Thank you Kalle_H,
The technique worked perfectly. It saved 3-5 FPS from 700 HISMC instances. It is however, rather fussy about the accuracy of the mask. I was using 256x256 color mask and it gave me bad streaks. I had to upgrade to 512x512 color mask to get error free. I have never known about this unusual method. Learned something new today. Thanks again! 
