Usage: immask [-thresh #] [-mask mask_image] [-pos] input_image output_image
* Masks the input_image and produces the output_image;
* Use of -thresh # means all pixels with absolute value below # in
   input_image will be set to zero in the output_image
* Use of -mask mask_image means that only locations that are nonzero
   in the mask_image will be nonzero in the output_image
* Use of -pos means only positive pixels from input_image will be used