P2D and set() behave strange

this code works OK with default render ( fills up the screen from top to down ), however with P2D it makes strange things (makes some strips like a comb). I think it is because of some incompactibility of set() function with P2D render (I guess something like bad synchronization of framebuffer operations...).

I use processing 2.0b6  (maybe in 2.0b7 it is already repaired, I don't know)

2. final int sz = 512;
3. final int sz2 = sz*sz;
4. final int n = 1000;
5. 
   
6. int npoints;
7. color c;
8. 
   
9. void setup(){
10.   //size(sz,sz    );   // this works OK
11.   size(sz,sz,P2D);   // this behave strange
12.   background(255);
13.   color c = color(random(255), random(255), random(255));
14. }
15. 
    
16. void draw(){
17.   for (int i=0;i<n; i++){    // loop over some chunk of workitems
18.     int id=npoints%sz2;    // workitem id
19.     if (id==0){ c=color(random(255), random(255), random(255));  }  // new screen start here
20.     int ix=id%sz; int iy=id/sz;    // indexes of pixel
21.     set( ix,iy, c  );
22.     npoints++;
23.   } 
24. }
 

NOTE - if you ask why the hell I tried such silly way of rastering over the whole screen, it is because I use processing to debug Kernels for OpenCL, and there varibale "id" is index of workitem