GraphicsView performance problems

[camel]

I tested with QLine but that didn't make a difference. Also i am not sure what the int cast does - i feel it just takes int part without rounding. But do you think this will make a difference since I am following one convention in all places in drawBackground() ?

It does make a difference:

void drawBackground(QPainter *painter, const QRectF &rect)
      {
         const int gridSize = 25;
 
         const int realLeft = static_cast<int>(std::floor(rect.left()));
         const int realRight = static_cast<int>(std::ceil(rect.right()));
         const int realTop = static_cast<int>(std::floor(rect.top()));
         const int realBottom = static_cast<int>(std::ceil(rect.bottom()));
 
 
         const int firstLeftGridLine = realLeft - (realLeft % gridSize);
         const int firstTopGridLine = realTop - (realTop % gridSize);
 
         QVarLengthArray<QLine, 100> lines;
 
         for (int x = firstLeftGridLine; x <= realRight; x += gridSize)
            lines.append(QLine(x, realTop, x, realBottom));
         for (int y = firstTopGridLine; y <= realBottom; y += gridSize)
            lines.append(QLine(realLeft, y, realRight, y));
 
 
         painter->setPen(QPen(Qt::darkGreen,0));
         painter->drawLines(lines.data(), lines.size());
      }

void drawBackground(QPainter *painter, const QRectF &rect)
      {
         const int gridSize = 25;

         const int realLeft = static_cast<int>(std::floor(rect.left()));
         const int realRight = static_cast<int>(std::ceil(rect.right()));
         const int realTop = static_cast<int>(std::floor(rect.top()));
         const int realBottom = static_cast<int>(std::ceil(rect.bottom()));


         const int firstLeftGridLine = realLeft - (realLeft % gridSize);
         const int firstTopGridLine = realTop - (realTop % gridSize);

         QVarLengthArray<QLine, 100> lines;

         for (int x = firstLeftGridLine; x <= realRight; x += gridSize)
            lines.append(QLine(x, realTop, x, realBottom));
         for (int y = firstTopGridLine; y <= realBottom; y += gridSize)
            lines.append(QLine(realLeft, y, realRight, y));


         painter->setPen(QPen(Qt::darkGreen,0));
         painter->drawLines(lines.data(), lines.size());
      }

To copy to clipboard, switch view to plain text mode 

As you see I make three changes
a) integer instead of float
b) using the containing integer rect as basis (i.e. the smallest integer rect that contains the float rect)
c) I changed the "<" in your for loops to a "<=", i thing part of your corruption was that it did not always repaint the grid when the line was at the border of the exposed rect.


The result: no more corruption of the grid...only weird color changes that I am trying to track down ;-)


EDIT: Of course you now need:

#include <cmath>

#include <cmath>

To copy to clipboard, switch view to plain text mode 

for the floor, and ceil to work ;-)