F_avg
case F_avg : cur[k] = raw[k] + (prior[k]>>1); break;
CASE(F_avg) cur[k] = raw[k] + ((prior[k] + cur[k-img_n])>>1);
CASE(F_avg) cur[k] = raw[k] + ((prior[k] + cur[k-out_n])>>1);