# Shifted curve rotation

Hi all,

I was just playing around with rhino by rotating a curve :

As you can see in the plane of the curve, the plane gets shifted all the time somehow (I just rotated like 2 deg. but the corresponding plane is rotated about 90 deg.). What could be the reason for this ?

Hello,

its a line not a curve!It has no curvature and so you cannot explicitly compute a normal of a line. There are infinite solutions orthogonal to the tangent. I have no idea how its computed within the library, but I guess its trying to guess a good normal by its position in relation to the world plane.

Hi Tom,

yeah, its a line. Instead of using a curve component, I changed it to a line component in my component. However, the problem is still there.

The issue isnât using curve vs line component, the issue is this

It has no curvature

2 Likes

A plane consists of an origin and two vectors. In the above images these vectors are y-axis and x-axis and the line is rotating around z- axis.
Why is it impossible to rotate this line without its plane jumping around ? And what does this have to that the line has no curvature?

I am not really understanding the issue with no curvatureâŚ

A frame on a curve is based on the point, tangent and the normal at a parameter. Since a line has no curvature (k=0) there is no hint for a normal. Any vector perpendicular to a tangent could be the normal vector of a line, so there are infinite solutions. If you are more strict with the definition of a normal => a normal is also known as curvature vector, you would even need to say a line has a null normal.

Make sure the gumball is set to local to the object before rotating. If your line isnât parallel to any of the world axis youâll get âunexpectedâ results.

edit:

Gumball set to local object vs cplane/world.

1 Like