Hi, I did some more testing on this. The zone_vector() had a wrong comment on it. Its really small, it is 8*(number of elements in the star shaped pattern - that is zone_outer_radius - zone_inner_radius) I added some extra lines to see where the thing breaks - it does the median filter to the full image. After the last pixel, it breaks. This is about the same time where the C code should end. Hence, my guess is there is a memory leak. IS there any way to check this? Pitam Mitra Deepspace Group, UCSB Physics.