Open Scene Graph 3D到2D坐标转换

OSG中实现3D世界坐标到2D屏幕坐标的转换是许多应用的基础功能，如HUD显示、对象拾取等。以下是详细的实现方法：

1. 基本坐标转换原理

3D到2D坐标转换流程图：

2D到3D坐标转换流程图：

2. 核心转换方法

2.1 使用osg::Matrix实现转换

// 3D世界坐标转2D屏幕坐标
bool worldToScreen(const osg::Vec3& worldPos, osg::Vec3& screenPos, 
                  const osg::Camera* camera)
{
    // 获取视图矩阵、投影矩阵和视口
    const osg::Matrix& viewMatrix = camera->getViewMatrix();
    const osg::Matrix& projMatrix = camera->getProjectionMatrix();
    const osg::Viewport* viewport = camera->getViewport();
    
    if(!viewport) return false;
    
    // 执行坐标转换
    osg::Vec3 eyePos = worldPos * viewMatrix;
    osg::Vec3 clipPos = eyePos * projMatrix;
    
    // 归一化设备坐标(NDC)
    osg::Vec3 ndcPos;
    ndcPos.x() = clipPos.x() / clipPos.w();
    ndcPos.y() = clipPos.y() / clipPos.w();
    ndcPos.z() = clipPos.z() / clipPos.w();
    
    // 转换为屏幕坐标
    screenPos.x() = (ndcPos.x() * 0.5f + 0.5f) * viewport->width() + viewport->x();
    screenPos.y() = (ndcPos.y() * 0.5f + 0.5f) * viewport->height() + viewport->y();
    screenPos.z() = (ndcPos.z() * 0.5f + 0.5f);
    
    // 检查点是否在视锥体内
    return (ndcPos.x() >= -1.0f && ndcPos.x() <= 1.0f &&
            ndcPos.y() >= -1.0f && ndcPos.y() <= 1.0f &&
            ndcPos.z() >= -1.0f && ndcPos.z() <= 1.0f);
}

2.2 使用osgUtil::SceneView简化转换

osg::Vec3 worldToScreen(osg::Vec3 worldPos, osgUtil::SceneView* sceneView)
{
    osg::Matrix MVPW = sceneView->getProjectionMatrix() * 
                      sceneView->getViewMatrix() * 
                      osg::Matrix::identity();
    
    osg::Viewport* viewport = sceneView->getViewport();
    osg::Vec3 screenPos = worldPos * MVPW;
    
    // 透视除法
    screenPos.x() = (screenPos.x() / screenPos.z() * 0.5f + 0.5f) * viewport->width() + viewport->x();
    screenPos.y() = (screenPos.y() / screenPos.z() * 0.5f + 0.5f) * viewport->height() + viewport->y();
    
    return screenPos;
}

3. 2D到3D坐标转换（逆向）

// 2D屏幕坐标转3D世界坐标（获取地面交点）
bool screenToWorld(const osg::Vec3& screenPos, osg::Vec3& worldPos, 
                  const osg::Camera* camera, const osg::Vec3& planeNormal, float planeDistance)
{
    if(!camera || !camera->getViewport()) return false;
    
    // 创建从屏幕到世界的射线
    osg::Matrix VPW = camera->getViewport()->computeWindowMatrix();
    osg::Matrix inverseMVPW = osg::Matrix::inverse(
        camera->getViewMatrix() * 
        camera->getProjectionMatrix() * 
        VPW);
    
    // 近平面和远平面点
    osg::Vec3 nearPoint = osg::Vec3(screenPos.x(), screenPos.y(), 0.0f) * inverseMVPW;
    osg::Vec3 farPoint = osg::Vec3(screenPos.x(), screenPos.y(), 1.0f) * inverseMVPW;
    
    // 计算与指定平面的交点
    osg::Plane plane(planeNormal, planeDistance);
    return plane.intersect(nearPoint, farPoint, worldPos);
}

