Lukas Stockner is working on some clever Cycles improvements that will denoise renders without rendering more samples or losing details in your scenes!
Greg Zaal writes:
Lukas Stockner has been working on yet another awesome Cycles feature, this time a sneaky method to reduce noise without actually rendering more samples.
To put it simply, “denoising” is a process of analysing your render and trying to shmoosh the noise/grain together and make it look clean.
A common side-effect of all denoising algorithms is that some detail is lost. It’s almost impossible for the computer to tell the difference between noise that you don’t want and noise that you do want (like details in a texture). To address this, as far as I can tell, Lukas has added options to denoise Direct and Indirect passes separately, which allow you to (potentially) smooth only the noise introduced by the rendering process and keep all the details of your textures in tact.