cesium-02 scene render2-1（updateAndRenderPrimitives）

function updateAndRenderPrimitives(scene) {
    
    
  var frameState = scene._frameState;

  // 下面是primitive.update逻辑，会单独开一个篇幅分析
  scene._groundPrimitives.update(frameState);
  scene._primitives.update(frameState);

  updateDebugFrustumPlanes(scene);
  updateShadowMaps(scene);

  if (scene._globe) {
    
    
    scene._globe.render(frameState);
  }
}

继续进入主线的Globe.prototype.render

Globe.prototype.render = function (frameState) {
    
    
  if (!this.show) {
    
    
    return;
  }

  if (defined(this._material)) {
    
    
    this._material.update(frameState.context);
  }

  this._surface.render(frameState);
};

再进入this._surface.render(frameState);

QuadtreePrimitive.prototype.render = function (frameState) {
    
    
  var passes = frameState.passes;
  var tileProvider = this._tileProvider;

  if (passes.render) {
    
    
    tileProvider.beginUpdate(frameState);

    selectTilesForRendering(this, frameState);
    createRenderCommandsForSelectedTiles(this, frameState);

    tileProvider.endUpdate(frameState);
  }

  if (passes.pick && this._tilesToRender.length > 0) {
    
    
    tileProvider.updateForPick(frameState);
  }
};

这里发现四个重要的步骤
tileProvider.beginUpdate(frameState);
selectTilesForRendering(this, frameState);
createRenderCommandsForSelectedTiles(this, frameState);
tileProvider.endUpdate(frameState);
一步一步来

tileProvider.beginUpdate(frameState);

selectTilesForRendering(this, frameState);

function selectTilesForRendering(primitive, frameState) {
    
    
  // 1.清空
  var tilesToRender = primitive._tilesToRender;
  tilesToRender.length = 0;
  ..
  // 2.创建两个0级瓦片，分别是x=0 y=0 z=0/ x=1 y=0 z=0
    if (!defined(primitive._levelZeroTiles)) {
    
    
    if (tileProvider.ready) {
    
    
      var tilingScheme = tileProvider.tilingScheme;
      primitive._levelZeroTiles = QuadtreeTile.createLevelZeroTiles(
        tilingScheme
      );
      var numberOfRootTiles = primitive._levelZeroTiles.length;
      if (rootTraversalDetails.length < numberOfRootTiles) {
    
    
        rootTraversalDetails = new Array(numberOfRootTiles);
        for (i = 0; i < numberOfRootTiles; ++i) {
    
    
          if (rootTraversalDetails[i] === undefined) {
    
    
            rootTraversalDetails[i] = new TraversalDetails();
          }
        }
      }
    } else {
    
    
      // Nothing to do until the provider is ready.
      return;
    }
  } 
  ...
  // 3.判断tile的可见性
    // Traverse in depth-first, near-to-far order.
  for (i = 0, len = levelZeroTiles.length; i < len; ++i) {
    
    
    tile = levelZeroTiles[i];
    primitive._tileReplacementQueue.markTileRendered(tile);
    if (!tile.renderable) {
    
    
      queueTileLoad(primitive, primitive._tileLoadQueueHigh, tile, frameState);
      ++debug.tilesWaitingForChildren;
    } else {
    
    
      visitIfVisible(
        primitive,
        tile,
        tileProvider,
        frameState,
        occluders,
        false,
        rootTraversalDetails[i]
      );
    }
  }
}

createRenderCommandsForSelectedTiles(this, frameState);

function createRenderCommandsForSelectedTiles(primitive, frameState) {
    
    
  var tileProvider = primitive._tileProvider;
  var tilesToRender = primitive._tilesToRender;// 待渲染tile 队列

  for (var i = 0, len = tilesToRender.length; i < len; ++i) {
    
    
    var tile = tilesToRender[i];
    tileProvider.showTileThisFrame(tile, frameState);
  }
}

tileProvider.endUpdate(frameState);

主要是各个tile构建自己的DrawCommand

GlobeSurfaceTileProvider.prototype.endUpdate = function (frameState) {
    
    

... 
addDrawCommandsForTile(this, tile, frameState);
}

function addDrawCommandsForTile(tileProvider, tile, frameState) {
    
    
...
    if (tileProvider._drawCommands.length <= tileProvider._usedDrawCommands) {
    
    
      command = new DrawCommand();
      command.owner = tile;
      command.cull = false;
      command.boundingVolume = new BoundingSphere();
      command.orientedBoundingBox = undefined;

      uniformMap = createTileUniformMap(frameState, tileProvider);

      tileProvider._drawCommands.push(command);
      tileProvider._uniformMaps.push(uniformMap);
    } else {
    
    
      command = tileProvider._drawCommands[tileProvider._usedDrawCommands];
      uniformMap = tileProvider._uniformMaps[tileProvider._usedDrawCommands];
    }
...
}