finger flexion sourced from oriels

sk_demo/src/Rig.cs

@@ -51,4 +51,42 @@ public class Rig
             held = down;
         }
     }
+
+    public float FingerFlex(Hand hand, FingerId id, float deadzone = 0.15f)
+    {
+        float fingerFlex = (Vec3.Dot(
+              Vec3.Direction(
+                  hand.Get(id, JointId.Tip).position,
+                  hand.Get(id, JointId.KnuckleMinor).position
+              ),
+              Vec3.Direction(
+                  hand.Get(id, JointId.KnuckleMid).position,
+                  hand.Get(id, JointId.KnuckleMajor).position
+              )
+          ) + 1f) / 2;
+        float fingerTrim = 0f + deadzone; // 180°
+        return Math.Max(fingerFlex - fingerTrim, 0f) / (1 - fingerTrim);
+    }
+
+    public float KnuckleFlex(Hand hand, FingerId id, float deadzone = 0.15f)
+    {
+        float knuckleFlex = (Vec3.Dot(
+          Vec3.Direction(
+            hand.Get(id, JointId.KnuckleMid).position,
+            hand.Get(id, JointId.KnuckleMajor).position
+          ),
+          Vec3.Direction(
+            hand.Get(id, JointId.KnuckleMajor).position,
+            hand.Get(id, JointId.Root).position
+          )
+        ) + 1f) / 2;
+        float knuckleTrim = 0.5f + deadzone; // 90°
+        return Math.Max(knuckleFlex - knuckleTrim, 0f) / (1 - knuckleTrim);
+    }
+
+    public float Flexion(Hand hand, FingerId id, float deadzone = 0.15f)
+    {
+        float flexion = KnuckleFlex(hand, id, deadzone) * FingerFlex(hand, id, deadzone);
+        return flexion * flexion; // why assume this curve?
+    }
 }