An opacity mask that moves gradually unhiding a mesh object?

Hello, is there any way to create a box mask that hides the mesh object, and that when moving it the part of the mesh object that is outside of this box is then visible? I’m still learning, but so far I have only done the typical hidden / visible instantaneous, playing with the opacity / translucency of the material but that is also the whole object not gradually from one part to another. A to B specific directional coordinates unhiding the mesh in a defined period of time.

Does anyone know where I should look?

Thank you
Have a nice day

Are you talking about a physical box?
If so, distance to nearest surface plus related stuff to get them going.

If you only need an x/y/z location, you can generate a mask based on color and then use a material parameter collection.
Commonly done with a sphere mask. Not sure a “box” one exists as I have never had to use it, but I would assume it does being easier than a sphere to calculate…

In either case, this is just visual. You cannot “hack” off collision partially. (Unless this is a landscape and you set it up right with the opacity mask).