Here we're using the Default Denoise settings but there are other useful settings:

• Volumes - used to denoise volume renderings (where the volume is rendered alone on transparent black)
• Alpha - used to denoise the alpha mask

You can also choose different modes:

• Frame - denoise a single frame
• Crossframe - denoise an animation of multiple frames (this requires multiple frames to be available to begin the denoise process instead of each frame immediately as it finishes)

Your pixel filter should be set to "importance". RenderMan for Maya and Houdini turns this on automatically when selecting Denoise on AOVs