OSG 渲染剖析之 Geometry 的 VBO生成

对于一个渲染引擎的渲染过程，我们一般关心的是渲染引擎如何组织、绘制场景对象。OSG 通过状态树与渲染树对渲染节点进行分组、排序与渲染。我们先抛开OSG 事件更新与处理等宏观渲染过程，这里我们从更细的角度研究OSG 是如何将场景对象数据与状态转换成OpenGL 具体实现。

如果让你设计一个渲染引擎渲染场景对象，你通常会怎么做？我们一般都会这样做：首先设计场景对象的组织与数据封装类，将数据从文件或程序处理过程中获取顶点、法向、颜色等数据初始化场景对象；其次，设计OpenGL数据实现层，并与场景对象组织封装对象关联起来，这样在渲染一开始，就可以从数据封装对象获取数据并生成底层OpenGL各种状态数据，例如VBO, VAO,TBO PBO 等；再次调用一个绘制处理过程，将数据与绘制调用传入到OpenGL 执行绘制。

上述的绘制过程OSG是如何设计或组织的呢？

那就是Geometry 类。它的实例对象缓存顶点，法向，颜色，纹理坐标等几何数据并通Geometry.compileGLObjects()生成VBO,VAO数据，通过Geometry.drawImplementation()执行具体的绘制。

Geometry 以及OSG 的OpenGL 相关类的设计比较巧妙，实现效率也较高，多处使用链表等缓存数据结构提高VBO（具体实现类 GLBufferObject）生成效率。

OSG数据接口类 osg::BufferData

class OSG_EXPORT BufferData : public Object
{virtual osg::Array* asArray() { return 0; }virtual const osg::Array* asArray() const { return 0; }virtual osg::PrimitiveSet* asPrimitiveSet() { return 0; }virtual const osg::PrimitiveSet* asPrimitiveSet() const { return 0; }virtual osg::Image* asImage() { return 0; }virtual const osg::Image* asImage() const { return 0; }void setBufferObject(BufferObject* bufferObject);BufferObject* getBufferObject() { return _bufferObject.get(); }    const BufferObject* getBufferObject() const { return _bufferObject.get(); }void setBufferIndex(unsigned int index) { _bufferIndex = index; }    unsigned int getBufferIndex() const { return _bufferIndex; }GLBufferObject* getGLBufferObject(unsigned int contextID) const { return _bufferObject.valid() ? _bufferObject->getGLBufferObject(contextID) : 0; }GLBufferObject* getOrCreateGLBufferObject(unsigned int contextID) const { return _bufferObject.valid() ? _bufferObject->getOrCreateGLBufferObject(contextID) : 0; }}

Osg::Image、osg::Array、 osg::PrimitiveSet 类派生于 BufferData；

osg::Array 具体派生类由模版类class TemplateArray : public Array, public MixinVector<T>给出；

顶点，法向，纹理坐标，颜色数组均派生于 osg::Array；

索引数据类为向量类：

class DrawElements : public PrimitiveSet

class DrawElementsUByte : public DrawElements, public VectorGLubyte

BufferData 实例会关联BufferObject 实例，BufferObject 是什么？ VBO? 我们往下看。

我们以顶点数组的生成为例，看看OSG 如何将BufferData数据转换成OpenGL 对应的数据，请看这段代码：

void Geometry::setVertexArray(Array* array)
{if (array && array->getBinding()==osg::Array::BIND_UNDEFINED) array->setBinding(osg::Array::BIND_PER_VERTEX);_vertexArray = array;dirtyGLObjects();dirtyBound();if (/*_useVertexBufferObjects && */array){_vertexArrayStateList.assignVertexArrayDispatcher();addVertexBufferObjectIfRequired(array);}
}void Geometry::addVertexBufferObjectIfRequired(osg::Array* array)
{if (/*_useVertexBufferObjects &&*/ array->getBinding()==Array::BIND_PER_VERTEX || array->getBinding()==Array::BIND_UNDEFINED){if (!array->getVertexBufferObject()){array->setVertexBufferObject(getOrCreateVertexBufferObject());}}
}osg::VertexBufferObject* Geometry::getOrCreateVertexBufferObject()
{ArrayList arrayList;getArrayList(arrayList);ArrayList::iterator vitr;for(vitr = arrayList.begin();vitr != arrayList.end();++vitr){osg::Array* array = vitr->get();if (array->getVertexBufferObject()) return array->getVertexBufferObject();}return new osg::VertexBufferObject;
}

