Drawing in QT

viniltc

Hi All,

I'm trying to understand how to draw in QT. I have a graph which looks like this:

I need to draw this in qt and control slope of the trapezoid with a QSlider.

It will be really helpful if someone can show me this with a simple example.

Thanks in advance

mrjj

Hi
Which part do you need help with ?
You can use QPainter and
https://doc.qt.io/qt-5/qtwidgets-painting-painterpaths-example.html
for the trapezoid

If i read here
https://stackoverflow.com/questions/59373961/draw-and-control-rectangle-in-qt

It seems you want the slider to adjust the 2 top points to be the same value according to a slider.

You can do this by updating the points for the path and call update()

mrjj

Hi
very basic example. I draw on pixmap and show in label.
Its just to show as a concept. A custom control might be better.

alt text

void MainWindow::on_verticalSlider_valueChanged(int value)
{

    int h = ui->label->height();
    int w = ui->label->width();

    QPixmap pix(w, h);
    QPainter paint(&pix);
    pix.fill( Qt::white );
    paint.setPen(QColor(0, 0, 0, 255));

    QPainterPath polygonPath;
    polygonPath.moveTo(30.0, h);
    int topy =  h - 100 - value;
    polygonPath.lineTo(60.0, topy);
    polygonPath.lineTo(160.0, topy );
    polygonPath.lineTo(260.0, h);
    polygonPath.closeSubpath();

    paint.drawPath(polygonPath);

    ui->label->setPixmap(pix);
}

test project
https://www.dropbox.com/s/cctwcsadkhq5x2m/trapezoidTest.zip?dl=0

viniltc

@mrjj Thanks a lot. That's exact info i need to start. I really appreciate your input.

viniltc

@mrjj

I need to show the Polygon path while the MainWindow is constructed with a default value and then it should change if I drag a Slider.
In the example, the figure is constructed under on_verticalSlider_valueChanged slot itself , so I wont see anything until I move the Slider.

If I move the polygon painting part under the MainWindow constructor, plot does not look right:

//this gives totally wrong figure
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    int h = ui->label->height();
    int w = ui->label->width();
    QPixmap pix(w, h);
    QPainter paint(&pix);
    pix.fill( Qt::white );
    paint.setPen(QColor(0, 0, 0, 255));


    polygonPath.moveTo(30.0, h);
    int topy =  h - 100 ;
    polygonPath.lineTo(60.0, topy);
    polygonPath.lineTo(160.0, topy );
    polygonPath.lineTo(260.0, h);
    polygonPath.closeSubpath();
    paint.drawPath(polygonPath);

    ui->label->setPixmap(pix);
}

basically i need to have a default figure with int topy = h - 100 then if I change Slider int topy = h - 100 - value. What's the best way to implement this?

mrjj

@viniltc

Hi
In the constructor, nothing has the right sizes yet.

So just overwrite showEvent function and call the
on_verticalSlider_valueChanged( 100);

to have it show up with a value from start.

viniltc

@mrjj

How do I overwrite showEvent function, should I call showEvent in the constructor?
I tried this:

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

    QWidget::showEvent(parent); 

    on_verticalSlider_valueChanged(100);

}

this gives error:

mainwindow.cpp:11:24: error: cannot initialize a parameter of type 'QShowEvent *' with an lvalue of type 'QWidget *'
qwidget.h:646:40: note: passing argument to parameter 'event' here

jsulm

@viniltc said in Drawing in QT:

How do I overwrite showEvent function

Like any other:

void MainWindow::showEvent(QShowEvent *event)
{
    QWidget::showEvent(event);
}

https://www.programiz.com/cpp-programming/function-overriding

mrjj

@viniltc
Hi
No, overwrite means to add that function to your class so
its called instead of the base class.

void MainWindow:showEvent(QShowEvent *event)
{
QMainWindow::showEvent(event); // call base  so we dont fux things up
on_verticalSlider_valueChanged(100);
}