CODEWITHSUNDEEP
c++qtembedded-linuxframebuffer
Josh Kurland
Josh Kurland

Reputation: 217

Qt: Why is the background of my image green on LinuxFb?

My Qt application is supposed to display a JPEG image to an HDMI monitor. When I run the application on my Linux desktop environment, the image is displayed correctly.

However, when I run this application in an embedded Linux environment under LinuxFB, there is a large amount of green surrounding the image.

NTSC (720x480) color bars on my 1080p monitor.

My application was written with QLabel instead of QWidget, and does not have any active window. I have attempted a number of solutions, mostly using QPainter, but so far nothing has made a difference to background appearance. 

int main (int argc, char *argv[])
{
    QApplication app(argc, argv);

    // Set the app palette to be transparent
    app.setPalette(QPalette(Qt::transparent));

    QPixmap input ("test.jpg");
    QImage image(input.size(), QImage::Format_ARGB32_Premultiplied);

    // Try to fill the QImage to be transparent
    image.fill(Qt::transparent);

    QPainter p(&image);

    // Try a few things to get the painter background to be transparent
    p.setOpacity(0.5);  // 0 is invisible, 1 is opaque
    p.setBackgroundMode(Qt::TransparentMode);
    p.setBackground(Qt::transparent);
    p.setBrush(Qt::transparent);

    p.drawPixmap(0,0,input);
    p.end();
    QPixmap output = QPixmap::fromImage(image);

    QLabel label (0, Qt::FramelessWindowHint | Qt::WindowStaysOnTopHint);

    // Set the style sheet background color as transparent as well
    label.setStyleSheet("background-color: transparent;");
    //label.setStyleSheet("background-color: rgba(255,255,255,0);");

    label.setPixmap(output);
    label.setScaledContents(true);
    label.show();

    return app.exec();
}

The application is executed with the following

./my-Qt-application -qws -nomouse -display LinuxFb:/dev/fb1

I do not believe this has anything to do with the Linux frame buffer itself because the green background is only present when I am running a Qt application.

Assuming the green background is coming from Qt, what can I do to turn it off (or make it transparent)?

Upvotes: 2

Views: 1080

Answers (2)

egonr
egonr

Reputation: 982

Qt 4.8 (probably 4.x)

The green background is coming from QWSServer. You can change it by using QWSServer::setBackground().

Qt 5.x

The origin of the green background is still Qt, but QWSServer doesn't exist anymore. I'd suggest using a fullscreen QWidget as a background. You can change to background color of the QWidget with a simple stylesheet containing background-color: black;.

Upvotes: 1

Kuba hasn't forgotten Monica
Kuba hasn't forgotten Monica

Reputation: 98485

You've done nothing to scale the label properly. The green background is where your label isn't - it's probably the default contents of the framebuffer. Fill the entire screen and you won't have that problem.

Upvotes: 0

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