How to fill the void with transparent background when rotating a QImage?

Question

When I rotate an image in Qt, It automatically enlarge the image to encapsulate the bigger dimensions which is great and intelligent. However, it fills the newly created void with white color which is not what I want. How can I make it fill with a totally transparent color?

Here is may code.

QImage lSource("/path/to/an/image/file");
QTransform lRotation;
lRotation.rotate(30.0);
QImage lRotated(lSource.transformed(lRotation, Qt::SmoothTransformation));
ui.labelImage->setPixmap(QPixmap::fromImage(lRotated));

Thanks a lot!

Answer 1

Based on my comment above...

I suspect, that the void areas of the rotated image are transparent, and the white color is the background color of your label. If you want to set a specific background to your rotated image, you can do the following:

// Create rotated image.
QImage lSource("source.png");
QTransform lRotation;
lRotation.rotate(30.0);
QImage lRotated(lSource.transformed(lRotation, Qt::SmoothTransformation));

// Create resulting image with specific background.
QImage img(lRotated.size(), QImage::Format_ARGB32);
img.fill(Qt::red); // Set the red background
QPainter p(&img);
p.drawImage(0, 0, lRotated);

QLabel l;
l.setPixmap(QPixmap::fromImage(img));
l.show();