I want an arrow to look at trajectory of a flight as in real life. So, the first thing I tried was to just directly manipulate rotation of RigidBody with formula I found online:
velocity = get_linear_velocity()
rotation = atan2(velocity.y, velocity.x)
This works visually just fine, but the collision shape doesn't rotate, so as soon as it hits something, it bounces as if it wasn't rotated. I soon found out that these kind of manipulations are not correct way to do things with RigidBodies.
Next thing I tried was to rotate the transform:
var m = state.get_transform()
This didn't work at all, movement was chaotic, seems like transforms shouldn't be applied every frame?
I think the right way to do this is to implement lookat method, but since with RigidBodies you have to do it with setangular_velocity(), which I did with the help of code from docs (https://docs.godotengine.org/en/3.1/tutorials/physics/rigid_body.html)
func look_follow(state, current_transform, target_position):
var up_dir = Vector3(0, 1, 0)
var cur_dir = current_transform.basis.xform(Vector3(0, 0, 1))
var target_dir = (target_position - current_transform.origin).normalized()
var rotation_angle = acos(cur_dir.x) - acos(target_dir.x)
state.set_angular_velocity(up_dir * (rotation_angle / state.get_step()))
var target_position = $my_target_spatial_node.get_global_transform().origin
look_follow(state, get_global_transform(), target_position)
I have trouble understanding how this works, but I think I managed to make this work in 2D (arrow looks at player). Now what should be target_position? How to make an arrow look at path it goes through? My understanding of transforms are weak.