SatView can extract a subregion of a satellite image. It is necessary to define a polygon on the screen that encloses the desired subset of image points. To define the subregion, hold down the shift key and click the left mouse button at each vertex of the desired polygon. Click the right to automatically close the polygon. When you select Write Subregion to File, a new file will be created containing only the points inside the polygon. Clicking the left button again after closing the polygon erases it and starts a new one. Please note that extracting a subregion from a large (or high-resolution) image involves a great deal of computation and can take a very long time. The same effect may be possible in less time by using the program si_blast to create a subregion file. See the si_blast man page for more information.