Drawing lines in QPainter takes a lot of time

[Lesiok]

Maybe I repeat myself but... This is the original code :

const int cst_nbLines = 5000;
 
    for (int i=0 ; i<cst_nbLines ; i++)
    {
        QLine myLine(i/10, i/10, i/10+300, i/10);
        myPainter.drawLine(myLine);
    }

const int cst_nbLines = 5000;

    for (int i=0 ; i<cst_nbLines ; i++)
    {
        QLine myLine(i/10, i/10, i/10+300, i/10);
        myPainter.drawLine(myLine);
    }

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This is the code giving the same final result but 10 times faster :

const int cst_nbLines = 5000;
 
    for (int i=0 ; i<cst_nbLines/10 ; i++)
    {
        QLine myLine(i, i, i+300, i);
        myPainter.drawLine(myLine);
    }

const int cst_nbLines = 5000;

    for (int i=0 ; i<cst_nbLines/10 ; i++)
    {
        QLine myLine(i, i, i+300, i);
        myPainter.drawLine(myLine);
    }

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[d_stranz]

This is the code giving the same final result but 10 times faster :

I think the OP understands he is drawing the same line 10 times. He misunderstood your original comment (as I did when I first read it) to say that he was executing the loop 10 times (to give 50k lines). His point is that even if these were 5000 individual lines with different coordinates, the time it takes to draw all of them is too long, no matter what method he uses.