Transparency effect between image and drawing

Furkas

I want to give a transparency effect to an image, so the underlying drawing could be seen through. Has anybody used this king of effect, often visible in Windows desktop, with Qt ?
I'm grateful for any indication.

? Offline

@Furkas Hi! Are we talking about QWidgets or QtQuick? Do you want to make a whole window transparent or just an object within a window?

Furkas

@Wieland,
I'm talking about QWidgets, and I want just one transparent object, I think a QLabel may be cool.

raven-worx

see QGraphicsOpacityEffect class in case you want to make the whole widget transparent
Its also possible to add a linear opacity (gradient-like), but there is more work involved.

Furkas

@raven-worx Thanks for your advice. It seems to be the right tool. But I have troubles with this class. I get an exception "Read acces violation" any time I try to call setGraphicsEffect() with this effect on a widget in my UI. I have not found in the manual what is the restriction.
It's ok if I add a QLabel outside of the UI.

raven-worx

hard to tell ... you're probably trying to accessing a uninitialized/deleted variable.
show some code pls.

Furkas

@raven-worx Here the significant code :
#include <QMessageBox>
#include <QStandardPaths>
#include <QFileDialog>
#include <QImageReader>
#include <QTimer>
#include <QtWidgets>

#include "radararea.h"
#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
imageLabel = new QLabel;
imageLabel->setBackgroundRole(QPalette::Base);
imageLabel->setSizePolicy(QSizePolicy::Ignored, QSizePolicy::Ignored);
imageLabel->setScaledContents(true);

effect = new QGraphicsOpacityEffect;
effect->setOpacity(0.6);

//ui->imaLabel->setGraphicsEffect(effect); // Nok
//ui->spinBoxGain->setGraphicsEffect(effect); // Nok
//ui->pushButtonACAuto->setGraphicsEffect(effect); // Nok
//imageLabel->setGraphicsEffect(effect);  // ok

ui->setupUi(this);

//ui->imaLabel->setGraphicsEffect(effect); // Nok
//ui->spinBoxGain->setGraphicsEffect(effect); // Nok
//ui->pushButtonACAuto->setGraphicsEffect(effect); // Nok
//imageLabel->setGraphicsEffect(effect);  // ok

ui->comboBoxAntType->addItem(tr("Antenne F1"));
ui->comboBoxAntType->addItem(tr("Antenne F2"));
ui->comboBoxAntType->addItem(tr("Antenne F3"));

connect(ui->comboBoxAntType, SIGNAL(currentIndexChanged(int)), SLOT(on_ComboChanged(int)));

......
I think UI is not available before the call to setupUI(), but it's the same after.

raven-worx

yes indeed as you said, dont access any member of the ui variable before calling setupUi() on it.
Also - as the docs of QWidget::setGraphicsEffect() are clearly stating - you cannot reuse a graphicseffect for multiple widgets.

Furkas

@raven-worx I noted. So, I tried one after another.