# How do I determine Clockwise rotation of vertexes

Can someone help me with a quick formula or even a short explanation on how I would determine the order of vertexes in a 3D space. I’d like to do this so that I can reorder all my vertexes to be in counter clockwise order.

for a little more explanation if needed, I am only using triangles, so I only have 3 points for each triangle which I would like to reorder.

So if I have (0,0,0), (1,0,0) and (1,1,0). How can I make sure they go in the right order as above or one of the other 2 possibilities and not in the wrong order such as (1,1,0) (1,0,0) (0,0,0) or (1,0,0) (0,0,0) (1,1,0).

Any ideas?

Thanks,

Korin Bampton

Well I only got 3 hours of sleep last night so if this is confusing bear with me…
This assumes the positive z axis is “up”.
Take your 3 vertices v1,v2,v3
(each of which has components .x, .y, .z)
vector1 = v2-v1;
vector2 = v3-v1
vector3 = vector1 X vector2 (cross product)
if vector3.z > 0, then all is well,
otherwise, swap vertices 2 and 3, then all will be well.
Oh and I think you meant COUNTER clockwise, as this is the default orientation of OpenGL.
This example ensures counterclockwise orientation.
If you don’t know what any of this means, I encourage you to learn linear algebra THEN come back to OpenGL - it will save you time in the long run.

Which side is the front face and which is the back face (along with the chirality of the axes), determines which direction is clockwise and which is counter-clockwise. Just using the z axis as a reference is not enough.