osgearth瓦片渲染

osgearth瓦片渲染

相关类：

• TileNode

/**
     * TileNode represents a single tile. TileNode has 5 children:
     * one SurfaceNode that renders the actual tile content under a MatrixTransform;
     * and four TileNodes representing the LOD+1 quadtree tiles under this tile.
     */

• TileDrawable: 只是瓦片的一个外壳，持有内部的SharedGeomtry进行最后渲染

 /**
     * TileDrawable is an osg::Drawable that represents an individual terrain tile
     * for the purposes of scene graph operations (like intersections, bounds
     * computation, statistics, etc.)
     * 
     * NOTE: TileDrawable does not actually render anything!
     * The TerrainRenderData object does all the rendering of tiles.
     *
     * Instead, it exposes various osg::Drawable Functors for traversing
     * the terrain's geometry. It also hold a pointer to the tile's elevation
     * raster so it can properly reflect the elevation data in the texture.
     */

• SharedGeometry： 瓦片渲染的最小单元，由GeometryPool::createGeometry创建

• SurfaceNode： the node to house the tile drawable:TtileDrawable的持有者

• RexTerrainEngine:派生自TerrainEngine，用于生成瓦片

   * TerrainEngineNode is the base class and interface for map engine implementations.
       *
       * A map engine lives under a MapNode and is responsible for generating the
       * actual geometry representing the Earth.
  

  RexTerrainEngine有个创建tile的接口：

  一个是创建heightmap的瓦片，一个是创建地球瓦片

      //! for standalone tile creation outside of a terrain
      osg::Node* createTile(const TileKey& key);
  
      //! Create a standalone tile from a tile model (experimental)
      osg::Node* createTile(
          const TerrainTileModel* model,
          int createTileFlags,
          unsigned referenceLOD);
  

• LayerDrawable:作为一个layer所拥有的DrawTileCommand，统一进行渲染

/**
     * Drawable for single "Layer" i.e. rendering pass. 
     * It is important that LayerDrawables be rendered in the order in which
     * they appear. Since all LayerDrawables share a common bounds, this 
     * should happen automatically, but let's keep an eye out for trouble.
     */

tile的渲染：

TerrainCuller，在执行cull流程时，遍历SurfaceNode,获取，::addDrawCommand 将可见的tile加入到待渲染列表中

TerrainCuller::apply(SurfaceNode& node)
{
    TileRenderModel& renderModel = _currentTileNode->renderModel();

    // push the surface matrix:
    osg::RefMatrix* matrix = createOrReuseMatrix(*getModelViewMatrix());
    node.computeLocalToWorldMatrix(*matrix,this);
    _cv->pushModelViewMatrix(matrix, node.getReferenceFrame());

    // now test against the local bounding box for tighter culling:
    if (!_cv->isCulled(node.getAlignedBoundingBox()))
    {
        if (!_isSpy)
        {
            node.setLastFramePassedCull(getFrameStamp()->getFrameNumber());
        }

        int order = 0;
        unsigned count = 0;

        // First go through any legit rendering pass data in the Tile and
        // and add a DrawCommand for each.
        for (unsigned p = 0; p < renderModel._passes.size(); ++p)
        {
            const RenderingPass& pass = renderModel._passes[p];
            DrawTileCommand* cmd = addDrawCommand(pass.sourceUID(), &renderModel, &pass, _currentTileNode);
            if (cmd)
            {
                if (_firstDrawCommandForTile == 0L)
                {
                    _firstDrawCommandForTile = cmd;
                }
                else if (cmd->_layerOrder < _firstDrawCommandForTile->_layerOrder)
                {
                    _firstDrawCommandForTile = cmd;
                }
            }
        }

        // If the culler added no draw commands for this tile... we still need
        // to draw something or else there will be a hole! So draw a blank tile.
        // UID = -1 is the special UID code for a blank.
        if (_firstDrawCommandForTile == 0L)
        {
            //OE_INFO << LC << "Adding blank render for tile " << _currentTileNode->getKey().str() << std::endl;
            DrawTileCommand* cmd = addDrawCommand(-1, &renderModel, 0L, _currentTileNode);
            if (cmd)
            {
                _firstDrawCommandForTile = cmd;
            }
        }

        // Set the layer order of the first draw command for this tile to zero,
        // to support proper terrain blending.
        if (_firstDrawCommandForTile)
        {
            _firstDrawCommandForTile->_layerOrder = 0;
        }

        // update our bounds
        _terrain._drawState->_bs.expandBy(node.getBound());
        _terrain._drawState->_box.expandBy(_terrain._drawState->_bs);
    }
                
    // pop the matrix from the cull stack
    _cv->popModelViewMatrix();

    if (node.getDebugNode())
    {
        node.accept(*_cv);
    }
}

生成DrawTilecommand，保存渲染状态、投影、pass，几何体（SharedGeometry）等参数，并将DrawTileCommand添加到与某个layer相关联的layerDrawable中

DrawTileCommand*
TerrainCuller::addDrawCommand(UID uid, const TileRenderModel* model, const RenderingPass* pass, TileNode* tileNode)
{
    SurfaceNode* surface = tileNode->getSurfaceNode();

    const RenderBindings& bindings = _context->getRenderBindings();

