Query in 40000 chip demo

[aamer4yu]

I am new to Qt and was exploring demos of Qt.
I was trying the 40000 chip demo code and faced some confusion

1) I want to know the significance of choosing -11000 to 11000 in the function void MainWindow::populateScene()

2) if i change the loop, the chip movemment gets slow


The loop in void MainWindow::populateScene() function is

for (int i = -11000; i < 11000; i += 110) {
        ++xx;
        int yy = 0;
        for (int j = -7000; j < 7000; j += 70) {
            ++yy;
            qreal x = (i + 11000) / 22000.0;
            qreal y = (j + 7000) / 14000.0;
 
            QColor color(image.pixel(int(image.width() * x), int(image.height() * y)));
            QGraphicsItem *item = new Chip(color, xx, yy);
            item->setPos(QPointF(i, j));
            scene->addItem(item);
 
            ++nitems;
        }
    }

for (int i = -11000; i < 11000; i += 110) {
        ++xx;
        int yy = 0;
        for (int j = -7000; j < 7000; j += 70) {
            ++yy;
            qreal x = (i + 11000) / 22000.0;
            qreal y = (j + 7000) / 14000.0;

            QColor color(image.pixel(int(image.width() * x), int(image.height() * y)));
            QGraphicsItem *item = new Chip(color, xx, yy);
            item->setPos(QPointF(i, j));
            scene->addItem(item);
       
            ++nitems;
        }
    }

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If i replace the loop with

for (int i = 0; i < 22000; i += 110) {
        ++xx;
        int yy = 0;
        for (int j = 0; j < 14000; j += 70) {
            ++yy;
            qreal x = (i) / 22000.0;
            qreal y = (j) / 14000.0;
 
	int w,h;
	w= image.width();
	h= image.height();
            QColor color(image.pixel(int(image.width() * x), int(image.height() * y)));
            QGraphicsItem *item = new Chip(color, xx, yy);
            item->setPos(QPointF(i, j));
            scene->addItem(item);
 
 
	++nitems;
        }
    }

for (int i = 0; i < 22000; i += 110) {
        ++xx;
        int yy = 0;
        for (int j = 0; j < 14000; j += 70) {
            ++yy;
            qreal x = (i) / 22000.0;
            qreal y = (j) / 14000.0;

	int w,h;
	w= image.width();
	h= image.height();
            QColor color(image.pixel(int(image.width() * x), int(image.height() * y)));
            QGraphicsItem *item = new Chip(color, xx, yy);
            item->setPos(QPointF(i, j));
            scene->addItem(item);


	++nitems;
        }
    }

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and move the chips in the view, the movement is much slow.

What am i missing here ???