[Bf-cycles] Complex lighting situations issues_light intensity decreases > number lights increases

[F Tamas]

'Setting clamp to 0 does not *make* fireflies. Setting clamp to values
other than 0 imposes a maximum brightness limit on individual samples. In
many cases, limiting that brightness hides fireflies, but in cases like
this, it can take away important contributions.'

You are right; I'm from practical side, sorry for being non-technical. The
question is how Cycles settings could be optimized for avoiding fireflies
and keep the intensity of lights with quick rendertimes. I'm working on
scene with thousands of lights (both mesh lights and 'real' lights) and
also noticed this issue.

Could you share any tips? Thanks.

2017-02-13 10:58 GMT+01:00 Stefan Werner <stewreo at gmail.com>:

>
> > On 13 Feb 2017, at 02:02, F Tamas <tamasf.3d at gmail.com> wrote:
> >
> > But setting clamp to 0 makes fireflies, so rendertime will be far
> longer, am I right?
>
> Setting clamp to 0 does not *make* fireflies. Setting clamp to values
> other than 0 imposes a maximum brightness limit on individual samples. In
> many cases, limiting that brightness hides fireflies, but in cases like
> this, it can take away important contributions.
>
> A simple case to demonstrate this is if you take an object with a bright
> and sharp specular highlight and have that reflection be blurred either by
> depth of field or motion blur. Setting a clamp value on direct light very
> quickly dims what otherwise is a bright spot/streak.
>
> With many lights and path tracing, the following situation takes place:
> In standard path tracing, only one light is being sampled per pixel
> sample, all other lights are ignored. To compensate for that, the
> contribution of that light must be divided by the probability of that light
> being sampled, that is, divided by the number of lights. With 100 light
> sources, that means any given light source will be considered for only 1 in
> 100 pixel samples, but its contribution to that sample will be 100 times
> that of what it would be if it were just one light source. When that sample
> is now being clamped, this brightness gets limited and the appears darker
> than it should be.
>
> Without looking at the Guido's scene, I cannot tell if this is what’s the
> issue there, but this is my educated guess.
>
> -Stefan
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