QLabel with shadow effect issue

[jiveaxe]

Hi, I have a problem when using QGraphicsDropShadowEffect onto a QLabel.

I want to draw a shadow around a label's text. The label's content is not static but changes over time.

When I start the application all seems ok:

shadow.png

but starting from the third change a shadow is drawn all around the label:

shadow2.png

Curiously, this does not happen when using the same string with QLabel::setText().

This is the code I'm using:

Dialog::Dialog(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Dialog)
{
    ui->setupUi(this);
 
    ui->frame->setBackgroundRole(QPalette::AlternateBase);
    ui->frame->setAutoFillBackground(true);
 
    QGraphicsDropShadowEffect *coverShadow = new QGraphicsDropShadowEffect(this);
    coverShadow->setBlurRadius(10.0);
    coverShadow->setColor(palette().color(QPalette::Shadow));
    coverShadow->setOffset(0.0);
 
    ui->label->setWindowFlags(Qt::FramelessWindowHint);
    ui->label->setAttribute(Qt::WA_TranslucentBackground);
    ui->label->setGraphicsEffect(coverShadow);
}

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

    ui->frame->setBackgroundRole(QPalette::AlternateBase);
    ui->frame->setAutoFillBackground(true);

    QGraphicsDropShadowEffect *coverShadow = new QGraphicsDropShadowEffect(this);
    coverShadow->setBlurRadius(10.0);
    coverShadow->setColor(palette().color(QPalette::Shadow));
    coverShadow->setOffset(0.0);

    ui->label->setWindowFlags(Qt::FramelessWindowHint);
    ui->label->setAttribute(Qt::WA_TranslucentBackground);
    ui->label->setGraphicsEffect(coverShadow);
}

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Any idea about the problem?

Very thanks

Quick update:

Noticed that resizing the dialog make shadow frame disappear. Unfortunately, no luck with QLabel::update(), QLabel::resize(), QLabel::adjustSize()...

[ChrisW67]

Curiously, this does not happen when using the same string with QLabel::setText().

How are you changing the text on the label if you are not currently calling setText()?

[jiveaxe]

No, I'm not saying that. I meant that the error is not triggered feeding QLabel::setText() with the same string.

A very ugly hack I found is the following (after setText() ) :

label->setVisible(false);
label->setVisible(true);

label->setVisible(false);
label->setVisible(true);

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This is a minimal but complete code:

#include <QApplication>
#include <QtGui>
 
class Dialog : public QDialog
{
    Q_OBJECT
public:
    Dialog(QWidget *parent = 0);
    ~Dialog() {}
 
private Q_SLOTS:
    void buttonClicked();
 
private:
    QLineEdit *m_lineEdit;
    QPushButton *m_pushButton;
    QLabel *m_label;
};
 
Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
    , m_lineEdit(new QLineEdit(this))
    , m_pushButton(new QPushButton(this))
    , m_label(new QLabel(this))
{
    QGraphicsDropShadowEffect *shadowEffect = new QGraphicsDropShadowEffect(this);
    shadowEffect->setBlurRadius(15.0);
    shadowEffect->setColor(palette().color(QPalette::Shadow));
    shadowEffect->setOffset(0.0);
 
    m_label->setWindowFlags(Qt::FramelessWindowHint);
    m_label->setAttribute(Qt::WA_TranslucentBackground);
    m_label->setGraphicsEffect(shadowEffect);
    m_label->setAlignment(Qt::AlignHCenter | Qt::AlignVCenter);
 
    QHBoxLayout *layout1 = new QHBoxLayout;
    layout1->addWidget(m_lineEdit);
    layout1->addWidget(m_pushButton);
 
    QFrame *frame = new QFrame(this);
    frame->setMinimumSize(QSize(400, 100));
    frame->setAutoFillBackground(true);
    frame->setBackgroundRole(QPalette::Base);
    frame->setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);
 
    QGridLayout *frameLayout = new QGridLayout(frame);
    frameLayout->addWidget(m_label);
    frameLayout->setContentsMargins(15,15,15,15);
 
    QVBoxLayout *mainLayout = new QVBoxLayout;
    mainLayout->addLayout(layout1);
    mainLayout->addWidget(frame);
 
    setLayout(mainLayout);
 
    m_lineEdit->setText("I'm a QLabel");
    m_pushButton->setText("&Change");
    m_label->setText("I'm a QLabel");
 
    connect(m_pushButton, SIGNAL(clicked()), SLOT(buttonClicked()));
}
 
void Dialog::buttonClicked()
{
    if(!m_lineEdit->text().isEmpty()) {
        m_label->setText(m_lineEdit->text());
        // uncomment the following to "fix" the issue
        // m_label->setVisible(false);
        // m_label->setVisible(true);
    }
}
 
int main(int argc, char * argv[])
{
    QApplication app(argc, argv);
 
    Dialog dialog;
    dialog.show();
 
    return app.exec();
}
 
#include "main.moc"

#include <QApplication>
#include <QtGui>

class Dialog : public QDialog
{
    Q_OBJECT
public:
    Dialog(QWidget *parent = 0);
    ~Dialog() {}
    
private Q_SLOTS:
    void buttonClicked();
    
private:
    QLineEdit *m_lineEdit;
    QPushButton *m_pushButton;
    QLabel *m_label;
};

Dialog::Dialog(QWidget *parent)
    : QDialog(parent)
    , m_lineEdit(new QLineEdit(this))
    , m_pushButton(new QPushButton(this))
    , m_label(new QLabel(this))
{
    QGraphicsDropShadowEffect *shadowEffect = new QGraphicsDropShadowEffect(this);
    shadowEffect->setBlurRadius(15.0);
    shadowEffect->setColor(palette().color(QPalette::Shadow));
    shadowEffect->setOffset(0.0);
    
    m_label->setWindowFlags(Qt::FramelessWindowHint);
    m_label->setAttribute(Qt::WA_TranslucentBackground);
    m_label->setGraphicsEffect(shadowEffect);
    m_label->setAlignment(Qt::AlignHCenter | Qt::AlignVCenter);
    
    QHBoxLayout *layout1 = new QHBoxLayout;
    layout1->addWidget(m_lineEdit);
    layout1->addWidget(m_pushButton);
    
    QFrame *frame = new QFrame(this);
    frame->setMinimumSize(QSize(400, 100));
    frame->setAutoFillBackground(true);
    frame->setBackgroundRole(QPalette::Base);
    frame->setSizePolicy(QSizePolicy::Preferred, QSizePolicy::Preferred);
    
    QGridLayout *frameLayout = new QGridLayout(frame);
    frameLayout->addWidget(m_label);
    frameLayout->setContentsMargins(15,15,15,15);
    
    QVBoxLayout *mainLayout = new QVBoxLayout;
    mainLayout->addLayout(layout1);
    mainLayout->addWidget(frame);
    
    setLayout(mainLayout);
    
    m_lineEdit->setText("I'm a QLabel");
    m_pushButton->setText("&Change");
    m_label->setText("I'm a QLabel");
    
    connect(m_pushButton, SIGNAL(clicked()), SLOT(buttonClicked()));
}

void Dialog::buttonClicked()
{
    if(!m_lineEdit->text().isEmpty()) {
        m_label->setText(m_lineEdit->text());
        // uncomment the following to "fix" the issue
        // m_label->setVisible(false);
        // m_label->setVisible(true);
    }
}

int main(int argc, char * argv[])
{
    QApplication app(argc, argv);
    
    Dialog dialog;
    dialog.show();
    
    return app.exec();
}

#include "main.moc"

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