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.
Denoising does not affect the alpha channel of images. Doing so can cause artifacts and halos to form and is therefore avoided.
Denoise can attempt to use GPU acceleration. You can activate this mode by adding
--override gpuIndex 0 to the command line, where the number indicates which GPU to use. In a single-GPU system, this will always be 0. If no compatible hardware is found it will failover to CPUs. Using the GPU requires CUDA 7.0 (compute capability 2.0 or later) and a capable graphics card with support.
Used to filter based on image alpha.
Used for volumes rendered alone onto transparent black.