Considering example:

I have a mesh(es) on the OpenGL screen and would like to select a part of it (say for delete purpose). There is a clear way to do the selction via Ray Tracing, or via Selection provided by OpenGL itself.

But, for my users, considering that meshes can get wired surfaces, I need to implement a selection via a Arbitrary closed region, so all triangles that appears present inside that region has to be selected.

To be more clear, here is screen shot:

I want all triangles inside black polygon to be selected, identified, whatever in some way.

How can I achieve that ?