C++学习（三一一）AttributeDispatchers和VertexAttribArrayDispatch关系

Posted 2021-08-15 hankern

顶点属性的位置数据通过VertexAttribArrayDispatch来派发，逐点绑定的顶点属性通过VertexAttribArrayDispatch派发，全局绑定或逐图元绑定的顶点属性通过AttributeDispatchers派发。他们两个的工作过程参考C++学习（三零七）AttributeDispatchers详解和C++学习（三零九）osg::VertexArrayState的作用。

这样设计的目的是，VertexAttribArrayDispatch会派发比较多的顶点属性信息，调用glVertexAttribPointer比较合适，这里Array就意味着数据量比较大，AttributeDispatchers由于需要派发的顶点属性信息比较少（因为所有顶点或所有图元共享一样的信息），调用glVertexAttrib1fv之类的函数比较合适。

这两个派发器的关系也就是glVertexAttribPointer和glVertexAttrib1fv的关系。

osg/Geometry.cpp
void Geometry::drawVertexArraysImplementation(RenderInfo& renderInfo) const
{
    State& state = *renderInfo.getState();
    VertexArrayState* vas = state.getCurrentVertexArrayState();

    AttributeDispatchers& attributeDispatchers = state.getAttributeDispatchers();


    // activate or dispatch any attributes that are bound overall
    attributeDispatchers.activateNormalArray(_normalArray.get());
    attributeDispatchers.activateColorArray(_colorArray.get());
    attributeDispatchers.activateSecondaryColorArray(_secondaryColorArray.get());
    attributeDispatchers.activateFogCoordArray(_fogCoordArray.get());


    // set up arrays
    if( _vertexArray.valid() )
        vas->setVertexArray(state, _vertexArray.get());

    if (_normalArray.valid() && _normalArray->getBinding()==osg::Array::BIND_PER_VERTEX)
        vas->setNormalArray(state, _normalArray.get());

    if (_colorArray.valid() && _colorArray->getBinding()==osg::Array::BIND_PER_VERTEX)
        vas->setColorArray(state, _colorArray.get());

    if (_secondaryColorArray.valid() && _secondaryColorArray->getBinding()==osg::Array::BIND_PER_VERTEX)
        vas->setSecondaryColorArray(state, _secondaryColorArray.get());

    if (_fogCoordArray.valid() && _fogCoordArray->getBinding()==osg::Array::BIND_PER_VERTEX)
        vas->setFogCoordArray(state, _fogCoordArray.get());

    
}