Thank you for this excellent answer and for taking time to have a look at my project and even to write me an expression, I really do apriciate. Unfortunately the problem was still there and I agree with you about the speed of the roll, I think that it’s to fast and that the monitors pixel simply don’t have time to completly get rid of the light they just received before some more light come to light the same pixel. Slowing down the whole thing made it a lot smoother.
Thank you very much moldyboot,Dave LaRonde and ribdog for you fantastic help, I hope I could help you back some time !
Nicolas