Usage: gdal_viewshed [--help] [--help-general]
                     [-b <band>]
                     [-a_nodata <value>] [-f <formatname>]
                     [-oz <observer_height>] [-tz <target_height>] [-md <max_distance>]
                     -ox <observer_x> -oy <observer_y>
                     [-vv <visibility>] [-iv <invisibility>]
                     [-ov <out_of_range>] [-cc <curvature_coef>]
                     [-co <NAME>=<VALUE>]...
                     [-q] [-om <output mode>]
                     <src_filename> <dst_filename>