I would like to share some of my experiences here with the intention of knowing more tricks to circumvent this issue in Blender.
I got Boundary Artifacts while using Cryptomatte.
Rendering was done at 2560x1440 and layered the image using an Alpha Over node that takes the Matte output of the Cryptomatte node as a factor.
Even when working at high resolutions, the 1px Boundary Artifacts are still noticeable, and the more distant the object from the camera, the more vulnerable it appears.
How to use Cryptomatte correctly?
I’ve attached images and blend file to help you understand.
ⅰ) comparison of the original and processed images
ⅱ) Compositor node tree
ⅲ) processed image with 2560x1440 resolution (Click for larger screen please.)
ⅳ) blend file
Open the file and render it in F12 and you will immediately see the artifact. You can check the raw and processed images by turning on/off the Switch node in the raw/fx frame.
Cryptomatte Boundary Artifacts.blend (1.6 MB)