Monochrome QImage transformation - void area looks black

Hi there,

I want to rotate a monochrome image of Type QImage using QImage::transformed method.
After applying rotation, the background color of the void area is always black.

The Image before Rotation lookes like this:
Attachment 12674

After rotation, of 45° clockwise, it lookes like this:
Attachment 12675

But it should look like this:
Attachment 12676

My question is how can I make sure that the void area color is white?
Thanks in advance!

Here is my sample code:

Code:

---

#include <QImage>
#include <QPainter>
 
int main()
{
   // size of square Image
   const int nSize = 100;
 
   // create monochrome Image
   QImage img = QImage(nSize, nSize, QImage::QImage::Format_Mono);
 
   QPainter pnt;
   pnt.begin(&img);
 
   // fill background with white
   pnt.fillRect(0, 0, nSize, nSize, Qt::color0);
   pnt.setPen(QPen(Qt::color1));
   // draw a rectangle
   pnt.drawRect(0, 0, nSize-1, nSize-1);
   pnt.end();
 
   img.save("original.bmp");
 
   // rotate image
   QMatrix m;
   m.rotate(45);
   img = img.transformed(m);
   img.save("rotated.bmp");
 
   return 0;
}

---

Please stop posting duplicate posts/threads.

To your question:
This is just a guess:
Since this is a monochrome image, when you transform it, the areas out side the rect are not being painted on - which means they stay 0, which means black.
Try using QImage::transformed().