My grasshopper is ver1.0.0007.

When I’m using goal-angle component of kangaroo 2, I found that it can’t work when the angle of input lines is 0 degree. Neither can the rod component. They can’t work until at least one of the input lines’ angles is bigger than 0 degree.

So is it a bug or something normal?

I think it’s normal. when two segments are collinear, two forces of equal magnitude and in opposite directions balance each other.

This is not a bug. The angle goal works in 3d, so if you start out with a pair of consecutive segments straight, then try and increase the angle, bending either way in plane is a valid solution. Relative to one fixed segment, you can think of the second segment as having to lie anywhere on the cone formed by revolving a line at the given RestAngle to the first:

When there are no other forces acting to make it bend one way or the other, the segments will stay straight.

I’m guessing you might be thinking of a 2d angle goal, which could actually be a useful thing to add, with an input for the plane the angle is measured in, as then one could distinguish between clockwise/counter-clockwise bending.

@DanielPiker it wouldn’t be a bad idea to include an optional plane parameter in the angle goals for when the dot product is zero, so you support 2d defined by the user with a plane, why not?

Thank you for your answer @DanielPiker

As you say, an input for a plane the angle is measured in will be useful under this special circumstance.