Creating a transparent image.

Jay_Ladhad

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.

Code:

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);
        }
    }
    //original.fill(Qt::transparent);
    imagePixmap.convertFromImage(original);
    //imagePixmap.fill(Qt::transparent);
    imagePixmap = imagePixmap.scaledToHeight(previewSize);
    /*QPainter painter(this);
    painter.begin(&imagePixmap);

    painter.setOpacity(0.5);
    painter.end();*/
    imageLabel->setPixmap(imagePixmap);
    
    prevDataSeries->replace(imageDataSeries->pointsVector());
}

Asperamanca

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.

Devopia53

@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.

Jay_Ladhad

@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.