没错，您看到了 VBO对象的生成与数组对象的关联是在Geometry设置顶点数组数据的时候。array->setVertexBufferObject(VBO),那您应该也看到Geometry::getOrCreateVertexBufferObject()里有个数组循环收集的过程，为什么如果一个数组对象有了VBO 立即返回了？而不是一个数组关联一个VBO对象？也许您猜对了，我们不能颜色数据，法向坐标数组，雾坐标数组，纹理坐标数组每个都生成一个VBO, 那将是VBO 灾难，VBO 数量剧增会导致渲染性能严重下降！OSG 的每个Geometry只生成一个缓存对象VBO, 其它数组数据也就是BufferData 在VBO中只记录偏移值和数据大小。

下面我们看看VBO 的相关的中介类BufferObject：

class OSG_EXPORT BufferObject : public Object
{public:void setTarget(GLenum target) { _profile._target = target; }GLenum getTarget() const { return _profile._target; }/** Set what type of usage the buffer object will have. Options are:*          GL_STREAM_DRAW, GL_STREAM_READ, GL_STREAM_COPY,*          GL_STATIC_DRAW, GL_STATIC_READ, GL_STATIC_COPY,*          GL_DYNAMIC_DRAW, GL_DYNAMIC_READ, or GL_DYNAMIC_COPY.*/void setUsage(GLenum usage) { _profile._usage = usage; }/** Get the type of usage the buffer object has been set up for.*/GLenum getUsage() const { return _profile._usage; }BufferObjectProfile& getProfile() { return _profile; }const BufferObjectProfile& getProfile() const { return _profile; }void dirty();/** Resize any per context GLObject buffers to specified size. */virtual void resizeGLObjectBuffers(unsigned int maxSize);/** If State is non-zero, this function releases OpenGL objects for* the specified graphics context. Otherwise, releases OpenGL objects* for all graphics contexts. */void releaseGLObjects(State* state=0) const;unsigned int addBufferData(BufferData* bd);void removeBufferData(unsigned int index);void removeBufferData(BufferData* bd);void setBufferData(unsigned int index, BufferData* bd);BufferData* getBufferData(unsigned int index) { return _bufferDataList[index]; }const BufferData* getBufferData(unsigned int index) const { return _bufferDataList[index]; }unsigned int getNumBufferData() const { return static_cast<unsigned int>(_bufferDataList.size()); }void setGLBufferObject(unsigned int contextID, GLBufferObject* glbo) { _glBufferObjects[contextID] = glbo; }GLBufferObject* getGLBufferObject(unsigned int contextID) const { return _glBufferObjects[contextID].get(); }GLBufferObject* getOrCreateGLBufferObject(unsigned int contextID) const;unsigned int computeRequiredBufferSize() const;protected:~BufferObject();typedef std::vector< BufferData* > BufferDataList;typedef osg::buffered_object< osg::ref_ptr<GLBufferObject> > GLBufferObjects;BufferObjectProfile     _profile;bool                    _copyDataAndReleaseGLBufferObject;BufferDataList          _bufferDataList;mutable GLBufferObjects _glBufferObjects;
};

看到 Profile , Usage, Target ,这不是我们熟悉的OpenGL 缓存对象 glBufferData 相关参数的内容？ VeryGood, 总算找到了OpenGL 具体实现。饿。。。还差那么一点，真正的OpenGL VBO具体实现类它在这呢 GLBufferObject* getOrCreateGLBufferObject(unsigned int contextID) const。 BufferObject 对象只是负责收集 BufferData，添加到 BufferDataList _bufferDataList 中。_glBufferObjects 是上下文相关的VBO 数据，我们知道 OpenGL 实例的生成是依托于窗口的，每个窗口（WINDOW）都有一个ID contextID. 您要是在一个窗口上调用其它窗口的openGL环境，不是崩溃就是渲染错误，调试器哇哇的报警，而且您要注意OpenGL 资源是与线程相关的。

