= extract_vertices =

Used to compute the valid vertices (corners) of an image. Defines a polygon - inside the polygon are valid sampled pixels outside are nulls. Scans the image, extracts vertices and writes results to a keyword list.
Name of the keyword list is the image_file with"_vertices.kwl" appended.
So if image = "foo.tif" then results file = "foo_vertices.kwl".

{{{
extract_vertices <image_file>
NOTE:
Scans the image, extracts vertices and writes results to a keyword list.
   Name of the keyword list is the image_file with"_vertices.kwl" appended.
   So if image = "foo.tif" then results file = "foo_vertices.kwl".
}}}