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

[Stefan Werner]

> 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