Post Processing with Imagers - Arnold User Guide
Post-processing nodes called imagers operate on pixels before the output driver. Imagers can be chained together.
Important:
The Intel® Open Image Denoiser imager (OIDN) is now added by default to new scenes. More information is available here.
Important:
Currently, Arnold refreshes the IPR when a new imager is added. It is, therefore, recommended to add imagers before starting an IPR session. Denoiser imagers should be applied before post-processing imagers if the post-processing is introducing new features in the image (ex: bloom). You should apply the imager_light_mixer before any denoiser imagers, as light AOVs won't necessarily be denoised by the denoiser.
An introduction video to imagers can be found here.