GraphicsView performance problems

[Gopala Krishna]

Just want to share a little thing I just read about on qt4-preview-feedback, for all those who like to play around with snapshots. (And to get it into the archive here too, if anyone looks this thread up later ;-)

In this message Andreas mentions a nice flag you can set on the GraphicsView QGraphicsView::SmartViewportUpdate

Hey thanks for the nice link. One of the thing qt is good at is to make us eagerly wait for the next version !!!!!
And hence it pulls you towards using the snapshots !
I am surely gonna try this weekend !

[Gopala Krishna]

Today I found some time to play again with all codes in this thread. Just then I thought of one more technique to draw the grid which I think should increase the performance. But I couldn't see major difference while trying it out.
The main idea is to create big pixmap and draw the grid to it in the constructor. In drawBackground() we can just draw relevant part of pixmap. I feel this way, even if the grid size is reduced, the performance remains same. Only thing is I compensate ram for speed which is ok for me. To combat this, probably we can reduce pixmap size to medium one and take necessary actions when the backgroundRect to be drawn is larger.
Anyway the code is here.

Note: This is modified version of camel's code

#include <QtGui>
#include <cmath>
 
namespace {
   static inline int gridFloor(const qreal& value)
   {
      if (value < 0) {
         return -1 * static_cast<int>(std::ceil(qAbs(value)));
      } else {
         return static_cast<int>(std::floor(value));
      }
   }
 
   static inline int gridCeil(const qreal &value)
   {
      if (value < 0) {
         return -1 * static_cast<int>(std::floor(qAbs(value)));
      } else {
         return static_cast<int>(std::ceil(value));
      }
   }
}
 
class GridScene : public QGraphicsScene
{
   public:
      GridScene(qreal x, qreal y, qreal w, qreal h)
         : QGraphicsScene(x, y, w, h)
      {
         setItemIndexMethod(QGraphicsScene::NoIndex);
         setupCache();
      }
 
   protected:
      void setupCache()
      {
         const int gridSize = 25;
         backgroundCache = QPixmap(1024,768);
         backgroundCache.fill(Qt::white);
 
         QPainter painter(&backgroundCache);
         painter.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
         painter.setPen(QPen(Qt::darkGreen,0));
         painter.setBrush(Qt::NoBrush);
 
         for (int x = 0; x < 1024; x += gridSize)
            painter.drawLine(x,0,x,768);
         for (int y = 0; y < 768; y += gridSize)
            painter.drawLine(0,y,1024,y);
 
      }
      void drawBackground(QPainter *painter, const QRectF &rect)
      {
         const int realLeft = gridFloor(rect.left());
         const int realRight = gridCeil(rect.right());
         const int realTop = gridFloor(rect.top());
         const int realBottom = gridCeil(rect.bottom());
         QRect realRect(realLeft,realTop,realRight-realLeft,realBottom-realTop);
         painter->drawPixmap(realRect,backgroundCache,realRect);
      }
 
   private:
      QPixmap backgroundCache;
};
 
namespace {
   static inline QPainterPath constructNodeShape(const QRectF& elipseRect)
   {
      QPainterPath path;
      path.addEllipse(elipseRect);
      return path;
   }
}
 
class Node : public QGraphicsItem
{
   public:
      Node(QGraphicsItem *par = 0, QGraphicsScene *sc = 0)
         : QGraphicsItem(par,sc),
           elipseRect(-4.0, -4.0, 8.0, 8.0),
           elipsePath(constructNodeShape(QRectF(-3.0, -3.0, 2.0*3.0, 2.0*3.0))),
           elipseShape(constructNodeShape(elipseRect)),
           nodePen(Qt::darkRed),
           nodeBrush(Qt::NoBrush)
      {
         setFlags(0);
         setAcceptedMouseButtons(0);
      }
 
      void paint(QPainter* p,const QStyleOptionGraphicsItem *o, QWidget *)
      {
         p->setPen(nodePen);
         p->setBrush(nodeBrush);
         p->setOpacity(1.0);
 
         p->setClipRect(o->exposedRect);
         p->drawPath(elipsePath);
      }
 
      QPainterPath shape() const
      {
         return elipseShape;
      }
 
      QRectF boundingRect() const
      {
         return elipseRect;
      }
 
   protected:
      const QRectF elipseRect;
      const QPainterPath elipsePath;
      const QPainterPath elipseShape;
      const QPen nodePen;
      const QBrush nodeBrush;
};
 
 
namespace {
   static inline QPainterPath constructResistorPath()
   {
      QPainterPath resistorPath;
      resistorPath.addRect(QRectF(-18.0, -9.0, 36.0, 18.0));
      resistorPath.moveTo(-27, 0);
      resistorPath.lineTo(-18, 0);
      resistorPath.moveTo(18, 0);
      resistorPath.lineTo(27, 0);
      return resistorPath;
   }
}
 
