Qt基礎——獲取QGraphicsScene的縮圖即匯出到圖片

Renders the source rect from scene into target, using painter. This function is useful for capturing the contents of the scene onto a paint device, such as a QImage (e.g., to take a screenshot), or for printing with QPrinter. For 

If source is a null rect, this function will use sceneRect() to determine what to render. If target is a null rect, the dimensions of painter's paint device will be used.

The source rect contents will be transformed according to aspectRatioMode to fit into the target rect. By default, the aspect ratio is kept, and source is scaled to fit in target.

        //get thumbnail
        QImage image(130 * mSceneSize.width()/ mSceneSize.height(),130 ,QImage::Format_ARGB32);   
        QString pngName = currentPageID+"_scene.png";
        QPainter painter(&image);
        painter.setRenderHint(QPainter::Antialiasing);
        scene->render(&painter);   
        bool saveSuccess =  image.save(pngName);  
        Q_ASSERT(saveSuccess == true);

• 如果你把QImage image(130 * mSceneSize.width()/ mSceneSize.height(),130 ,QImage::Format_ARGB32);   寫成QImage image。無論如何image都是無法save成功的。因為image沒有初始化。
• 如果你的scene裡有的item/widget的座標位於目前可顯示的外面，意思是你的scene現在大小是100*100，但是有一個item的座標位於100*101，那麼呼叫scene->render時會有ASSERT錯誤：

  --------------------------- Microsoft Visual C++ Debug Library --------------------------- Debug Error! Program: ...\Win32\Debug\Maker.exe Module: 5.0.1 File: global\qglobal.cpp Line: 1951 ASSERT: "!item->d_ptr->itemDiscovered" in file graphicsview\qgraphicsscenebsptreeindex.cpp, line 343 (Press Retry to debug the application) --------------------------- Abort   Retry   Ignore    ---------------------------

  你當然可以忽略掉他，在release模式下是沒有此類問題的。