How to draw on QLabel with every QTimer emission - c++

Question

My goal is to draw on QLabel with every QTimer emission.
Here's how i'm trying to achieve this:
so, timer is created by triggering an action.
I want measures, parameters of which are updated as user inputs data to the dialog, to be drawn with every timer emission.
void ImageViewer::on_measuresAct_triggered()
{
dialog = new Measures;
dialog->show();
Ymax = Origin = Xmax = QPoint(30,30);
measuresflag = true;
pixtemp = imageLabel->pixmap();
timer = new QTimer;
connect(timer, SIGNAL(timeout()), this, SLOT(drawontimer()));
timer->start(100);
if(!dialog->isVisible())
timer->stop();
}
Here's the slot that is to draw the measures.
void ImageViewer::drawontimer()
{
pixmap = pixtemp;
qDebug()<<"Hey there";
pix = (*pixmap);
QPainter paint(&pix);
QPen MeasurePen (Qt::magenta);
MeasurePen.setWidth(5);
QBrush MeasureBrush (Qt::magenta,Qt::SolidPattern);
paint.setPen(MeasurePen);
paint.setBrush(MeasureBrush);
paint.drawLine(Ymax,Origin);
paint.drawLine(Origin, Xmax);
QString originpoint = "(" + dialog->ui->olinex->text() + ", " + dialog->ui->oliney->text() + ")";
QString xmaxpoint = "(" + dialog->ui->xline->text() + ", " + dialog->ui->oliney->text() + ")";
QString ymaxpoint = "(" + dialog->ui->yline->text() + ", " + dialog->ui->olinex->text() + ")";
paint.setPen(QPen(Qt::green));
paint.setFont(QFont("Arial", 15, QFont::Bold));
paint.drawText(Origin.x() + 8, Origin.y() + 18, originpoint);
paint.drawText(Xmax.x() - 10, Xmax.y() - 8, xmaxpoint);
paint.drawText(Ymax.x() + 8, Ymax.y() + 8, ymaxpoint);
QPolygon poly1, poly2;
poly1 << Xmax << QPoint(Xmax.x() - 12, Xmax.y() - 6)
<< QPoint(Xmax.x() - 12, Xmax.y() + 6)<< Xmax;
poly2 << Ymax << QPoint(Ymax.x() + 6, Ymax.y() + 12) << QPoint(Ymax.x() - 6, Ymax.y() + 12) << Ymax;
// style(), width(), brush(), capStyle() and joinStyle().
QPen ArrowPen(Qt::magenta, 1);
paint.setPen(ArrowPen);
// Brush
QBrush brush;
brush.setColor(Qt::magenta);
brush.setStyle(Qt::SolidPattern);
// Fill polygon
QPainterPath path1, path2;
path1.addPolygon(poly1);
path2.addPolygon(poly2);
// Draw polygon
paint.drawPolygon(poly1);
paint.fillPath(path1, brush);
paint.drawPolygon(poly2);
paint.fillPath(path2, brush);
imageLabel->setPixmap(pix);
}
But my program crashes. I figured out where my problem is by using "Hey there" message output with QDebug. It's in
pix = (*pixmap);
line of my code. If i comment this line program doesn't crash, nothing is drawn neither.
Declaration in .h file:
const QPixmap* pixmap;
QPixmap pix;
const QPixmap * pixtemp;
"Hey there" message is written 2 times most of the time, but 1 time out of 10 it's written 3 or 4 times. That confuses even more.
So, the problem is how i draw on a label (i suppose). Is there more convinient or "right" way to draw on label, which won't lead to crash of my program.
Thanks in advance!

Answer 1

Do not try to draw into QLabel's pixmap or mangling with the pointers. Instead:
Create a new pixmap p on stack (local variable, no pointer) using the copy constructor
Check if your pixmap is valid (p.isNull())
Draw onto the pixmap p
Call setPixmap(p) on your label
A general note on Qt properties: Qt properties are not to be modified directly. The access to Qt properties always works through getting and setting. So, if you want to manipulate a property, you initialize a local variable with the getter, then modify your local variable, then use the setter.
A note about drawing into widgets: If you don't want to work on a copy, you can draw on a widget directly by deriving and implementing the paint() method. Furthermore, you can call update() on the widget to trigger a repaint (which will call your paint() method). This will be more efficient than using a label with a pixmap.