To get this value, you need to "uncurl" the finger direction based on the current metacarpal direction. It's a little involved in the end; you have to construct some basis vectors in order to uncurl the hand along juuust the right axis. Hopefully the comments in this example script will explain everything.
using Leap;
using Leap.Unity;
using UnityEngine;
public class MeasureIndexSplay : MonoBehaviour {
// Update is called once per frame
void Update () {
var hand = Hands.Get(Chirality.Right);
if (hand != null) {
Debug.Log(GetIndexSplayAngle(hand));
}
}
// Some member variables for drawing gizmos.
private Ray _metacarpalRay;
private Ray _proximalRay;
private Ray _uncurledRay;
/// <summary>
/// This method returns the angle of the proximal bone of the index finger relative to
/// its metacarpal, when ignoring any angle due to the curling of the finger.
///
/// In other words, this method measures the "side-to-side" angle of the finger.
/// </summary>
public float GetIndexSplayAngle(Hand h) {
var index = h.GetIndex();
// These are the directions we care about.
var metacarpalDir = index.bones[0].Direction.ToVector3();
var proximalDir = index.bones[1].Direction.ToVector3();
// Let's start with the palm basis vectors.
var distalAxis = h.DistalAxis(); // finger axis
var radialAxis = h.RadialAxis(); // thumb axis
var palmarAxis = h.PalmarAxis(); // palm axis
// We need a basis whose forward direction is aligned to the metacarpal, so we can
// uncurl the finger with the proper uncurling axis. The hand's palm basis is close,
// but not aligned with any particular finger, so let's fix that.
//
// We construct a rotation from the palm "finger axis" to align it to the metacarpal
// direction. Then we apply that same rotation to the other two basis vectors so
// that we still have a set of orthogonal basis vectors.
var metacarpalRotation = Quaternion.FromToRotation(distalAxis, metacarpalDir);
distalAxis = metacarpalRotation * distalAxis;
radialAxis = metacarpalRotation * radialAxis;
palmarAxis = metacarpalRotation * palmarAxis;
// Note: At this point, we don't actually need the distal axis anymore, and we
// don't need to use the palmar axis, either. They're included above to clarify that
// we're able to apply the aligning rotation to each axis to maintain a set of
// orthogonal basis vectors, in case we wanted a complete "metacarpal-aligned basis"
// for performing other calculations.
// The radial axis, which has now been rotated a bit to be orthogonal to our
// metacarpal, is the axis pointing generally towards the thumb. This is our curl
// axis.
// If you're unfamiliar with using directions as rotation axes, check out the images
// here: https://en.wikipedia.org/wiki/Right-hand_rule
var curlAxis = radialAxis;
// We want to "uncurl" the proximal bone so that it is in line with the metacarpal,
// when considered only on the radial plane -- this is the plane defined by the
// direction approximately towards the thumb, and after the above step, it's also
// orthogonal to the direction our metacarpal is facing.
var proximalOnRadialPlane = Vector3.ProjectOnPlane(proximalDir, radialAxis);
var curlAngle = Vector3.SignedAngle(metacarpalDir, proximalOnRadialPlane,
curlAxis);
// Construct the uncurling rotation from the axis and angle and apply it to the
// *original* bone direction. We determined the angle of positive curl, so our
// rotation flips its sign to rotate the other direction -- to _un_curl.
var uncurlingRotation = Quaternion.AngleAxis(-curlAngle, curlAxis);
var uncurledProximal = uncurlingRotation * proximalDir;
// Upload some data for gizmo drawing (optional).
_metacarpalRay = new Ray(index.bones[0].PrevJoint.ToVector3(),
index.bones[0].Direction.ToVector3());
_proximalRay = new Ray(index.bones[1].PrevJoint.ToVector3(),
index.bones[1].Direction.ToVector3());
_uncurledRay = new Ray(index.bones[1].PrevJoint.ToVector3(),
uncurledProximal);
// This final direction is now uncurled and can be compared against the direction
// of the metacarpal under the assumption it was constructed from an open hand.
return Vector3.Angle(metacarpalDir, uncurledProximal);
}
// Draw some gizmos for debugging purposes.
public void OnDrawGizmos() {
Gizmos.color = Color.white;
Gizmos.DrawRay(_metacarpalRay.origin, _metacarpalRay.direction);
Gizmos.color = Color.blue;
Gizmos.DrawRay(_proximalRay.origin, _proximalRay.direction);
Gizmos.color = Color.red;
Gizmos.DrawRay(_uncurledRay.origin, _uncurledRay.direction);
}
}
For what it's worth, while the index finger is curled, tracked Leap hands don't have a whole lot of flexibility on this axis.