    // skip layers that are not visible:
    if (pass && 
        pass->visibleLayer() && 
        pass->visibleLayer()->getVisible() == false)
    {
        //OE_DEBUG << LC << "Skipping " << pass->visibleLayer()->getName() << " because it's not visible." << std::endl;
        return 0L;
    }

    // add a new Draw command to the appropriate layer
    osg::ref_ptr<LayerDrawable> drawable = _terrain.layer(uid);
    if (drawable.valid())
    {
        // Layer marked for drawing?
        if (drawable->_draw)
        {
            // Cull based on the layer extent.
            if (drawable->_layer)
            {
                const LayerExtent& le = (*_layerExtents)[drawable->_layer->getUID()];
                if (le._computed && 
                    le._extent.isValid() &&
                    le._extent.intersects(tileNode->getKey().getExtent()) == false)
                {
                    // culled out!
                    //OE_DEBUG << LC << "Skippping " << drawable->_layer->getName() 
                    //    << " key " << tileNode->getKey().str()
                    //    << " because it was culled by extent." << std::endl;
                    return 0L;
                }            
            }

            drawable->_tiles.push_back(DrawTileCommand());
            DrawTileCommand* tile = &drawable->_tiles.back();

            // install everything we need in the Draw Command:
            tile->_colorSamplers = pass ? &(pass->samplers()) : 0L;
            tile->_sharedSamplers = &model->_sharedSamplers;
            tile->_modelViewMatrix = _cv->getModelViewMatrix();
            tile->_keyValue = tileNode->getTileKeyValue();
            tile->_geom = surface->getDrawable()->_geom.get();
            tile->_morphConstants = tileNode->getMorphConstants();
            tile->_key = &tileNode->getKey();

            osg::Vec3 c = surface->getBound().center() * surface->getInverseMatrix();
            tile->_range = getDistanceToViewPoint(c, true);

            tile->_layerOrder = drawable->_drawOrder;

            const osg::Image* elevRaster = tileNode->getElevationRaster();
            if (elevRaster)
            {
                float bias = _context->getUseTextureBorder() ? 1.5 : 0.5;

                // Compute an elevation texture sampling scale/bias so we sample elevation data on center
                // instead of on edge (as we do with color, etc.)
                //
                // This starts out as:
                //   scale = (size-1)/size : this shrinks the sample area by one texel since we're sampling on center
                //   bias = 0.5/size : this shifts the sample area over 1/2 texel to the center.
                //
                // But, since we also have a 1-texel border, we need to further reduce the scale by 2 texels to
                // remove the border, and shift an extra texel over as well. Giving us this:
                float size = (float)elevRaster->s();
                tile->_elevTexelCoeff.set((size - (2.0*bias)) / size, bias / size);
            }

            return tile;
        }
    }
    else if (pass)
    {
        // The pass exists but it's layer is not in the render data draw list.
        // This means that the layer is no longer in the map. Detect and record
        // this information so we can run a cleanup visitor later on.
        ++_orphanedPassesDetected;
    }
    else
    {
        OE_WARN << "Added nothing for a UID -1 darw command" << std::endl;
    }
    
    return 0L;
}

LayerDrawable的渲染，执行真正的瓦片渲染：

LayerDrawable::drawImplementation(osg::RenderInfo& ri) const
{
    //OE_INFO << LC << (_layer ? _layer->getName() : "[empty]") << " tiles=" << _tiles.size() << std::endl;

    // Get this context's state values:
    PerContextDrawState& ds = _drawState->getPCDS(ri.getContextID());

    ds.refresh(ri, _drawState->_bindings);

    if (ds._layerUidUL >= 0)
    {
        GLint uid = _layer ? (GLint)_layer->getUID() : (GLint)-1;
        ds._ext->glUniform1i(ds._layerUidUL, uid);
    }
    else
    {
        // This just means that the fragment shader for this layer doesn't use oe_layer_uid
    }

    for (DrawTileCommands::const_iterator tile = _tiles.begin(); tile != _tiles.end(); ++tile)
    {
        tile->draw(ri, *_drawState, 0L);
    }

    // If set, dirty all OSG state to prevent any leakage - this is sometimes
    // necessary when doing custom OpenGL within a Drawable.
    if (_clearOsgState)
    {
        // Dirty the texture attributes so OSG can properly reset them
        // NOTE: cannot call state.dirtyAllAttributes, because that would invalidate
        // positional state like light sources!
        reinterpret_cast<StateEx*>(ri.getState())->dirtyAllTextureAttributes();

        // NOTE: this is a NOOP in OSG 3.5.x, but not in 3.4.x ... Later we will need to
        // revisit whether to call disableAllVertexArrays() in 3.5.x instead.
        ri.getState()->dirtyAllVertexArrays();
        
        // unbind local buffers when finished.
        ds._ext->glBindBuffer(GL_ARRAY_BUFFER_ARB,0);
        ds._ext->glBindBuffer(GL_ELEMENT_ARRAY_BUFFER_ARB,0);

        // gw: no need to do this, in fact it will cause positional attributes
        // (light clip planes and lights) to immediately be reapplied under the
        // current MVM, which will by definition be wrong!)
        //ri.getState()->apply();
    }
}