## Regarding directional dipole

Practical and theoretical implementation discussion.
citadel
Posts: 21
Joined: Fri Oct 17, 2014 4:19 am

### Regarding directional dipole

Following the shader code from http://people.compute.dtu.dk/jerf/code/dirsss/,
I have compared bssrdf using dipole (Jensen'01) and dirpole (Hachisuka'15),
cdipole.jpg (29.46 KiB) Viewed 3836 times
cdirpole.jpg (31.25 KiB) Viewed 3836 times

Both are sufficiently converged using a very small solid angle threshold(<0.25^2 sr) for hierarchical integration.
The problem is that dipole is noticeably darker than dirpole. And they both look quite far from volumetric path tracing.
But I have not tried single scattering, so the error may largely come from that.

ingenious
Posts: 274
Joined: Mon Nov 28, 2011 11:11 pm
Location: London, UK
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### Re: Regarding directional dipole

If I remember correctly, the dirpole model includes single scattering, so in theory its result should be closer to the path traced reference. Can you actually please post that reference here for completeness?
Click here. You'll thank me later.

citadel
Posts: 21
Joined: Fri Oct 17, 2014 4:19 am

### Re: Regarding directional dipole

ingenious wrote:If I remember correctly, the dirpole model includes single scattering, so in theory its result should be closer to the path traced reference. Can you actually please post that reference here for completeness?

You're right, the directional solution has included single scattering.

I had some problem with normalization, in the environment sampling PDF.
Now I think I get it working right, as I made a comparison between BRDF and light sampling,
compare_sampling.jpg (132.55 KiB) Viewed 3778 times

I then compared between dipole, dirpole and vpt (~1k spp):
compare_buddha.jpg (595.48 KiB) Viewed 3778 times

But the medium is not thick enough for the diffusion approximation to work efficiently, both for dipole and dirpole.
My volume path tracing routine is not unbiased. I limit the tracing depth to int(log(0.03)/log(albedo)) for each channel.
FYI, the material is skin2 from Jensen'01, and scaled by 30. The index of refraction is 1.49 for all channels.

And I used 32^2 samples for irradiance with a very naive sampler, without stratification, so that may also add to the error.
Last edited by citadel on Fri May 06, 2016 6:24 am, edited 2 times in total.

citadel
Posts: 21
Joined: Fri Oct 17, 2014 4:19 am

### Re: Regarding directional dipole

Comparing using the XYZ dragon model (up to down, dipole, dirpole and vpt):
compare_dragon.jpg (882.55 KiB) Viewed 3778 times

Mitsuba in (almost) the same setting:
mts_ref.jpg (312.31 KiB) Viewed 3778 times

citadel
Posts: 21
Joined: Fri Oct 17, 2014 4:19 am

### Re: Regarding directional dipole

Tonemapped using Reinhard '05 Dynamic Range Reduction Inspired by Photoreceptor Physiology

tonemapped_dipole.jpg (218 KiB) Viewed 3704 times

tonemapped_dirpole.jpg (211.5 KiB) Viewed 3704 times

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