While some RenderMan plugins integrate the Denoise feature, RenderMan does ship with a tool for processing images after rendering. Keep in mind that the appropriate AOVs must be included for the Denoise filter to work.
denoise -h results in the following 'help' output:
There are three main sections of note in the configuration file. The
kernel section defines the properties of individual kernels filter kernels – namely the size and tolerances. The
filterbank section refers to the filter kernels by name and collects them together into filter banks. By default, there are two main ones: one for filtering diffuse layers and one for specular layers (there is also a generic specularAndDiffuse for both in special cases). Finally, there is a
layergroups section which defines the names of channels in a layer image recognized as belonging together in a layer and gives the names of filterbanks from the
filterbank section to be used to filter the diffuse and specular elements of the layer.
See the Layered Filtering section here and the comments in
default.filter.json for more details.