How do I mask out bad pixel regions in GALFIT?

Masking out pixels one by one is tedious, how do I mask out a region more easily?

Sometimes you may come across a situation where you need to mask out a whole lot of pixels, but the region you're trying to mask has a complex shape. In that situation, it would be nice if one can draw some shape around that region. That is possible, as shown in the following figure:

Above: an example of an image mask. The green outline is the polygon drawn into SAOimage DS9, and the filling is done by the ``fillpoly'' program.
The easiest way to mask out a large region interactively is to use SAOimage DS9 in conjunction of two algorithms ``ds9poly.c'' and ``fillpoly.f'' that I provide, with instructions below. The steps look long, but once everything is set up, the only steps to remember are: draw polygon, run ``ds9poly'', and run``fillpoly''.

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