Polygon Intersection

When you say the triangles overlap, do you mean they''re parallel and they occupy the same space, or do you mean they intersect?

It might be both. But I managed to get rid of the triangles that are "parallell and occupy the same space". It''s the intersecting ones that are giving me problems. Sorry for being so vague.

Perhaps I misunderstand you, but why are the intersecting triangles giving you problems? Unless you need a closed mesh for some reason, Z buffered rendering should take care of them nicely.

If you''re saying it looks wrong, perhaps a screenshot would help.

Here''s a detailed explanation of what I''m doing. I''ve got a 3D object and I want to add some "logical" information to parts of its surface (nothing to do with openGL, this information is used by an AI engine). So anyway, I use a "click and drag" selection to select a "square" on the surface of the objectusing the feedback buffer; so far so good. Now if I want to add another "square" that intersects the first one I get twice the info of the intersected part and I don''t want that. The way it looks is great, but it doesn''t look like it''s drawn, if you see what I mean.

Let me try and explain by example. If I select a perfect square with bottom left (1,1) and top right (3,3). The feedback buffer tells me there are 2 triangles (1,1)(1,3)(3,3) and (1,1)(3,1)(3,3). Now if I add to my selection another perfect square from (2,2) to (4,4) I get 2 new triangles (2,2)(2,4)(4,4) and (2,2)(4,2)(4,4). So now I have 4 overlapping triangles in my selection. But what I would like to have are, for example:
(1,1)(1,3)(3,3)
(1,1)(3,1)(3,3)
(3,2)(4,2)(3,3)
(4,2)(3,3)(4,4)
(4,4)(3,3)(2,4)
(2,4)(2,3)(3,3)

Sorry about all this, but I don''t have a personal webpage where I can post images. And, as I said, this is definitely not an openGL question but I really dont know where else to post this since I think this must be a common problem (and it apears it''s not...)

Ok, I think I follow you: you want to retesselate the mesh where it intersects your selection squares in eye space, and then retreive the triangles which are within the squares. It''s definitely not a question I''ve seen before.

I''m not sure about multiple squares (which may be non-convex, and therefore harder), but you could split the triangles for a single square by treating the four lines as planes which run outwards from the eyepoint to form a frustum. Then, for each plane, you could you could split each triangle whose lines intersect the plane. The intersection point can be found like this.

You could do this for each selection square.

Finally you could build a list of of the triangles which fall within all four plane equations by plugging their points into each plane and selecting triangles which are inside each plane.

Hope that helps. I''m moving this to graphics programming and theory, so someone else might be able to help you there.