improved MB (proportional, not 50/50)

src/main/java/com/elphel/imagej/gpu/GpuQuad.java

@@ -4380,12 +4380,19 @@ public class GpuQuad{ // quad camera description
 							}
 							dx *= mb_offs;
 							dy *= mb_offs;
-							centerXY_sub = new double[] {centerXY[0]+0.5*dx,centerXY[1]+0.5*dy};
-							centerXY =     new double[] {centerXY[0]-0.5*dx,centerXY[1]-0.5*dy};
-							tp_task_sub.setCenterXY(centerXY_sub);
 							double exp_offs = Math.exp(-mb_offs/mb_len);
+							// centroid offset from the first point along the {dx,dy}, in the range 0..1
+							// using that centroid of the half-exponent (-inf to 0) is at -1
+							double rel_cent = mb_len* (1.0 - (mb_offs/mb_len + 1.0) * exp_offs) / (mb_offs * (1 - exp_offs)); 
+							
+							centerXY =     new double[] {centerXY[0]-rel_cent*    dx, centerXY[1]-rel_cent*    dy};
+							centerXY_sub = new double[] {centerXY[0]+(1-rel_cent)*dx, centerXY[1]+(1-rel_cent)*dy};
+							
+							tp_task.setCenterXY(centerXY);
+							tp_task_sub.setCenterXY(centerXY_sub);
 							double gain =    1.0/(1.0 - exp_offs);
 							double gain_sub = -gain * exp_offs;
+							
 							if (gain_sub > -min_sub) {
 								gain_sub = -min_sub;
 							}