calculateTbRadius( camera ) {

		const distance = camera.position.distanceTo( this._gizmos.position );

		if ( camera.type == 'PerspectiveCamera' ) {

			const halfFovV = MathUtils.DEG2RAD * camera.fov * 0.5; //vertical fov/2 in radians
			const halfFovH = Math.atan( ( camera.aspect ) * Math.tan( halfFovV ) ); //horizontal fov/2 in radians
			return Math.tan( Math.min( halfFovV, halfFovH ) ) * distance * this.radiusFactor;

		} else if ( camera.type == 'OrthographicCamera' ) {

			return Math.min( camera.top, camera.right ) * this.radiusFactor;

		}

	}
Calculates the trackball radius so that gizmo's diameter will be 2/3 of the minimum side of the camera frustum.

@private
@param {Camera} camera - The camera.
@returns {number} The trackball radius.