If you put a title over your image that says to right click to cancel,
title('Left click vertices then right click to finish');
[x, y] = roipolyold();
then you can check if the returned point set is empty or less than 2, and if so, they bailed out.
if isempty(x) || length(x) <= 2
% Not a complete polygon. Better not continue with code or you'll get an error.
return;
end
You can also use questdlg() to ask the user if the polygon is acceptable or not, and if not, have them redraw it.
