3dTiles 最後一塊拼圖：幾何偏差與屏幕空間偏差

// Cesium3DTile.js Cesium3DTile.prototype.getScreenSpaceError()
Cesium3DTile.prototype.getScreenSpaceError = function (
  frameState,
  useParentGeometricError,
  progressiveResolutionHeightFraction
) {
  var tileset = this._tileset;
  var heightFraction = defaultValue(progressiveResolutionHeightFraction, 1.0);
  var parentGeometricError = defined(this.parent)
    ? this.parent.geometricError
    : tileset._geometricError;
  var geometricError = useParentGeometricError
    ? parentGeometricError
    : this.geometricError;
  if (geometricError === 0.0) {
    // Leaf tiles do not have any error so save the computation
    return 0.0;
  }
  var camera = frameState.camera;
  var frustum = camera.frustum;
  var context = frameState.context;
  var width = context.drawingBufferWidth;
  var height = context.drawingBufferHeight * heightFraction;
  var error;
  if (
    frameState.mode === SceneMode.SCENE2D ||
    frustum instanceof OrthographicFrustum
  ) {
    if (defined(frustum._offCenterFrustum)) {
      frustum = frustum._offCenterFrustum;
    }
    var pixelSize =
      Math.max(frustum.top - frustum.bottom, frustum.right - frustum.left) /
      Math.max(width, height);
    error = geometricError / pixelSize;
  } else {
    // Avoid divide by zero when viewer is inside the tile
    var distance = Math.max(this._distanceToCamera, CesiumMath.EPSILON7);
    var sseDenominator = camera.frustum.sseDenominator;
    error = (geometricError * height) / (distance * sseDenominator);
    if (tileset.dynamicScreenSpaceError) {
      var density = tileset._dynamicScreenSpaceErrorComputedDensity;
      var factor = tileset.dynamicScreenSpaceErrorFactor;
      var dynamicError = CesiumMath.fog(distance, density) * factor;
      error -= dynamicError;
    }
  }

  error /= frameState.pixelRatio;

  return error;
};

上述源碼是獲取當前 Tile 的屏幕空間偏差的，在這裏，能夠很直觀看出 幾何偏差 與 屏幕空間偏差 的關係。
屏幕空間偏差是指當前攝像機狀態下（若是是2D，那麼就是當前高度的視角下），實時計算出來的一個值
而幾何偏差，則用於計算當前狀態下的屏幕空間偏差，見這兩行：

error = geometricError / pixelSize;

error = (geometricError * height) / (distance * sseDenominator);

而具體的這個屏幕空間偏差的含義，待後續研究。