16, 2018 8:34 am

I think I squashed the worst bugs. This is fun stuff.
parabolic_reflector_volumetric.jpg
parabolic_reflector_volumetric.jpg (477.89 KiB) Viewed 1164 times

A perfectly parallel beam send through a scattering medium. The sphere is made of a much denser medium. It is sub surface scattering with brute force BDPT.
boxy_laser.jpg
boxy_laser.jpg (428.54 KiB) Viewed 1164 times

dawelter
Posts: 21
Joined: Sun Oct 29, 2017 3:15 pm

Re: My little path tracer

Postby dawelter » Sat Mar 24, 2018 8:38 am

I threw a "real world" scene at it. Thinking about how else I could exploit volume caustics in lack of refractive materials, I figure it would be cool to build a solar thermal power plant model. :-D

Modelled in Stud.io and Cinema 4D.
677761 Triangles, most of them occluded due to the bricks being stuck in each other.
8700 SPP in 34 h :?

The scene is contained in a box with a scattering medium, obviously. Fireflys due to SDS paths? I'm not sure, but I suppose there can be paths from camera to mirror to volume scattering to mirror to sun. The sun spans a small solid angle, so there is a non-zero chance to hit it with a "forward path" like that and it would be the only technique in Veach's sense to generate the path. Makes sense?

There is also some sort of "light leak" on the first mirror.
solarlego.jpg
solarlego.jpg (358.62 KiB) Viewed 1052 times


EDIT: Hm .. after some testing the strange "leak" appears to be no bug after all. I'll write it off as caustic cast by the mirror behind the one with the "leak".

Here is another shot. Moved the rear mirror a bit to see more of the caustics. Tuned down brightness. Converges and looks like normal illumination.
solarlego_bug2.jpg
solarlego_bug2.jpg (8.34 KiB) Viewed 1010 times

beason
Posts: 49
Joined: Sat Dec 10, 2011 1:58 am
Location: Los Angeles, CA

Re: My little path tracer

Postby beason » Wed Apr 18, 2018 1:07 am

Your explanation makes sense. Looks very nice! I like the planar laser beam. :D

dawelter
Posts: 21
Joined: Sun Oct 29, 2017 3:15 pm

Re: My little path tracer

Postby dawelter » Fri Apr 20, 2018 7:44 am

Thank you beason!

Actually there was a bug. I wrongly assumed that pdfs for BSDF sampling are symmetric which they are clearly not. p(wi,wo) = p(wo,wi)? No! no! Ah well, how does one say, assumptions are the mother of all f'ups. I finally realized my mistake when ...

I implemented statistical tests (chi-sqr and t-test) for my scatter functions in the style of PBRT and Nori. I found it quite tricky. Because the chance to have at least one false alarm grows quickly with the number of tests. On the other hand, if the threshold for the p-value is made too low one runs the risk of false negatives. My solution is to change the RNG seed rather than lowering the p-value. :lol:

Still have trouble with the microfacet BSDF. I get bad number of samples at the edge of a bin where incidentally wi*h = wo*h = 0. At that place the pdf also seems to diverge because of a 1/|wi*h| term. I'm unsure if bug or not. Probably not. The material looks okay ... ;)

dawelter
Posts: 21
Joined: Sun Oct 29, 2017 3:15 pm

Re: My little path tracer

Postby dawelter » Tue Apr 24, 2018 8:49 pm

I haven't had implemented a refractive material till now. That needed to be rectified badly.

RefractiveCaustics.jpg
RefractiveCaustics.jpg (244.13 KiB) Viewed 575 times

Not too shabby, I suppose.

(Although, since SDS paths are not rendered, parts are missing. I.e. in lower row, areas behind the glass. And top right, the beam behind the glass.)


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