6.6.1 Degrain Filtering attributes

Post Effects tab in the Render Settigns Window when Iray Photoreal is selected
Degrain Filtering can reduce low frequency noise without sacrificing overall sharpness. These are intended to be used in the final stage of the rendering phase, mainly to reduce remaining subtle grain in difficult areas of a scene.
Unfortunately there is no general recommendation on which degrain filter mode will perform best on a specific scene, thus it is necessary to cycle through the different modes manually to receive the best results for a given scene. As a rule of thumb, modes 3 and 5 are usually the most reliable ones, with 5 being the more aggressive of the two. Also note that the overall rendering performance can be reduced noticeably on low end CPU cores, thus the filter should only be enabled in the final phase of the rendering process.
Mode 0 disables the degrain function. Modes 1 to 3 are working very conservative and should thus be safe to use in general, modes 4 and 5 are considered to be more aggressive and should be used with caution, especially if the scene features fine details in either geometry or applied materials. As these filters can be tweaked during the rendering process, it is recommended to experiment with different radius settings to achieve best results.
This value should be reduced if the filter smooths out edges, and increased if some noise still remains in the image.
Blur Difference
Modes 4 and 5 feature an additional setting that limits the influence of neighboring pixels if the brightness is too different. Higher values create a more blurry effect.