Function copySign

Three.js/js/ThreeDebug.js:3601–3605

( a, b )

		// Adapted from: http://www.euclideanspace.com/maths/geometry/rotations/conversions/matrixToQuaternion/index.htm

		function copySign( a, b ) {

			return b < 0 ? -Math.abs( a ) : Math.abs( a );

		}

		var absQ = Math.pow( m.determinant(), 1.0 / 3.0 );
		this.w = Math.sqrt( Math.max( 0, absQ + m.elements[0] + m.elements[5] + m.elements[10] ) ) / 2;