[Bf-cycles] Lightprobe importance sampling

[Brecht Van Lommel]

Hi,

On Thu, Jan 5, 2012 at 6:55 PM, Mike Farnsworth
<mike.farnsworth at gmail.com> wrote:
> In kernel_path.h it samples the background on ray misses.  But now the
> background is also sampled as a light (when that light is chosen), so
> sampling the background on a ray miss amounts to doubling up the light
> from the background at the last bounce.  I added a check in
> direct_emission where it'll notify the calling integrator if the light
> it sampled was the background, so it can avoid sampling the background
> again the next time through the path trace loop if the next ray
> misses.  So far so good.

I don't think this is needed, it should not disable sampling the
background the next time, they should just be weighted with MIS.

> However, it ends up being darker than it should be, especially for the
> more specular surfaces.  I think I figured out why.  In
> kernel_emission.h light sampling routines, we sample the light and its
> PDF, then ask the BSDF for its PDF given that direction, and then we
> perform MIS.  However, we never ask the BSDF for a direction and then
> ask the light for its PDF for *that* direction as well (with a shadow
> ray check), and add that in.  We have to have both, or else the tight
> glossy surfaces never have a chance to add in light based on the
> BSDF's preferred direction.

For mesh lights, this is done in indirect_emission. Other lights don't
use MIS yet.

> I double-checked both PBRT and RenderSpud, and they both have that
> second direction's contribution mixed in when doing MIS for lights.  I
> think the technically correct thing to do is generate the BSDF's
> preferred direction, cast a ray, and if it hits *any* area light (area
> light, mesh/triangle light, background light) then factor that in.
> But for now, I'm just going to have it cast a ray for the BSDF
> direction to see if it hits nothing, and then sample the background,
> and mix that in with MIS with respect to the BSDF's PDF for the second
> direction.

The way I see this is, you have two ways to render the emission, one
using BSDF sampling and one using Light sampling. When leaving out the
MIS factor, they should both converge to the same result.

It is not needed that for every Light sample ray, there is a matching
BSDF sample ray, all that is needed is that the pdf's match. Avoiding
that extra BSDF sample and instead doing it using the BSDF sample that
is already done for the next path vertex gives a nice speedup.

Brecht.