cocos2dx源码分析：渲染队列RenderQueue

当遍历场景树，搜索可渲染的节点，并添加到渲染器时，渲染指令就被添加到了渲染器的渲染队列中

void Renderer::addCommand(RenderCommand* command, int renderQueue) { CCASSERT(!_isRendering, "Cannot add command while rendering");
    CCASSERT(renderQueue >=0, "Invalid render queue");
    CCASSERT(command->getType() != RenderCommand::Type::UNKNOWN_COMMAND, "Invalid Command Type");
    /*渲染队列按栈的方式存在，这里添加到栈顶的渲染队列中*/
    _renderGroups[renderQueue].push_back(command);
}

一个渲染队列又分为5个组：

   enum QUEUE_GROUP
    {
        /**Objects with globalZ smaller than 0.*/
        GLOBALZ_NEG = 0,
        /**Opaque 3D objects with 0 globalZ.*/
        OPAQUE_3D = 1,
        /**Transparent 3D objects with 0 globalZ.*/
        TRANSPARENT_3D = 2,
        /**2D objects with 0 globalZ.*/
        GLOBALZ_ZERO = 3,
        /**Objects with globalZ bigger than 0.*/
        GLOBALZ_POS = 4,
        QUEUE_COUNT = 5,
    };

渲染顺序为GLOBALZ_NEG->OPAQUE_3D->TRANSPARENT_3D->GLOBALZ_ZERO->GLOBALZ_POS，后渲染的会盖住先渲染的，透明物体除外。当渲染指令添加到渲染队列时，会根据渲染指令的情况添加到对应的渲染组中。

void RenderQueue::push_back(RenderCommand* command)
{
    float z = command->getGlobalOrder();
    if(z < 0)
    {
        /*存入globalz<0的渲染组*/
        _commands[QUEUE_GROUP::GLOBALZ_NEG].push_back(command);
    }
    else if(z > 0)
    {
        /*存入globalz>0的渲染组*/
        _commands[QUEUE_GROUP::GLOBALZ_POS].push_back(command);
    }
    else
    {
        /*globalz=0又分3种情况*/
        if(command->is3D())
        {
            /*3d透明渲染组*/
            if(command->isTransparent())
            {
                /*3d透明渲染组*/
                _commands[QUEUE_GROUP::TRANSPARENT_3D].push_back(command);
            }
            else
            {
                /*3d不透明渲染组*/
                _commands[QUEUE_GROUP::OPAQUE_3D].push_back(command);
            }
        }
        else
        {
            /*2d渲染组*/
            _commands[QUEUE_GROUP::GLOBALZ_ZERO].push_back(command);
        }
    }
}

渲染前，会对渲染对象进行排序，并不会对所有的渲染组进行排序，globalzorder为0的已经排好序，但是透明3d物体需要按距视点的深度排序。

void RenderQueue::sort()
{
    // Don't sort _queue0, it already comes sorted
    std::sort(std::begin(_commands[QUEUE_GROUP::TRANSPARENT_3D]), std::end(_commands[QUEUE_GROUP::TRANSPARENT_3D]), compare3DCommand);
    std::sort(std::begin(_commands[QUEUE_GROUP::GLOBALZ_NEG]), std::end(_commands[QUEUE_GROUP::GLOBALZ_NEG]), compareRenderCommand);
    std::sort(std::begin(_commands[QUEUE_GROUP::GLOBALZ_POS]), std::end(_commands[QUEUE_GROUP::GLOBALZ_POS]), compareRenderCommand);
}

渲染renderqueue时，需要改变一些全局设置，需要保存改变前的全局设置，渲染后再恢复全局设置

void RenderQueue::saveRenderState()
{
    _isDepthEnabled = glIsEnabled(GL_DEPTH_TEST) != GL_FALSE;
    _isCullEnabled = glIsEnabled(GL_CULL_FACE) != GL_FALSE;
    glGetBooleanv(GL_DEPTH_WRITEMASK, &_isDepthWrite);

    CHECK_GL_ERROR_DEBUG();
}

void RenderQueue::restoreRenderState()
{
    if (_isCullEnabled)
    {
        glEnable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setCullFace(true);
    }
    else
    {
        glDisable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setCullFace(false);
    }

    if (_isDepthEnabled)
    {
        glEnable(GL_DEPTH_TEST);
        RenderState::StateBlock::_defaultState->setDepthTest(true);
    }
    else
    {
        glDisable(GL_DEPTH_TEST);
        RenderState::StateBlock::_defaultState->setDepthTest(false);
    }

    glDepthMask(_isDepthWrite);
    RenderState::StateBlock::_defaultState->setDepthWrite(_isDepthEnabled);

    CHECK_GL_ERROR_DEBUG();
}

处理renderqueue中的渲染指令，也就是进行渲染，这里可以看出渲染顺序，和渲染状态的保存恢复

void Renderer::visitRenderQueue(RenderQueue& queue)
{
    /*保存全局渲染状态*/
    queue.saveRenderState();

