Many filters alter not only the filling of an object, but also its shape. For example, the "rough paper" filter shrinks the edges. How can I avoid affecting the edges when applying this filter ? For example, I would like to fill a perfect rectangle with "rough paper" texture filter, keeping it rectangle. Any advice ?

One way is to make it bigger than you require, then clip it with a rectangular shape.

Take a look at Part 13 of my tutorial series (over here: viewtopic.php?f=6&t=11981) which covers clipping and shows a couple of examples that might be relevant. Part 14 covers masking and may also be worth reading if you want a softer edge.