class Resistor : public QGraphicsItem
{
   public:
      Resistor(QGraphicsItem *par = 0, QGraphicsScene *scene = 0)
         : QGraphicsItem(par,scene),
           resistorPath(constructResistorPath()),
           boundingBox(resistorPath.boundingRect().adjusted(-1, -1, 1, 1)),
           resistorSelectedPen(Qt::darkBlue),
           resistorOpenPen(Qt::darkGray,1),
           resistorBrush(Qt::NoBrush)
      {
         setFlags(ItemIsMovable | ItemIsSelectable | ItemIsFocusable);
         // comment the following  4 lines to see the performance difference
         QGraphicsTextItem *t = new QGraphicsTextItem("R1 = 100k",this,scene);
         t->setPos(0,-35);
         Node * node = new Node(this,scene);
         node->setPos(QPointF(-30,0));
         node = new Node(this,scene);
         node->setPos(QPointF(30,0));
      }
 
      void paint(QPainter *p, const QStyleOptionGraphicsItem *o, QWidget *)
      {
         if(!(o->state & QStyle::State_Open))
            p->setPen(resistorOpenPen);
         if(o->state & QStyle::State_Selected)
            p->setPen(resistorSelectedPen);
 
         p->setClipRect( o->exposedRect );
         p->setBrush(resistorBrush);
         p->drawPath(resistorPath);
      }
 
      QRectF boundingRect() const
      {
         return boundingBox;
      }
 
   private:
      const QPainterPath resistorPath;
      const QRectF boundingBox;
      const QPen resistorSelectedPen;
      const QPen resistorOpenPen;
      const QBrush resistorBrush;
};
 
int main(int argc,char *argv[])
{
   QApplication app(argc,argv);
   GridScene scene(0,0,1024,768);
   for(int j = 2; j < 4; ++j)
      for(int i = 1; i <11; ++i)
      {
         Resistor *r = new Resistor(0,&scene);
         r->setPos(j*100, i * 50);
      }
   QGraphicsView *view = new QGraphicsView(&scene);
   view->setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
   view->setDragMode(QGraphicsView::RubberBandDrag);
   view->show();
   return app.exec();
}

#include <QtGui>
#include <cmath>
       
namespace {
   static inline int gridFloor(const qreal& value)
   {
      if (value < 0) {
         return -1 * static_cast<int>(std::ceil(qAbs(value)));
      } else {
         return static_cast<int>(std::floor(value));
      }
   }
       
   static inline int gridCeil(const qreal &value)
   {
      if (value < 0) {
         return -1 * static_cast<int>(std::floor(qAbs(value)));
      } else {
         return static_cast<int>(std::ceil(value));
      }
   }
}
       
class GridScene : public QGraphicsScene
{
   public:
      GridScene(qreal x, qreal y, qreal w, qreal h)
         : QGraphicsScene(x, y, w, h)
      {
         setItemIndexMethod(QGraphicsScene::NoIndex);
         setupCache();
      }
       
   protected:
      void setupCache()
      {
         const int gridSize = 25;
         backgroundCache = QPixmap(1024,768);
         backgroundCache.fill(Qt::white);
         
         QPainter painter(&backgroundCache);
         painter.setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
         painter.setPen(QPen(Qt::darkGreen,0));
         painter.setBrush(Qt::NoBrush);

         for (int x = 0; x < 1024; x += gridSize)
            painter.drawLine(x,0,x,768);
         for (int y = 0; y < 768; y += gridSize)
            painter.drawLine(0,y,1024,y);
                
      }
      void drawBackground(QPainter *painter, const QRectF &rect)
      {
         const int realLeft = gridFloor(rect.left());
         const int realRight = gridCeil(rect.right());
         const int realTop = gridFloor(rect.top());
         const int realBottom = gridCeil(rect.bottom());
         QRect realRect(realLeft,realTop,realRight-realLeft,realBottom-realTop);
         painter->drawPixmap(realRect,backgroundCache,realRect);
      }
       
   private:
      QPixmap backgroundCache;
};
       
namespace {
   static inline QPainterPath constructNodeShape(const QRectF& elipseRect)
   {
      QPainterPath path;
      path.addEllipse(elipseRect);
      return path;
   }
}
       
class Node : public QGraphicsItem
{
   public:
      Node(QGraphicsItem *par = 0, QGraphicsScene *sc = 0)
         : QGraphicsItem(par,sc),
           elipseRect(-4.0, -4.0, 8.0, 8.0),
           elipsePath(constructNodeShape(QRectF(-3.0, -3.0, 2.0*3.0, 2.0*3.0))),
           elipseShape(constructNodeShape(elipseRect)),
           nodePen(Qt::darkRed),
           nodeBrush(Qt::NoBrush)
      {
         setFlags(0);
         setAcceptedMouseButtons(0);
      }
       
