2018 will be bookmarked as a turning point for Monte Carlo rendering due to the wide availability of fast, high quality denoising algorithms, which can be attributed for a large part to Nvidia Research: Nvidia just released OptiX 5.0 to developers, which contains a new GPU accelerated post-processing denoising filter.
The new denoiser was trained with machine learning on a database of thousands of rendered images and works pretty much in real-time. The OptiX 5.0 SDK contains a sample program of a simple path tracer with the denoiser running on top (as a post-process). The results are nothing short of stunning: noise disappears completely, even difficult indirectly lit surfaces like refractive (glass) objects and shadowy areas clear up remarkably fast and the image progressively get closer to the ground truth.
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| The OptiX denoiser works great for glass and dark, indirectly lit areas |
While in general the denoiser does a fantastic job, it's not yet optimised to deal with areas that converge fast, and in some instances overblurs and fails to preserve texture detail as shown in the screen grab below (perhaps this can be solved with more training for the machine learning):
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| Overblurring of textures |
The denoiser is provided free for commercial use (royalty-free), but requires an Nvidia GPU. It works with both CPU and GPU rendering engines and is already implemented in Iray (Nvidia's own GPU renderer), V-Ray (by Chaos Group), Redshift Render and Clarisse (a CPU based renderer for VFX by Isotropix).
Some videos of the denoiser in action in Optix, V-Ray, Redshift and Clarisse:
Optix 5.0: youtu.be/l-5NVNgT70U
Iray: youtu.be/yPJaWvxnYrg
This video provides a high level explanation of the deep learning algorithm behind the OptiX/Iray denoiser based on the Nvidia research paper "Interactive Reconstruction of Monte Carlo Image Sequences using a Recurrent Denoising Autoencoder"
V-Ray 4.0: youtu.be/nvA4GQAPiTc
Redshift: youtu.be/ofcCQdIZAd8 (and a post from Redshift's Panos explaining the implementation in Redshift)
ClarisseFX: youtu.be/elWx5d7c_DI
Other renderers like Cycles and Corona already have their own built-in denoisers, but will probably benefit from the OptiX denoiser as well (especially Corona which was acquired by Chaos Group in September 2017).
The OptiX team has indicated that they are researching an optimised version of this filter for use in interactive to real-time photorealistic rendering, which might find its way into game engines. Real-time noise-free photorealistic rendering is tantalisingly close.