I hate to do this to everyone because I normally search through the other posts for help before I make one of my own, but the search dialog doesn't seem to work at the moment - I get an error about the full-text index not available(?). I know this topic has been discussed over and over. I've always relied on my 3D-card to do back-face culling in my program. Now, I need to learn it myself (eg. do it in software). If I have a number of vertices and the order they were selected from the screen by the user, how do I work out whether the order the points were selected in, was clockwise or counter-clockwise? I know it has something to do with the plane normal (and I know how to get that) but I don't know what to do with the plane normal once I have it (align the plane with the camera's x/y axis plane and test the z direction?). I need a more detailed description. Thanks, Paulcoz. Edited by - paulcoz on January 9, 2001 6:37:14 PM

(normal|one_of_the_vertices - cammera_pos)>0 ? cull

The above won''t work as you expect because the you are testing to
see if the plane is back-culled against your entire screen, not
just a single camera point. You need more than one test to test
the entire frustum area.

Fortunately for me I only need to know whether the shape made by the vertices was drawn clockwise or counter-clockwise when the user is drawing it on the screen. That's a lot easier because in the standard left, behind, and above views (it's for a level editor) you are aligned with the world axes and all of the vertices are on the same axis-aligned plane (have the same camera depth).

I've had a chance to think about this now, and what I am going to do is this:

(a) Calculate the x and y difference between the first point and the second point in camera space.

(b) Calculate the x and y difference between the first point and the last point in camera space.

(c) Get the z value (which is the only one I need in this case) of the normal for the intersection point made by these two vectors by:

1.Multiplying the x value in (a) by the y value in (b).
2,Multiplying the x value in (b) by the y value in (a).
Subtract 2 from 1 to get the z value.
(I used a determinant to get the order)

(d) Check the z value - a negative value indicates clockwise order, a positive value indicates counter-clockwise order.

It worked on paper anyway, and is accurate enough for my purposes.
Paulcoz.

Edited by - paulcoz on January 11, 2001 1:36:24 AM

Sorry, that reply was for a perspective view, not an ortho one.