Degenerate Polygons?

Hey,

From what I understand of degenerate polygons, they are basically a polygon with two or more points that are the same.

For example, if a triangle had two points, it would become a line, or if all three points are the same, it would be a point.

I hope this sheds some light, and that I have answered your question
-Boblin

Edit: hey, its 4 in the afternoon, I just woke up :-P

[edited by - Boblin on July 21, 2003 4:23:32 PM]

Yes, degenerate polygons are triangles with two or more points being the same. Graphics drivers / hardware recognize degerates and know not to try to render them.

I like pie.

I''ll give ya an example of how degenerate triangles are sometimes used, just so you can see why they''re very useful. Graphics cards are usually optimized to process triangle strips faster than any other drawing primitive (triangles, quads, polygons, triangle fans, etc.).

Basically, you define the first triangle by it''s three points, and then each successive triangle only needs one additional point defined as it uses the last two points from the previous triangle.

Example.
Triangle 1: (0, 0, 0) (0, 1, 0) (1, 0, 0)
You would then define a fourth point, say (1, 1, 0)
This would create a second triangle with points
(0, 1, 0) (1, 0, 0) (1, 1, 0)

How do degenerate triangles help? If you want to have two objects drawn using one triangle strip instead of having to have two separate ones. The less calls you make to the video card to draw, the better it usually is. So what you would do, is you would repeat the last point of the first object, and also repeat the first point of the second object.You then take these two separate objects, and stick them together.

This is basically how it works. A degenerate triangle is any triangle with two or more points that are exactly the same. This would result in either a line (2 points the same) or a point (3 points the same). If you''re telling the graphics card to draw triangles, it won''t draw any degenerate triangles. So the extra degenerate triangles that connect the two objects wouldn''t be drawn, so you''d only see the two objects you wanted. And you''ve used one triangle strip instead of two.

Of course, there is some overhead in skipping over degenerate triangles, and extra memory usage. You have to balance the pros and cons of using degenerate polygons.

This might seem kind of strange. I know it did the first time I was shown it. But it''s a pretty cool way to use the graphics card''s built in twists to your advantage.

A couple books I could recommend are:
Computer Graphics by Hearn and Bake
Mathematics for 3D Game Programming & Computer Graphics by Lengyel

This are math heavy, so if that''s a little daunting for you yet, they''re just something to keep in mind

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Keith
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