Is there someone that can help me to find the tangent of a point on a spline with VOPs (I know the primduv() expr but I need it in the VOPs)?
I already found the position of that point with a Primitive_attribute VOP but I'm not able to find the tangent… :?
Any help?

I am not sure if this will help, but you can add normals to the points on the spline first. In VOPs (VOP SOP definition) bring in a global variable VOP along with a Parameter VOP. You would use N from Global Variable VOP since you are using the normals. Set the Parameter VOP to be a vector3 used as a reference vector (Pointing say in Y). Then pipe both VOPs into a cross product VOP and input the value into the N of the output vop. This would spit out a normal that is tangent to the surface/spline. Then wire your spline with points into the new VEX SOP (created from the VOPnet).

Or just slide on over to the exchange and download my nifty Tangent HDA!

It is all in there for all to see. I generate the tangent two ways: One a real cool nifty hack and the other is the correct way using expressions and I think VOPs. There is also an example hip file that shows you how to use it.

It works for any number of input curves with varying points.

Check it out.

http://www.sidefx.com/exchange/display.php?description=tangent [sidefx.com]

Thanks jeff for your help
I'm going to download your HDA just now.
That's what I call a great support
Thank you