QT opacity on background

[DevM]

Hey,

I'm new to QT and I have been struggling to put a QPixmap transparent, I currently have this code:

void MainWindow::resizeEvent(QResizeEvent *event){
    //Gets background images and resizes it on window resize
    QPixmap backgroundImage(":/Images/Background/img.jpg");
    backgroundImage = backgroundImage.scaled(this->size(), Qt::IgnoreAspectRatio);

    QPalette p = palette();
    p.setBrush(QPalette::Background,backgroundImage);
    this->setPalette(p);

    //Calls base implementation
    QMainWindow::resizeEvent(event);
}

That resizes the image when moving the window around, but I have been unable to set that background image to have some kind of opacity. What is the best way to go about this, I am aware that Im not the only one that has had this problem before since I have searched around but their implementations didn't seem to work for me.

Also if someone could explain the difference in using QPalette to paint versus QImage and the few others I have seen around that would be great as well, Im confused as why there are so many.

[mrjj]

Hi
Do you mean opacity so that Desktop can be seen through the mainwindow?

On windows 10 , i could only do that with

setAttribute(Qt::WA_TranslucentBackground);
setWindowFlags(Qt::FramelessWindowHint); // with borders, its always just black.

void MainWindow::paintEvent(QPaintEvent *event)
{
QPainter p(this);
p.setOpacity(0.3);
p.drawPixmap(0, 0, QPixmap(":/bg.jpg"));
}

[DevM]

@mrjj

I mean the image itself being transparent, I want to have this effect ( the image in the back )

image url)

[mrjj]

Hi
Ah, im not tried with QPalette::Background and transparent but if
you combine the shown paintEvent and your resizeEvent, it should look
just like that.

backgroundImage should be a member so you scale it in resizeEvent and paintEvent paints it with
p.setOpacity(0.3);

[DevM]

@mrjj

Currently have it like this then

void MainWindow::resizeEvent(QResizeEvent *event){
    //Gets background images and resizes it on window resize
    backgroundImage = backgroundImage.scaled(this->size(), Qt::IgnoreAspectRatio);

    /*QPalette p = palette();
    p.setBrush(QPalette::Background,backgroundImage);
    this->setPalette(p);*/

    //Calls base implementation
    QMainWindow::resizeEvent(event);
}

void MainWindow::paintEvent(QPaintEvent *event){
    QPainter p(this);
    p.setOpacity(0.5);
    p.drawPixmap(0,0,QPixmap(backgroundImage));
}

for some reason though the image on resize now acts weird

Okay:

Not okay(after resizing):

[mrjj]

Hi
I think it comes from scaling the scaled image. ( Scale the scaled so to speak)
Else i don't know why it would look that odd suddenly.

So try with keeping the original image in new variable org_backgroundImage
and do

backgroundImage = org_backgroundImage.scaled(this->size(), Qt::IgnoreAspectRatio);

so each time we scale from org image and not the last scaled version.

[DevM]

@mrjj

Yep that makes sense that it was what was happening, thank you very much for the help!