Drawing a rectangle over Qlabel image

[cheyanne]

I have a map as a image in my Qlabel, now i want to draw a rectangle over the image how to i do it? Because at the end of the day i need the rect(which is my robot footprint to be able to move accordingly. i tried :

MainWindow::MainWindow(Tqt_interface* tqt, QWidget *parent ) : QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QPixmap pix("/home/liqi/Qt_robotino/src/image/ariccmap.png");
ui->labelmap->setPixmap(pix);

QRectF rectangle(10.0, 20.0, 200.0, 200.0);
QPainter painter(this);
painter.drawRect(rectangle); // drawing code
}

but the rectangle does not appear. Any helps is appreciated.

[wysota]

You are trying to draw on the widget instead of the pixmap.

[cheyanne]

Then how do I make it to draw on the image instead?

[wysota]

Open the painter on the image and not on the widget.

[joginder11singh]

I am new Qt . I tried to draw a line on image using Qpainter on QPixmapBut image did not load just everything was black on Qlabel conataining Qpixmap;
I dont why
my code is

QPixmap pix(500,500);
ui->label->resize(pix.size());
pix.load(QString::fromUtf8("C:\raster.jpg"));
QPainter painter(&pix);
QPen Red((QColor::QColor(255,0,0)),10);
painter.setPen(Red);
painter.drawLine(10,10,50,50);
ui->label->setPixmap(pix);

QPixmap pix(500,500);
ui->label->resize(pix.size());
pix.load(QString::fromUtf8("C:\raster.jpg"));
QPainter painter(&pix);
QPen Red((QColor::QColor(255,0,0)),10);
painter.setPen(Red);
painter.drawLine(10,10,50,50);
ui->label->setPixmap(pix);

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problem is that raster.jpg image is not loading ....only whole black is appearing

[cheyanne]

MainWindow::MainWindow(Tqt_interface* tqt, QWidget *parent ) : QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QImage image("/home/liqi/Qt_robotino/src/image/ariccmap.png");

// tell the painter to draw on the QImage
QPainter* painter = new QPainter(&image); // sorry i forgot the "&"
painter->setPen(Qt::red);
painter->setFont(QFont("Arial", 10));
// you probably want the to draw the text to the rect of the image
painter->drawText(image.rect(), Qt::AlignBottom, "Text on Image");

QLabel* imageLabel = new QLabel();
ui->labelmap->setPixmap(QPixmap::fromImage(image));
}

Ok, now i managed to at least get text over the image, now, how do i go about placing one image over the previous one instead of text?

[wysota]

Such step by step approach is not a good way of solving problems. QLabel might be fit for displaying a simple pixmap however if you keep increasing complexity like that, you'll reach a state when you'll want to implement a 3d game and QLabel is not the best candidate for it. Maybe you should first say what your goal is and then we'll suggest a solution (I can already suggest using QGraphicsView instead of QLabel).

[cheyanne]

Okay, im actually trying to display a map on Qt, then insert a "robot footprint' on top of the map. The laptop with Qt design is client side, it will receive information from the server side(another laptop on the robot) and the robot footprint will move accordingly on the map. my server side is able to receive the x,y,z position now.

[wysota]

Use Graphics View.

[cheyanne]

Alright, but when i switched to QgraphicsView, my image is not being displayed again. Here is my code:

MainWindow::MainWindow(Tqt_interface* tqt, QWidget *parent ) : QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

QPixmap pixMap("/home/liqi/Qt_robotino/src/image/ariccmap.png");

QGraphicsScene scene;
scene.addPixmap(pixMap);
ui->graphicsViewmap->setScene(&scene);
ui->graphicsViewmap->show();

}

[wysota]

Your scene object goes out of scope.

[cheyanne]

Sorry but can you elaborate more on it? I don't really understand this. how do know that my scene object goes out of scope?

[wysota]

Because you created it on the stack as a local variable. This is plain C++ you know...

[cheyanne]

It might be plain c++ to you but i dont really study much of it cos all i was thought is those simple cin cout etc. this is the first time dealing with this so im here asking for help. but anyway, thanks for your replies.

[wysota]

We can't assume here that you know nothing of programming when writing our replies, we have to assume you know how to program and you only have problems with using Qt. You need a decent knowledge of C++ to be able to use Qt efficiently. So far it seemed you didn't have problems with programming as such so it was safe to assume you just didn't notice you were using a local variable or that you didn't know it mattered.

[cheyanne]

im able to draw over the image now but how do i put it to keep refreshing and change its position ?? I was thinking of putting while loop at first but it will just keep looping the same thing. any idea?

[wysota]

Use QGraphicsItem::setPos() to modify the position of an item.