# How to determine if two straight lines are collinear

How to determine if two straight lines are collinear， And can set tolerances for distance and angle，What method should I use to implement this function

I would probably make an A -B testing of the tangency of the two lines and compare the tangency vectors with each other. You might have to first check if the tangency needs to be flipped first… For the tolerance I would take the x, y, z val and use a tolerance value and if it’s inside this tolerance it would return true. Are you using pure Grasshopper or C# / Python?

1 Like

Using Python

LineCo-Linear.gh (3.7 KB)

Here would be a solution (Just with angle tolerance and in c#)

1 Like

This seems to have some issues when the tolerance is very small

You can compute the distance from the endpoints of one line onto the infinite of the other:

231023_Colinear_00.gh (5.8 KB)

If you also want to check angles, you can use the properties/methods we talked about over here:

3 Likes

What is this

Test for both of:

Vector → Vector → Cross Product, L == 0 (or L < tol)
Curve → Analysis → Curve Proximity, and D == 0

I don’t think I’ve overlooked an edge case. The Cross Product ==0 test is easily thrown off by floating point inaccuracies, so < tol may be preferable.

two_curves_colinear.gh (7.5 KB)

I don’t know what the Python equivalents of cross product and proximity are (it’s probably a Numpy or Scipy thing), but if calls to NodeInCode to recreate the above GH definition work, then they’re not needed and you don’t need to process the Rhino or Grasshopper curve information yourself in Python then either.

1 Like

Thank you, it has been resolved

Even if it is closed, maybe it helps someone at some point:

You could cast the line to Vector3D to Point3d and than measure the distance between the points:

I am pretty sure there are still way more ways to test if they are parallel.