Sergio

/***** QMainWindow constructor *****/
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow){

ui->setupUi(this);

// Add two subwindow to test the problem
this->ui->mdiArea->addSubWindow(new subWnd());
this->ui->mdiArea->addSubWindow(new subWnd());
}

/***** subWnd class definition *****/

class subWnd : public QMdiSubWindow
{
public:
subWnd();
protected:
virtual void paintEvent(QPaintEvent * event);
};

/***** and class implementation *****/
#include "subwnd.h"
#include <QPainter>
#include <QPaintEvent>

subWnd::subWnd(){

setAttribute(Qt::WA_DeleteOnClose);

}

int cordToGrid(int v)
{
int ret;
ret = v / 5;
ret = ret * 5;
return ret;
}
void subWnd::paintEvent(QPaintEvent * event){

QPen lapiz(Qt::black);
QBrush relleno(Qt::white);
QPainter painter(this);
lapiz.setWidth(1);
lapiz.setStyle(Qt::DashLine);
relleno.setStyle(Qt::NoBrush);
painter.setPen(lapiz);
painter.setBrush(relleno);
int xIni = cordToGrid(event->rect().x());
int yIni = cordToGrid(event->rect().y());
for(int x = xIni; x < event->rect().width();x += 5)
   for(int y = yIni; y < event->rect().height(); y += 5)
        painter.drawPoint(x, y);
painter.end();
QMdiSubWindow::paintEvent(event);

}