    //
    //Process Global-Z < 0 Objects
    //
    const auto& zNegQueue = queue.getSubQueue(RenderQueue::QUEUE_GROUP::GLOBALZ_NEG);
    if (zNegQueue.size() > 0)
    {
        if(_isDepthTestFor2D)
        {
            glEnable(GL_DEPTH_TEST);
            glDepthMask(true);
            glEnable(GL_BLEND);
            RenderState::StateBlock::_defaultState->setDepthTest(true);
            RenderState::StateBlock::_defaultState->setDepthWrite(true);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        else
        {
            glDisable(GL_DEPTH_TEST);
            glDepthMask(false);
            glEnable(GL_BLEND);
            RenderState::StateBlock::_defaultState->setDepthTest(false);
            RenderState::StateBlock::_defaultState->setDepthWrite(false);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        glDisable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setCullFace(false);

        for (auto it = zNegQueue.cbegin(); it != zNegQueue.cend(); ++it)
        {
            processRenderCommand(*it);
        }
        flush();
    }

    //
    //Process Opaque Object
    //
    const auto& opaqueQueue = queue.getSubQueue(RenderQueue::QUEUE_GROUP::OPAQUE_3D);
    if (opaqueQueue.size() > 0)
    {
        //Clear depth to achieve layered rendering
        glEnable(GL_DEPTH_TEST);
        glDepthMask(true);
        glDisable(GL_BLEND);
        glEnable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setDepthTest(true);
        RenderState::StateBlock::_defaultState->setDepthWrite(true);
        RenderState::StateBlock::_defaultState->setBlend(false);
        RenderState::StateBlock::_defaultState->setCullFace(true);


        for (auto it = opaqueQueue.cbegin(); it != opaqueQueue.cend(); ++it)
        {
            processRenderCommand(*it);
        }
        flush();
    }

    //
    //Process 3D Transparent object
    //
    const auto& transQueue = queue.getSubQueue(RenderQueue::QUEUE_GROUP::TRANSPARENT_3D);
    if (transQueue.size() > 0)
    {
        glEnable(GL_DEPTH_TEST);
        glDepthMask(false);
        glEnable(GL_BLEND);
        glEnable(GL_CULL_FACE);

        RenderState::StateBlock::_defaultState->setDepthTest(true);
        RenderState::StateBlock::_defaultState->setDepthWrite(false);
        RenderState::StateBlock::_defaultState->setBlend(true);
        RenderState::StateBlock::_defaultState->setCullFace(true);


        for (auto it = transQueue.cbegin(); it != transQueue.cend(); ++it)
        {
            processRenderCommand(*it);
        }
        flush();
    }

    //
    //Process Global-Z = 0 Queue
    //
    const auto& zZeroQueue = queue.getSubQueue(RenderQueue::QUEUE_GROUP::GLOBALZ_ZERO);
    if (zZeroQueue.size() > 0)
    {
        if(_isDepthTestFor2D)
        {
            glEnable(GL_DEPTH_TEST);
            glDepthMask(true);
            glEnable(GL_BLEND);

            RenderState::StateBlock::_defaultState->setDepthTest(true);
            RenderState::StateBlock::_defaultState->setDepthWrite(true);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        else
        {
            glDisable(GL_DEPTH_TEST);
            glDepthMask(false);
            glEnable(GL_BLEND);

            RenderState::StateBlock::_defaultState->setDepthTest(false);
            RenderState::StateBlock::_defaultState->setDepthWrite(false);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        glDisable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setCullFace(false);

        for (auto it = zZeroQueue.cbegin(); it != zZeroQueue.cend(); ++it)
        {
            processRenderCommand(*it);
        }
        flush();
    }

    //
    //Process Global-Z > 0 Queue
    //
    const auto& zPosQueue = queue.getSubQueue(RenderQueue::QUEUE_GROUP::GLOBALZ_POS);
    if (zPosQueue.size() > 0)
    {
        if(_isDepthTestFor2D)
        {
            glEnable(GL_DEPTH_TEST);
            glDepthMask(true);
            glEnable(GL_BLEND);

            RenderState::StateBlock::_defaultState->setDepthTest(true);
            RenderState::StateBlock::_defaultState->setDepthWrite(true);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        else
        {
            glDisable(GL_DEPTH_TEST);
            glDepthMask(false);
            glEnable(GL_BLEND);

            RenderState::StateBlock::_defaultState->setDepthTest(false);
            RenderState::StateBlock::_defaultState->setDepthWrite(false);
            RenderState::StateBlock::_defaultState->setBlend(true);
        }
        glDisable(GL_CULL_FACE);
        RenderState::StateBlock::_defaultState->setCullFace(false);

        for (auto it = zPosQueue.cbegin(); it != zPosQueue.cend(); ++it)
        {
            processRenderCommand(*it);
        }
        flush();
    }
    /*恢复全局渲染状态*/
    queue.restoreRenderState();
}