var trimx;
     var trimy;
     # the original values of trimx and trimy
     var ox, oy;
     if (firstscan) {
          #center position is zero
          trimx=128;
          trimy=128;
          ox=128;
          oy=128;
     }
     #check for trimming button pressed
     if (js0.b[0]) {
          trimx=128-js0.a[0]+ox;
          trimy=128-js0.a[1]+oy;
     } else {
          ox=trimx;
          oy=trimy;
     }
     #check for reset button pressed
     if (js0.b[1]) {
          #reset center position is zero
          trimx=128;
          trimy=128;
          ox=128;
          oy=128;
     } 
     a[0]=js0.a[0]-trimx+128;
     a[1]=js0.a[1]-trimy+128;