4. 实际应用示例

4.1 在3D对象上显示2D标签

class LabelUpdateCallback : public osg::NodeCallback {
public:
    LabelUpdateCallback(osgText::Text* label, osg::Node* target, osg::Camera* camera)
        : _label(label), _target(target), _camera(camera) {}
    
    virtual void operator()(osg::Node* node, osg::NodeVisitor* nv) {
        // 获取目标的世界坐标
        osg::Matrix worldMat = _target->getWorldMatrices()[0];
        osg::Vec3 worldPos = osg::Vec3(0,0,0) * worldMat;
        
        // 转换为屏幕坐标
        osg::Vec3 screenPos;
        if(worldToScreen(worldPos, screenPos, _camera)) {
            _label->setPosition(osg::Vec3(screenPos.x(), screenPos.y(), 0));
            _label->setNodeMask(0xFFFFFFFF); // 显示
        } else {
            _label->setNodeMask(0x0); // 隐藏
        }
        
        traverse(node, nv);
    }
    
private:
    osg::ref_ptr<osgText::Text> _label;
    osg::ref_ptr<osg::Node> _target;
    osg::ref_ptr<osg::Camera> _camera;
};

4.2 实现3D对象的屏幕空间固定效果

osg::Camera* createHUDCamera(osgViewer::Viewer* viewer)
{
    // 创建HUD相机
    osg::Camera* camera = new osg::Camera;
    camera->setReferenceFrame(osg::Transform::ABSOLUTE_RF);
    camera->setProjectionMatrix(osg::Matrix::ortho2D(0, viewer->getCamera()->getViewport()->width(), 
                                                   0, viewer->getCamera()->getViewport()->height()));
    camera->setViewMatrix(osg::Matrix::identity());
    camera->setClearMask(GL_DEPTH_BUFFER_BIT);
    camera->setRenderOrder(osg::Camera::POST_RENDER);
    camera->setAllowEventFocus(false);
    return camera;
}

void addScreenFixedNode(osg::Group* root, osgViewer::Viewer* viewer, osg::Node* node, const osg::Vec3& screenPos)
{
    osg::Camera* hudCamera = createHUDCamera(viewer);
    
    // 更新回调保持屏幕位置
    node->setUpdateCallback(new ScreenPositionCallback(viewer->getCamera(), screenPos));
    
    hudCamera->addChild(node);
    root->addChild(hudCamera);
}

5. 高级应用：拾取(Picking)实现

class PickHandler : public osgGA::GUIEventHandler {
public:
    bool handle(const osgGA::GUIEventAdapter& ea, osgGA::GUIActionAdapter& aa) {
        if(ea.getEventType() != osgGA::GUIEventAdapter::RELEASE || 
           ea.getButton() != osgGA::GUIEventAdapter::LEFT_MOUSE_BUTTON)
            return false;
        
        osgViewer::View* view = dynamic_cast<osgViewer::View*>(&aa);
        if(!view) return false;
        
        // 2D屏幕坐标转3D世界射线
        osgUtil::LineSegmentIntersector::Intersections intersections;
        if(view->computeIntersections(ea.getX(), ea.getY(), intersections)) {
            for(auto& hit : intersections) {
                osg::Vec3 worldPos = hit.getWorldIntersectPoint();
                osg::Vec3 screenPos;
                
                // 3D到2D转换验证
                worldToScreen(worldPos, screenPos, view->getCamera());
                std::cout << "Picked at: World(" << worldPos << ") Screen(" << screenPos.x() << "," << screenPos.y() << ")\n";
            }
        }
        return true;
    }
};

6. 性能优化建议

1. 批量转换：对多个点进行转换时，预计算MVPW矩阵

2. 缓存结果：对于静态对象，缓存转换结果避免每帧计算

3. 视锥体剔除：先检查点是否在视锥体内再执行转换

4. 使用着色器：对大量点的大规模转换，考虑使用GPU计算

7. 常见问题解决

问题1：转换后的坐标不准确

• 检查是否使用了正确的模型矩阵（对于变换节点）

• 确认视口(viewport)设置正确

• 验证投影矩阵是否匹配当前相机设置

问题2：点在屏幕外时计算错误

• 添加视锥体检查代码

• 使用clipPos.w()进行透视除法而非直接使用z值

问题3：正交投影下的转换问题

• 正交投影下可以简化计算，不需要透视除法

• 检查正交投影矩阵的near/far平面设置