code: function() {
      var gl = this._canvas.gl();
      gl.clearColor(0.0, 0.0, 0.0, 0.0);
      gl.disable(gl.BLEND);
      gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);
      gl.clearColor(this._options.background[0], this._options.background[1], 
                    this._options.background[2], 1.0);
      gl.cullFace(gl.FRONT);
      gl.enable(gl.CULL_FACE);
      this._drawWithPass('select');
    }
docs: INTERNAL: draws scene into offscreen pick buffer with the "select" shader.