setAxisAngleFromRotationMatrix( m ) {

		// http://www.euclideanspace.com/maths/geometry/rotations/conversions/matrixToAngle/index.htm

		// assumes the upper 3x3 of m is a pure rotation matrix (i.e, unscaled)

		let angle, x, y, z; // variables for result
		const epsilon = 0.01,		// margin to allow for rounding errors
			epsilon2 = 0.1,		// margin to distinguish between 0 and 180 degrees

			te = m.elements,

			m11 = te[ 0 ], m12 = te[ 4 ], m13 = te[ 8 ],
			m21 = te[ 1 ], m22 = te[ 5 ], m23 = te[ 9 ],
			m31 = te[ 2 ], m32 = te[ 6 ], m33 = te[ 10 ];

		if ( ( Math.abs( m12 - m21 ) < epsilon ) &&
		     ( Math.abs( m13 - m31 ) < epsilon ) &&
		     ( Math.abs( m23 - m32 ) < epsilon ) ) {

			// singularity found
			// first check for identity matrix which must have +1 for all terms
			// in leading diagonal and zero in other terms

			if ( ( Math.abs( m12 + m21 ) < epsilon2 ) &&
			     ( Math.abs( m13 + m31 ) < epsilon2 ) &&
			     ( Math.abs( m23 + m32 ) < epsilon2 ) &&
			     ( Math.abs( m11 + m22 + m33 - 3 ) < epsilon2 ) ) {

				// this singularity is identity matrix so angle = 0

				this.set( 1, 0, 0, 0 );

				return this; // zero angle, arbitrary axis

			}

			// otherwise this singularity is angle = 180

			angle = Math.PI;

			const xx = ( m11 + 1 ) / 2;
			const yy = ( m22 + 1 ) / 2;
			const zz = ( m33 + 1 ) / 2;
			const xy = ( m12 + m21 ) / 4;
			const xz = ( m13 + m31 ) / 4;
			const yz = ( m23 + m32 ) / 4;

			if ( ( xx > yy ) && ( xx > zz ) ) {

				// m11 is the largest diagonal term

				if ( xx < epsilon ) {

					x = 0;
					y = 0.707106781;
					z = 0.707106781;

				} else {

					x = Math.sqrt( xx );
					y = xy / x;
					z = xz / x;

				}

			} else if ( yy > zz ) {

				// m22 is the largest diagonal term

				if ( yy < epsilon ) {

					x = 0.707106781;
					y = 0;
					z = 0.707106781;

				} else {

					y = Math.sqrt( yy );
					x = xy / y;
					z = yz / y;

				}

			} else {

				// m33 is the largest diagonal term so base result on this

				if ( zz < epsilon ) {

					x = 0.707106781;
					y = 0.707106781;
					z = 0;

				} else {

					z = Math.sqrt( zz );
					x = xz / z;
					y = yz / z;

				}

			}

			this.set( x, y, z, angle );

			return this; // return 180 deg rotation

		}

		// as we have reached here there are no singularities so we can handle normally

		let s = Math.sqrt( ( m32 - m23 ) * ( m32 - m23 ) +
			( m13 - m31 ) * ( m13 - m31 ) +
			( m21 - m12 ) * ( m21 - m12 ) ); // used to normalize

		if ( Math.abs( s ) < 0.001 ) s = 1;

		// prevent divide by zero, should not happen if matrix is orthogonal and should be
		// caught by singularity test above, but I've left it in just in case

		this.x = ( m32 - m23 ) / s;
		this.y = ( m13 - m31 ) / s;
		this.z = ( m21 - m12 ) / s;
		this.w = Math.acos( ( m11 + m22 + m33 - 1 ) / 2 );

		return this;

	}
Sets the x, y and z components of this
vector to the axis of rotation and w to the angle.

@param {Matrix4} m - A 4x4 matrix of which the upper left 3x3 matrix is a pure rotation matrix.
@return {Vector4} A reference to this vector.