well, that's more or less what I thought. This patch is just a simplified part of a much bigger patch where I use a lot the fourth argument of the erase_color (1. 1. 1.and 0. to 1. for the 4th number) attribute of the jit.render object to create some "blur" or "feedback".
The visual artist I work with would like to project the "sound-altered" moving pictures (see : http://nicolascanot.tumblr.com/ for an idea of the pictures) produced by this patch, superimposed on fixed pictures (prepared photographs in Photoshop/InDesign) using only one VP.
The problem is that, when doing so (which works when using the videoplane + alphablend objects), the "erase_color feedback" doesn't work anymore so I try to find some solutions to create feedback in the patch "before" the "GL picture" is mixed with the photographs.
Maybe the solution is simply to use two VPs unless there's some solutions to remove the (white, in this case) background of the jit.render object (I mean, working with transparencies) while keeping the erase_color 4th attribute functional (which seems contradictory !).
Not sure it's more clear now but it's not easy to explain...