empty pixmap as a QLabel

[wysota]

Subclass QLabel and either reimplement its sizeHint() so that when you remove the pixmap its size is still returned by the label or reimplement paintEvent() so that when you set some flag, the pixmap is not drawn (but don't remove it so that a proper size hint is returned).

Or set a minimum size on the label so that even when it is empty, it won't collapse to a smaller size.

[tommy]

Thank you!
How do you set minimum QLabel size?

[pherthyl]

QWidget::setMinimumSize

[tommy]

Thank you, that worked.

My other question is:
Is it possible to give the QLabel a default color; so that when it does not display a pixmap it shows a default background color (instead of being invisible)?

[wysota]

Yes. set its window palette role to the colour you want and make sure autoFillBackground property is set to true.

Note - if you continue to expand requirements on the label, it might prove much simpler to write a custom widget that does all you want - it's just a few lines of code.

[tommy]

Thanks but I still don't know hot to actually specify the background color of the QLabel. The below code is not doing it:

pixmapLabel->setBackgroundRole(QPalette::Dark);
  pixmapLabel->autoFillBackground();

pixmapLabel->setBackgroundRole(QPalette::Dark);
  pixmapLabel->autoFillBackground();

To copy to clipboard, switch view to plain text mode 

[jpn]

Nowadays the easiest way is to use style sheets:

pixmapLabel->setStyleSheet("background-color: red");
// or
pixmapLabel->setStyleSheet("background-color: palette(dark)");

pixmapLabel->setStyleSheet("background-color: red");
// or
pixmapLabel->setStyleSheet("background-color: palette(dark)");

To copy to clipboard, switch view to plain text mode 

[tommy]

Thank you JPN!

The style-sheets work fine to make the background a certain color. However, after doing
this (in the constructor):

pixmapLabel->setStyleSheet("background-color: black");

pixmapLabel->setStyleSheet("background-color: black");

To copy to clipboard, switch view to plain text mode 

everything is always black and the actual pixmap that the QLabel (pixmapLabel) is supposed to show is all black and the picture is not seen.Looks like the background is sitting on top of the pixmap, not the other way around. Seems like I need to specify something more?

[jpn]

everything is always black and the actual pixmap that the QLabel (pixmapLabel) is supposed to show is all black and the picture is not seen.Looks like the background is sitting on top of the pixmap, not the other way around. Seems like I need to specify something more?

No, it should work. Double check your code.

Just to verify it works:

// main.cpp
#include <QtGui>
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QLabel label;
    label.setPixmap(QPixmap(":/trolltech/styles/commonstyle/images/standardbutton-help-128.png"));
    label.setStyleSheet("background-color: black");
    label.setAlignment(Qt::AlignCenter);
    label.show();
    return app.exec();
}

// main.cpp
#include <QtGui>
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QLabel label;
    label.setPixmap(QPixmap(":/trolltech/styles/commonstyle/images/standardbutton-help-128.png"));
    label.setStyleSheet("background-color: black");
    label.setAlignment(Qt::AlignCenter);
    label.show();
    return app.exec();
}

To copy to clipboard, switch view to plain text mode 

[tommy]

JPN,

Indeed, your suggestion is great and it works but for some reason it is not working for me. My QLabel is declared in the *.h file using pointers:

QLabel* pixmapLabel;

QLabel* pixmapLabel;

To copy to clipboard, switch view to plain text mode 

and I try to specify the background color in the constructor of my class in the same file as my main() function:

pixmapLabel = new QLabel;
pixmapLabel->setStyleSheet("background-color: black");

pixmapLabel = new QLabel;
pixmapLabel->setStyleSheet("background-color: black");

To copy to clipboard, switch view to plain text mode 

These are the only differences from what you are suggesting. I wonder if these could make a difference? Unfortunately I have to use the pointer apprach in my program.

[tommy]

I did some more digging and got my problem solved this way:

QPalette Pal2(pixmapLabel->palette()); 
          Pal2.setColor(QPalette::Window, Qt::black);
          pixmapLabel->setAutoFillBackground(true); 
          pixmapLabel->setPalette(Pal2);

QPalette Pal2(pixmapLabel->palette()); 
          Pal2.setColor(QPalette::Window, Qt::black);
          pixmapLabel->setAutoFillBackground(true); 
          pixmapLabel->setPalette(Pal2);

To copy to clipboard, switch view to plain text mode 

This code sets the background of my QLabel (pixmapLabel) to black as I wanted it. But this code doesn't work on layouts. I guess this is because layouts don't paint. However, sometimes you need to change the color of a layout even if it doesn't paint. How do you do that. For example: My layout consists of 2 sub-layouts (each with its own widgets etc). I want the top one to be red and bottom to be green. How do you do it?

[wysota]

But this code doesn't work on layouts.

Hmm?? There is no such thing as a color of a layout. Layouts don't have colours - it's like you wanted to change the colour of a tcp socket... If you want to change the colour of some area, you have to have a widget there.