It is “Difference Matte” indeed ! Thank you !
Unfortunately it didn’t work well with the footage I had.
Many issues. For exemple : having a subject with a white T-shirt in front of a gray wall. With so, the effects is not doing great.
But I must say that it might come from my camera. I’ve shot with my Sony HXR-MC2000e which compress the rushes in a 1440p resolution. There is a noise on the image due to (I think) the low quality of the camera lens (and of course it’s a fix camera lens !).
Got this camera since nearly 10 years now on. It’s maybe time to switch aha !
Thank you for the quick answer. (and sorry for the delay of my re-answer)