Change pixel color of QPixmap

Christian Ehrlicher

@Jakob-Clausen said in Change pixel color of QPixmap:

Is there another way of doing this?

Without relying on internals? No. You have to go over all pixels, no matter if you're doing it your way or some smarter by relying on internal stuff.

Jakob Clausen

@Christian-Ehrlicher Can you see what I am doing wrong. Why is it not working?

Christian Ehrlicher

@Jakob-Clausen said in Change pixel color of QPixmap:

Why is it not working?

Don't know - add some debug output, use a debugger and step trough to see the values. Do what a software developer does in such a case.

Jakob Clausen

@Christian-Ehrlicher I don't understand. How is a debugger going to help me? What do you think I should look for?

Christian Ehrlicher

@Jakob-Clausen said in Change pixel color of QPixmap:

What do you think I should look for?

You should look if e.g. setPixel() is called ...

Jakob Clausen

@Christian-Ehrlicher It is being called. If I remove the line, then the picture looks like the original. For me setPixel() is a magic function that should replace one pixel color with another. If it does not, then I am lost.

Christian Ehrlicher

So do you actually use the pixmap returned by the function?

kkoehne

Note the caveats mentioned in https://doc.qt.io/qt-5/qimage.html#setPixel :

If the image's format is either monochrome or paletted, the given index_or_rgb value must be an index in
the image's color table, otherwise the parameter must be a QRgb value.

If position is not a valid coordinate pair in the image, or if index_or_rgb >= colorCount() in the case of monochrome and paletted images, the result is undefined.

So, is your image maybe using a palette? There's more details here: https://doc.qt.io/qt-5/qimage.html#pixel-manipulation

JonB

@Jakob-Clausen
As @Christian-Ehrlicher has just asked --- if you think your code is changing the passed-in QPixmap& pixmap, it is not!

Jakob Clausen

@Christian-Ehrlicher yes.

Jakob Clausen

If I comment out the line setPixel(), then the picture will look like the original:

If I do as described, then it looks like this:

So it is changing the pixelvalues. But it is a little rough on the edges.

Christian Ehrlicher

Try to use setPixelColor(int x, int y, const QColor &color).

Jakob Clausen

@Christian-Ehrlicher If I do that, then it works perfectly. But setPixelColor() is introduced before 5.6, and I am forced to use 5.3.

Christian Ehrlicher

@Jakob-Clausen Did you read @kkoehne 's comment? What pixel format do you have?

Jakob Clausen

@Christian-Ehrlicher QImage::Format_ARGB32_Premultiplied

KroMignon

@Jakob-Clausen Perhaps you should try to force pixel format to what you need:

QPixmap IconWidget::changeColor(QPixmap& pixmap, QColor to, QColor from)
{
    QImage temp = pixmap.toImage();
    
    qDebug() << "Pixel format is:" << temp.pixelFormat();
    
    if(temp.pixelFormat() != QImage::Format_ARGB32)
        temp.convertTo(QImage::Format_ARGB32);
    
    for (int y = 0; y < temp.height(); ++y)
    {
        QRgb* s = reinterpret_cast<QRgb *>(temp.scanLine(y));

        for (int x= 0; x < temp.width(); ++x, ++s)
        {
            QColor color = QColor::fromRgba(*s);
            uint8_t alpha = color.alpha();

            to.setAlpha(alpha);
            from.setAlpha(alpha);

            if (color == from)
               *s = to.rgba();
        }
    }

    return QPixmap::fromImage(temp);
}

Christian Ehrlicher

QImage::Format_ARGB32_Premultiplied is not a good idea to do the pixel stuff then... see the docs: https://doc.qt.io/qt-5/qimage.html#Format-enum

Jakob Clausen

@KroMignon Thats a good idea. convertTo() is introduced in 5.13. But if I use

temp = temp.convertToFormat(QImage::Format_ARGB32);

Then it seems to work.
Thank you very much.