Creating a transparent image.

  • I am working on a project that requires me to make an image semi-transparent so that I can then plot a graph on that image.

    Problem: I am unable to make the whole image semi-transparent. The image is displayed using QLabel->setPixmap. I open the image and save it as a QImage and try to manipulate its alpha channel and then convert it to Pixmap.


    void Interface::updateImageDataGraph()
        QRgb value;
        QImage original(imageSetPath + "/" + openImageName);
        original.load(imageSetPath + "/" + openImageName);
        original = QImage(previewSize, previewSize, QImage::Format_ARGB32);
        for(int i=0; i<original.height(); i++){
            QRgb *line = (QRgb *)original.scanLine(i);
            for(int j=0; j<original.width(); j++){
                QRgb pixelData = line[j];
                unsigned int red = qRed(pixelData);
                unsigned int green = qGreen(pixelData);
                unsigned int blue = qBlue(pixelData);
                qDebug()<<"RGB at ("<<i<<","<<j<<") is "<<red<<green<<blue<<endl;
                value = qRgba(red, green, blue, 127);
                original.setPixel(i,j, value);
        imagePixmap = imagePixmap.scaledToHeight(previewSize);
        /*QPainter painter(this);

  • I would try creating a new QPixmap using fromImage, instead of reusing the original pixmap. Maybe you retain some kind of color information that affects the outcome.

  • @Jay_Ladhad

    When using the setPixel () function in the current code, the x, y coordinate system is being used incorrectly. Change like this:

    original.setPixel(j, i, value);

    And it is simple to using the combination of subclassing QLabel / overloading initPainter() / QPainter::setOpacity() or QGraphicsOpacityEffect.

  • @Devopia53 Thank you so much. I am completely new to this environment and would love to improve the efficiency of my program using the predefined functions.

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