这里我们看到了OpenGL VBO 具体实现类GLBufferObject : public GraphicsObject，我们在看看它是咋初始化的:

class OSG_EXPORT GLBufferObject : public GraphicsObject
{public:GLBufferObject(unsigned int contextID, BufferObject* bufferObject, unsigned int glObjectID=0);void setProfile(const BufferObjectProfile& profile) { _profile = profile; }const BufferObjectProfile& getProfile() const { return _profile; }void setBufferObject(BufferObject* bufferObject);BufferObject* getBufferObject() { return _bufferObject; }struct BufferEntry{BufferEntry(): numRead(0), modifiedCount(0),dataSize(0),offset(0),dataSource(0) {}BufferEntry(const BufferEntry& rhs):numRead(rhs.numRead),modifiedCount(rhs.modifiedCount),dataSize(rhs.dataSize),offset(rhs.offset),dataSource(rhs.dataSource) {}BufferEntry& operator = (const BufferEntry& rhs){if (&rhs==this) return *this;numRead = rhs.numRead;modifiedCount = rhs.modifiedCount;dataSize = rhs.dataSize;offset = rhs.offset;dataSource = rhs.dataSource;return *this;}unsigned int getNumClients() const;unsigned int        numRead;unsigned int        modifiedCount;unsigned int        dataSize;unsigned int        offset;BufferData*         dataSource;};inline unsigned int getContextID() const { return _contextID; }inline GLuint& getGLObjectID() { return _glObjectID; }inline GLuint getGLObjectID() const { return _glObjectID; }inline GLsizeiptr getOffset(unsigned int i) const { return _bufferEntries[i].offset; }inline void bindBuffer();inline void unbindBuffer(){_extensions->glBindBuffer(_profile._target,0);}/** release GLBufferObject to the orphan list to be reused or deleted.*/void release();inline bool isDirty() const { return _dirty; }void dirty() { _dirty = true; }void clear();void compileBuffer();void deleteGLObject();void assign(BufferObject* bufferObject);bool isPBOSupported() const { return _extensions->isPBOSupported; }bool hasAllBufferDataBeenRead() const;void setBufferDataHasBeenRead(const osg::BufferData* bd);protected:virtual ~GLBufferObject();unsigned int computeBufferAlignment(unsigned int pos, unsigned int bufferAlignment) const{return osg::computeBufferAlignment(pos, bufferAlignment);}unsigned int            _contextID;GLuint                  _glObjectID;BufferObjectProfile     _profile;unsigned int            _allocatedSize;bool                    _dirty;typedef std::vector<BufferEntry> BufferEntries;BufferEntries           _bufferEntries;BufferObject*           _bufferObject;public:GLBufferObjectSet*      _set;GLBufferObject*         _previous;GLBufferObject*         _next;unsigned int            _frameLastUsed;public:GLExtensions*          _extensions;};看到 GLBufferObject.compileBuffer()? 这就是真正的OpenGL VBO  生成并初始化的地方。
void GLBufferObject::compileBuffer()
{_dirty = false;_bufferEntries.reserve(_bufferObject->getNumBufferData());bool compileAll = false;bool offsetChanged = false;unsigned int bufferAlignment = 4;unsigned int newTotalSize = 0;unsigned int i=0;for(; i<_bufferObject->getNumBufferData(); ++i){BufferData* bd = _bufferObject->getBufferData(i);if (i<_bufferEntries.size()){BufferEntry& entry = _bufferEntries[i];if (offsetChanged ||entry.dataSource != bd ||entry.dataSize != bd->getTotalDataSize()){unsigned int previousEndOfBufferDataMarker = osg::computeBufferAlignment(entry.offset + entry.dataSize, bufferAlignment);// OSG_NOTICE<<"GLBufferObject::compileBuffer(..) updating BufferEntry"<<std::endl;entry.numRead = 0;entry.modifiedCount = 0xffffff;entry.offset = newTotalSize;entry.dataSize = bd->getTotalDataSize();entry.dataSource = bd;newTotalSize += entry.dataSize;if (previousEndOfBufferDataMarker!=newTotalSize){offsetChanged = true;}}else{newTotalSize = osg::computeBufferAlignment(newTotalSize + entry.dataSize, bufferAlignment);}}else{BufferEntry entry;entry.offset = newTotalSize;entry.modifiedCount = 0xffffff;entry.dataSize = bd ? bd->getTotalDataSize() : 0;entry.dataSource = bd;
#if 0OSG_NOTICE<<"entry"<<std::endl;OSG_NOTICE<<"   offset "<<entry.offset<<std::endl;OSG_NOTICE<<"   dataSize "<<entry.dataSize<<std::endl;OSG_NOTICE<<"   dataSource "<<entry.dataSource<<std::endl;OSG_NOTICE<<"   modifiedCount "<<entry.modifiedCount<<std::endl;
#endifnewTotalSize = computeBufferAlignment(newTotalSize + entry.dataSize, bufferAlignment);_bufferEntries.push_back(entry);}}if (i<_bufferEntries.size()){// triming end of bufferEntries as the source data is has less entries than the originally held._bufferEntries.erase(_bufferEntries.begin()+i, _bufferEntries.end());}_extensions->glBindBuffer(_profile._target, _glObjectID);_extensions->debugObjectLabel(GL_BUFFER, _glObjectID, _bufferObject->getName());if (newTotalSize > _profile._size){OSG_INFO<<"newTotalSize="<<newTotalSize<<", _profile._size="<<_profile._size<<std::endl;unsigned int sizeDifference = newTotalSize - _profile._size;_profile._size = newTotalSize;if (_set){_set->moveToSet(this, _set->getParent()->getGLBufferObjectSet(_profile));_set->getParent()->getCurrGLBufferObjectPoolSize() += sizeDifference;}}if (_allocatedSize != _profile._size){_allocatedSize = _profile._size;OSG_INFO<<"    Allocating new glBufferData(), _allocatedSize="<<_allocatedSize<<std::endl;_extensions->glBufferData(_profile._target, _profile._size, NULL, _profile._usage);compileAll = true;}for(BufferEntries::iterator itr = _bufferEntries.begin();itr != _bufferEntries.end();++itr){BufferEntry& entry = *itr;if (entry.dataSource && (compileAll || entry.modifiedCount != entry.dataSource->getModifiedCount())){// OSG_NOTICE<<"GLBufferObject::compileBuffer(..) downloading BufferEntry "<<&entry<<std::endl;entry.numRead = 0;entry.modifiedCount = entry.dataSource->getModifiedCount();const osg::Image* image = entry.dataSource->asImage();if (image && !(image->isDataContiguous())){unsigned int offset = entry.offset;for(osg::Image::DataIterator img_itr(image); img_itr.valid(); ++img_itr){_extensions->glBufferSubData(_profile._target, (GLintptr)offset, (GLsizeiptr)img_itr.size(), img_itr.data());offset += img_itr.size();}}else{_extensions->glBufferSubData(_profile._target, (GLintptr)entry.offset, (GLsizeiptr)entry.dataSize, entry.dataSource->getDataPointer());}}}
}

