So I’ve been using Cycles now for quite a few years and have always wondered about this: Why does a single frame always have a fixed noise pattern seed? Many fireflies virtually never disappear in certain circumstances when all samples in a frame are using the same seed. If, for every sample, for even every x samples, the seed is changed, you can basically eliminate, or greatly reduce, fireflies.
A quick way to test this idea is to render a comparison between an image rendered with X samples, compared to the same image rendered many times with different seeds, but with equivalent fewer samples.
In this example, I rendered an image with 300 samples and compared it to 10 images of 30 samples each with different noise seed, which gives you the same 300 samples in total. These images are combined using mean blending between them so each noise pattern has an equal influence. As you can see, the noise is more subdued, more even and the fireflies are basically eliminated.
I wonder, is there a particular reason why Cycles or other path tracers don’t already do this kind of thing internally? Seems like an obvious and very easy fix for an annoying rendering problem.
1 x 300 samples, fixed seed:
10 x 30 samples (300 samples total), 10 different seeds mean combined: