bone.js

var Pointable = require('./pointable'),
  glMatrix = require("gl-matrix")
  , vec3 = glMatrix.vec3
  , mat3 = glMatrix.mat3
  , mat4 = glMatrix.mat4
  , _ = require('underscore');


var Bone = module.exports = function(finger, data) {
  this.finger = finger;

  this._center = null, this._matrix = null;

  /**
  * An integer code for the name of this bone.
  *
  * * 0 -- metacarpal
  * * 1 -- proximal
  * * 2 -- medial
  * * 3 -- distal
  * * 4 -- arm
  *
  * @member type
  * @type {number}
  * @memberof Leap.Bone.prototype
  */
  this.type = data.type;

  /**
   * The position of the previous, or base joint of the bone closer to the wrist.
   * @type {vector3}
   */
  this.prevJoint = data.prevJoint;

  /**
   * The position of the next joint, or the end of the bone closer to the finger tip.
   * @type {vector3}
   */
  this.nextJoint = data.nextJoint;

  /**
   * The estimated width of the tool in millimeters.
   *
   * The reported width is the average width of the visible portion of the
   * tool from the hand to the tip. If the width isn't known,
   * then a value of 0 is returned.
   *
   * Pointable objects representing fingers do not have a width property.
   *
   * @member width
   * @type {number}
   * @memberof Leap.Pointable.prototype
   */
  this.width = data.width;

  var displacement = new Array(3);
  vec3.sub(displacement, data.nextJoint, data.prevJoint);

  this.length = vec3.length(displacement);


  /**
   *
   * These fully-specify the orientation of the bone.
   * See examples/threejs-bones.html or https://developer.leapmotion.com/gallery/bone-basis-arrows for more info
   *
   * Three vec3s:
   *  x (red): The rotation axis of the finger, pointing outwards.  (In general, away from the thumb )
   *  y (green): The "up" vector, orienting the top of the finger
   *  z (blue): The roll axis of the bone.
   *
   *  Most up vectors will be pointing the same direction, except for the thumb, which is more rightwards.
   *
   *  The thumb has one fewer bones than the fingers, but there are the same number of joints & joint-bases provided
   *  the first two appear in the same position, but only the second (proximal) rotates.
   *
   *  Normalized.
   *
   */
  this.basis = data.basis;
};

Bone.prototype.left = function(){

  if (this._left) return this._left;

  this._left =  mat3.determinant(this.basis[0].concat(this.basis[1]).concat(this.basis[2])) < 0;

  return this._left;

};


/**
 * The Affine transformation matrix describing the orientation of the bone, in global Leap-space.
 * It contains a 3x3 rotation matrix (in the "top left"), and center coordinates in the fourth column.
 *
 * Unlike the basis, the right and left hands have the same coordinate system.
 *
 */
Bone.prototype.matrix = function(){

  if (this._matrix) return this._matrix;

  var b = this.basis,
      t = this._matrix = mat4.create();

  // open transform mat4 from rotation mat3
  t[0] = b[0][0], t[1] = b[0][1], t[2]  = b[0][2];
  t[4] = b[1][0], t[5] = b[1][1], t[6]  = b[1][2];
  t[8] = b[2][0], t[9] = b[2][1], t[10] = b[2][2];

  t[3] = this.center()[0];
  t[7] = this.center()[1];
  t[11] = this.center()[2];

  if ( this.left() ) {
    // flip the basis to be right-handed
    t[0] *= -1;
    t[1] *= -1;
    t[2] *= -1;
  }

  return this._matrix;
};

/**
 * Helper method to linearly interpolate between the two ends of the bone.
 *
 * when t = 0, the position of prevJoint will be returned
 * when t = 1, the position of nextJoint will be returned
 */
Bone.prototype.lerp = function(out, t){

  vec3.lerp(out, this.prevJoint, this.nextJoint, t);

};

/**
 *
 * The center position of the bone
 * Returns a vec3 array.
 *
 */
Bone.prototype.center = function(){

  if (this._center) return this._center;

  var center = vec3.create();
  this.lerp(center, 0.5);
  this._center = center;
  return center;

};

// The negative of the z-basis
Bone.prototype.direction = function(){

 return [
   this.basis[2][0] * -1,
   this.basis[2][1] * -1,
   this.basis[2][2] * -1
 ];

};