      void paint(QPainter* p,const QStyleOptionGraphicsItem *o, QWidget *)
      {
         p->setPen(nodePen);
         p->setBrush(nodeBrush);
         p->setOpacity(1.0);
       
         p->setClipRect(o->exposedRect);
         p->drawPath(elipsePath);
      }
       
      QPainterPath shape() const
      {
         return elipseShape;
      }
       
      QRectF boundingRect() const
      {
         return elipseRect;
      }
       
   protected:
      const QRectF elipseRect;
      const QPainterPath elipsePath;
      const QPainterPath elipseShape;
      const QPen nodePen;
      const QBrush nodeBrush;
};
       
       
namespace {
   static inline QPainterPath constructResistorPath()
   {
      QPainterPath resistorPath;
      resistorPath.addRect(QRectF(-18.0, -9.0, 36.0, 18.0));
      resistorPath.moveTo(-27, 0);
      resistorPath.lineTo(-18, 0);
      resistorPath.moveTo(18, 0);
      resistorPath.lineTo(27, 0);
      return resistorPath;
   }
}
       
class Resistor : public QGraphicsItem
{
   public:
      Resistor(QGraphicsItem *par = 0, QGraphicsScene *scene = 0)
         : QGraphicsItem(par,scene),
           resistorPath(constructResistorPath()),
           boundingBox(resistorPath.boundingRect().adjusted(-1, -1, 1, 1)),
           resistorSelectedPen(Qt::darkBlue),
           resistorOpenPen(Qt::darkGray,1),
           resistorBrush(Qt::NoBrush)
      {
         setFlags(ItemIsMovable | ItemIsSelectable | ItemIsFocusable);
         // comment the following  4 lines to see the performance difference
         QGraphicsTextItem *t = new QGraphicsTextItem("R1 = 100k",this,scene);
         t->setPos(0,-35);
         Node * node = new Node(this,scene);
         node->setPos(QPointF(-30,0));
         node = new Node(this,scene);
         node->setPos(QPointF(30,0));
      }
       
      void paint(QPainter *p, const QStyleOptionGraphicsItem *o, QWidget *)
      {
         if(!(o->state & QStyle::State_Open))
            p->setPen(resistorOpenPen);
         if(o->state & QStyle::State_Selected)
            p->setPen(resistorSelectedPen);
       
         p->setClipRect( o->exposedRect );
         p->setBrush(resistorBrush);
         p->drawPath(resistorPath);
      }
       
      QRectF boundingRect() const
      {
         return boundingBox;
      }
       
   private:
      const QPainterPath resistorPath;
      const QRectF boundingBox;
      const QPen resistorSelectedPen;
      const QPen resistorOpenPen;
      const QBrush resistorBrush;
};
       
int main(int argc,char *argv[])
{
   QApplication app(argc,argv);
   GridScene scene(0,0,1024,768);
   for(int j = 2; j < 4; ++j)
      for(int i = 1; i <11; ++i)
      {
         Resistor *r = new Resistor(0,&scene);
         r->setPos(j*100, i * 50);
      }
   QGraphicsView *view = new QGraphicsView(&scene);
   view->setRenderHints(QPainter::Antialiasing | QPainter::SmoothPixmapTransform);
   view->setDragMode(QGraphicsView::RubberBandDrag);
   view->show();
   return app.exec();
}

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[camel]

What happens if you make the window larger than the pixmap? :-)

Thing is, I would imagine that most draws are actually rather small pieces. (Fixing for moving items), so I do not know if it is faster to always work with the one big pixmap.

Probably best would be something like 3 or 4 gridsizes as a cache...but I have not tested it out...

[Gopala Krishna]

While experimenting further I noticed one of the major neglected thing.
Just add this

view->scale(4.0,4.0);

view->scale(4.0,4.0);

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And now it is very very slow!!! Why ? Can someone explain this ?
Also the the gridlines are quite thicker which I don't want .
Please some one help me

EDIT: This happens only in pixmap version of grids.

[wysota]

Just add this

view->scale(4.0,4.0);

view->scale(4.0,4.0);

To copy to clipboard, switch view to plain text mode 

And now it is very very slow!!! Why ? Can someone explain this ?

Sure, you introduced matrix transformations which makes everything more computation intensive (the pixmap is scaled).

Also the the gridlines are quite thicker which I don't want .
Please some one help me

There is nothing you can do when using a pixmap to change it. The pixmap just gets scaled and all pixels get four times bigger in each direction.