[Bf-committers] About Final Gathering

[Rob Cozzens]

First of all, thanks for all you hard work on Blender Farsthary.

I've never implemented a final gather renderer, but I have read about them.
So take the following with a grain of salt.

My understanding is that the rays you shoot out to gather the illumination
samples sum several photons that are within a certain distance to the ray.
Setting how close a photon has to be before it is summed should affect the
noise. Of course, the number of rays you shoot would also affect the noise.
But interpolation is a big thing. I know Mental Ray using interpolation. You
set the spacing (in screen space) of the final gather points and then it
interpolates that lighting information on the rest of the rendering. With
the latest version of Mental Ray they convert the final gather info into
spherical harmonics--that way the interpolated values include the direction
of the light and not just the brightness. Apparently that improves the
quality of bumps, especially in areas only lit by bounced light.

One problem you run into with interpolation is lower frequency noise. In
still images, it isn't that noticeable, but in animation it can be very
objectionable. Mental Ray lets you cache final gather samples from frame to
frame, but that can be a problem with network rendering.

I would be very happy to get Mental Ray style final gathering in
Blender--that renderer is the closest thing I have ever seen to a "Make it
look better" button!

-Rob

On Mon, Apr 6, 2009 at 9:37 AM, <echelon at infomail.upr.edu.cu> wrote:

> Hi all!
>
>  for the photon mapping build I have implemented now a first draft of
> final gathering according to the algorithm definition but I have still
> several doubts about it.
>   with FG you shoot random rays to gather illumination samples from the
> photon map (done), but that way you get high frecyuency noise and that
> is not observed in other softwares final gather algoritms.
>  did they use some sort of interpolation?
>
>  I have searched for the web about final gathering papers but the vast
> majority only talk about the photon mapping and briefly cover final
> gather
>
>       any suggestion/help on that?
>
>                                      Cheers to all Farsthary
>
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