Fireflies are the bright spots that you can find in an image, don’t think that it is a bug… not at all, it is a real physical effect of the light. It can be caused by caustics, extreme accuracy cases, corners, sampling etc… so, it is totaly expected !
Even if this physical correct situation happend it is something we want to get rid, because visually it looks like an error and does not bring anything good to the image, we can of course wait the it dissappear but then you have to wait for a very long time !
There are several techniques that allow to handle this, like MIS, BDPT, MLT etc… but even with theses techniques it appear that sometimes we continue to got such bright spots !
There are several ways to improve such situation, like concentrating more samples in such area, detecting such “high power” pixels, using visual techniques to detect them, use basic statistics to compare frames to frames etc… and of course the easiest and fastest way is to simply filter such bright spots.
In our internal renderer, aka Spectral Studio, we have added some “filters” to remove the fireflies during the rendering, in addition to all the other techniques to improve such situation.
The following picture show the same scene but rendered with and without fireflies.