this.position);

          this._value = 0.0;
          this._pressed = false;
          this._touched = false;
          this._dx = 0.0;
          this._dy = 0.0;
        }

        _updateTransform() {
          mat4.identity(this.matrix);
          mat4.translate(this.matrix, this.matrix, this.position);
          mat4.translate(this.matrix, this.matrix, [
            this._dx * BOX_MOTION_RANGE,
            -this._value * BOX_SIZE * 0.5,
            this._dy * BOX_MOTION_RANGE
          ]);
          ma