ok i found it !
if i put my ellipse on ellipseMode(CENTRE) it work.
public void display(PApplet p){
for(PVector g : grass){
p.ellipseMode(PConstants.CENTER);
p.fill(0);
p.ellipse(g.x, g.y, r, r);
p.text("x " + g.x + " y " + g.y, g.x+16 , g.y);
}
}
but why ?