There is another issue in dividing the pixmap in parts and rendering only one at time.
When I play the sound the wave should be shifted from right to left ( simulating the time that spends ).
Doing so I should have infinite ( or many ) pixmaps to have a continuous effect and not a "discrete" effect.
So my idea is to fill the full image in a thread and render fragment of it.
So how can I do it?

Bye