Video footage is typically encoded with a non-linear brightness adjustment. This can be a gamma correction like in Rec. 709 footage or a Log curve. While this encoding is helpful for our cameras to capture additional shadow detail while keeping data sizes (bit depth) manageable, it is not helpful for compositing.
In compositing tools like Fusion, we need to work in linear color space. Working in non-linear space is technically possible but can lead to errors whenever pixel interpolation is required. A simple blur or transform node is enough to see the difference and cause mathematical erros.
In this video, I demonstrate where the problems occur and show you how to manually set up a linear workflow with the Gamut node and the Cineon log tool.
You may also be interested in these tutorials about linear workflow:
great explanation.