Drawing grids efficiently in QGraphicsScene

[Bitto]

Using a cosmetic 0-width pen avoids unnecessary tesselation, calling QPainter::drawLines() is usually faster than QPainter::drawLine(), and using QGraphicsScene::drawBackground() is almost always faster than creating items. Here's an example that performs pretty well for me:

class GridScene : public QGraphicsScene
{
public:
    GridScene(qreal x, qreal y, qreal w, qreal h)
        : QGraphicsScene(x, y, w, h)
    { }
 
protected:
    void drawBackground(QPainter *painter, const QRectF &rect)
    {
        const int gridSize = 25;
 
        qreal left = int(rect.left()) - (int(rect.left()) % gridSize);
        qreal top = int(rect.top()) - (int(rect.top()) % gridSize);
 
        QVarLengthArray<QLineF, 100> lines;
 
        for (qreal x = left; x < rect.right(); x += gridSize)
            lines.append(QLineF(x, rect.top(), x, rect.bottom()));
        for (qreal y = top; y < rect.bottom(); y += gridSize)
            lines.append(QLineF(rect.left(), y, rect.right(), y));
 
        qDebug() << lines.size();
 
        painter->drawLines(lines.data(), lines.size());
    }
};
 
int main(int argc, char **argv)
{
    QApplication app(argc, argv);
 
    GridScene scene(-1000, -1000, 2000, 2000);
    QGraphicsView view(&scene);
    view.rotate(33);
    view.show();
 
    return app.exec();
}

class GridScene : public QGraphicsScene
{
public:
    GridScene(qreal x, qreal y, qreal w, qreal h)
        : QGraphicsScene(x, y, w, h)
    { }

protected:
    void drawBackground(QPainter *painter, const QRectF &rect)
    {
        const int gridSize = 25;
        
        qreal left = int(rect.left()) - (int(rect.left()) % gridSize);
        qreal top = int(rect.top()) - (int(rect.top()) % gridSize);

        QVarLengthArray<QLineF, 100> lines;

        for (qreal x = left; x < rect.right(); x += gridSize)
            lines.append(QLineF(x, rect.top(), x, rect.bottom()));
        for (qreal y = top; y < rect.bottom(); y += gridSize)
            lines.append(QLineF(rect.left(), y, rect.right(), y));

        qDebug() << lines.size();
        
        painter->drawLines(lines.data(), lines.size());
    }
};

int main(int argc, char **argv)
{
    QApplication app(argc, argv);

    GridScene scene(-1000, -1000, 2000, 2000);
    QGraphicsView view(&scene);
    view.rotate(33);
    view.show();

    return app.exec();
}

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Notice the use of QVarLengthArray, that's a trick to avoid allocating memory. As long as no more than 100 grid lines are visible at the same time, it'll use the stack and nothing else. You can play with the size; a QLineF uses 32 bytes of memory so 100 of those means 3200 bytes of stack space; there's room for more.