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polygon/triangle in clockwise order

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Depends on what you mean. There's no such thing as a polygon that is inheritly "clockwise" in order. It depends on the angle from which you view it at. That's how backface culling works in most graphics API's.

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First, note that a triangle ABC can only be classified as clockwise or counterclockwise as viewed from a given position P not in the plane of the triangle.

So, given ABC and P, you can do this:


Vector normal = Cross(B - A, C - A);
bool counterclockwise = Dot(normal, P - A) > 0;



What P is depends on your application. It might be the camera position, the world origin, a vertex of some other triangle, or something else entirely.

If your triangle is not in 3D but in 2D, you can simplify the test accordingly (knowing that z = 0).

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