How to optimize QGraphicsView's performance?

Question

I'm developing a CAD application using Qt 5.6.2 that is required to run in cheap computers at the same time that it needs to handle thousands of items in the same scene. Because of that, I had to make a lot of experimentations in order to get the best performance.

I decided to create this post to help others and also myself, as long as other people also contribute with more optimization tips.

My text is still a work in progress and I may update it if I discover better techniques (or that I said something really stupid).

Answer 1

My application, while not exactly a CAD program, is CAD-like in that it allows the user to construct a "blueprint" of various items in a space, and the user is allowed to add as many items as he likes, and some users' designs can get quite crowded and elaborate, with hundreds or thousands of items present at once.

Most of the items in the view are going to be more or less static (i.e. they move or change appearance only when the user clicks/drags on them, which is rarely). But there are often also a few foreground items in the scene that are constantly animated and moving around at 20fps.

To avoid having to re-render complex static elements on a regular basis, I pre-render all the static elements into the QGraphicsView's background-cache whenever any of them change, or whenever the zoom/pan/size settings of the QGraphicsView change, and exclude them from being rendered as part of the normal foreground-view-redrawing process.

That way, when there are moving elements running around the QGraphicsView at 20fps, all of the very-numerous-and-elaborate-static-objects are drawn (by the code in QGraphicsScene::drawBackground()) via a single call to drawPixmap() rather than having to algorithmically re-render each item individually. The always-moving elements can then be drawn on top in the usual fashion.

Implementing this involves calling setOptimizationFlag(IndirectPainting) and setCacheMode(CacheBackground) on the QGraphicsView(s), and also calling resetCachedContent() on them whenever any aspect of the any of the static-items changes (so that the cached-background-image will be re-rendered ASAP).

The only tricky part is getting all of the "background" QGraphicsItems to render inside the QGraphicsScene's drawBackground() callback, and also to not render inside the usual QGraphicsScene::drawItems() callback (which is typically called much more often than QGraphicsScene::drawBackground()).

In my stress-test this reduces my program's steady-state CPU usage by about 50% relative to the "vanilla" QGraphicsScene/QGraphicsView approach (and by about 80% if I'm using OpenGL via calling setViewport(new QOpenGLWidget) on my QGraphicsView).

The only downside (other than added code complexity) is that this approach relies on using QGraphicsView::setOptimizationFlag(IndirectPainting) and QGraphicsView::drawItems(), both of which have been more-or-less deprecated by Qt, so this approach might not continue to work with future Qt versions. (It does work at least as far as Qt 5.10.1 though; that's the latest Qt version I've tried it with)

Some illustrative code:

void MyGraphicsScene :: drawBackground(QPainter * p, const QRectF & r)
{
   if (_isInBackgroundUpdate == false)  // anti-infinite-recursion guard
   {
      QGraphicsScene::drawBackground(p, r);

      const QRectF rect = sceneRect();

      p->fillRect(rect, backgroundBrush().color());

      // Render the scene's static objects as pixels 
      // into the QGraphicsView's view-background-cache
      this->_isInBackgroundUpdate = true;  // anti-infinite-recursion guard
      render(p, sceneRect());
      this->_isInBackgroundUpdate = false;
   }
}

// overridden to draw only the items appropriate to our current
// mode (foreground items OR background items but not both!)
void MyGraphicsScene :: drawItems(QPainter *painter, int numItems, QGraphicsItem *items[], const QStyleOptionGraphicsItem options[], QWidget *widget)
{
   // Go through the items-list and only keep items that we are supposed to be
   // drawing in this pass (either foreground or background, depending)
   int count = 0;
   for (int i=0; i<numItems; i++)
   {
      const bool isItemBackgroundItem = (_backgroundItemsTable.find(items[i]) != _backgroundItemsTable.end());
      if (isItemBackgroundItem == this->_isInBackgroundUpdates) items[count++] = items[i];
   }

   QGraphicsScene::drawItems(painter, count, items, options, widget);
}