代码过程长，但比较简单明了，我就不叨叨了。

它们之间不是单向关联关系，array 关联 BufferObject, GLBufferObject 初始化时父为 BufferObject, BufferObject 创建GLBufferObject 时通过glObjectList 记录。

那啥时机生成VBO 并进行初始化？

有两个地方：一是在整帧开始时, draw() 函数会判断当前是不是第一帧开始，如果是就执行图形编译：

void Renderer::compile()
{DEBUG_MESSAGE<<"Renderer::compile()"<<std::endl;_compileOnNextDraw = false;osgUtil::SceneView* sceneView = _sceneView[0].get();if (!sceneView || _done) return;sceneView->getState()->checkGLErrors("Before Renderer::compile");if (sceneView->getSceneData()){osgUtil::GLObjectsVisitor glov;glov.setState(sceneView->getState());glov.compile(*(sceneView->getSceneData()));}sceneView->getState()->checkGLErrors("After Renderer::compile");
}

osgUtil::GLObjectsVisitor 负责调用 Geometry 的 compileGLObject() 函数生成 VBO 。

二是，当个别Geomery 数据dirty() 时， VAS （VertextArrayState）绑定VBO或设置VAO时

class OSG_EXPORT VertexArrayState : public osg::Referenced
｛inline void bindVertexBufferObject(osg::GLBufferObject* vbo){if (vbo->isDirty()){vbo->compileBuffer();_currentVBO = vbo;}else if (vbo != _currentVBO){vbo->bindBuffer();_currentVBO = vbo;}｝

Geometry 类中为了兼容性的需要存在很多DisplayList显示列表相关的代码，显示列表是OpenGL1.0~OpenGL 1.5中的产物，历史有点悠久，包括顶点数组，法线数组这些不符合现代图形硬件的东西也应该统统的去掉，OpenGL 3.0 都是10年前的规范，固定管线那套API也没有存在的必要。