Change pixel color of QPixmap

[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.