Christoph Hormann <chr### [at] gmxde> wrote:
> George Pantazopoulos wrote:
> >
> > In the ideal case, you would want to use completely random rays, but POV-Ray
> > does not do this.
>
> This is not really correct.  Completely random directions would not lead
> to better but worse results (you can easily try this).  There are
> sometimes artefacts due to the sample set being the same everywhere -
> MegaPOV has a patch for this purpose.  But apart from that using an
> optimized set of sampling directions is the best method.
>
>

Hi Christoph,

I'm willing to take your word on this, that an optimized distribution is
best, with one reservation. Specifically that it can cause artefacts when
used with the irradiance caching system.

When an illuminance sample is gathered, information about the distance to
all objects hit by the gather rays is cached along with the illuminance
value. This information includes the minimum distance and the (harmonic)
mean distance to all the objects encountered.

Later when this sample is a candidate for reuse, these statistics are used
to calculate the effective radius of the sample to see how reusable it is.
The optimized sample set used by default has fewer rays as the angle to the
normal gets larger. This means that low-lying objects near a sample point
can get missed by the gather rays. The effect of this would be a too-large
radius of reuse, resulting in artefacts.

George

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