OrientOnSrf can't get curves to be evenly projected(rhino mac)

Having trouble putting curve on surface…

Used unrollsrf to get a flat surface of the cylinder.
Made a line from edge to edge on the flat surface.
Using OrientOnSrf, tried to get the line on the cylinder, but can’t seem to
get it go around 360deg.
Orientonsrf.3dm (58.3 KB)

Try flowAlongSrf and choose the reference and target surfaces on their edges near the same corner.
I think it will work for what you want

Thank you. Tried with FlowAlongSrf & works well.

Could I ask, when would OrientOnSrf useful for? putting on non-basic surfaces?
Trying to figure out which tools are good for which situations…
For the OrientOnSrf, I just can’t grasp the rules on the deformation…

OrientOnSrf is used for aligning difficult objects on freeform surfaces based on a direction you choose and (usually) by keeping your copies Rigid.
It’s like a NormalAlign in other 3D programs.
Check the classic rubberduck exercise in Rhino Level 1 manual. It is used for the placement of the eyes.

Orient3Pt is better for objects that are going to be placed on a surface based on one of their planar rectangular sides.

Got it. Thank you!

Just used Orient3Pt. Simpler tool for the job.
So OrientOnSrf is more for the flexible usage.
The problem I faced with OrientOnSrf seems a little bit clear now after using the Orient3Pt.
Just guessing how it works but, it seems the third point is problematic if the part is like a cylinder where the 3rd point becomes same position as the 1st or 2nd. As you suggested the flowalongsrf is the better approach for the cylinder.

If you have to place many objects in columns and rows on a surface you should also check the command ArraySrf.

The more you work in Rhino it will become easier to develop better strategies for every situation.