Monochrome QImage transformation - void area looks black

KBA_

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:

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

But it should look like this:

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

Here is my sample 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;
}

VRonin

Top of my head: call img.invertPixels() both before and after the transformation

P.S.
QMatrix is deprecated, use QTransform instead

KBA_

First of all, thank you for your reply!

I already tried your approach before, but my application is very time critical and I want to avoid unnecessary computation time! Is there a way to tell the transformation process which color to take for the void area pixels?

By the way, the approach doesn't work, it produces the following:

1_original.bmp
2_original_inverted.bmp
3_rotated_inverted.bmp
4_rotated_normal.bmp:

Using this code fragment:

#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("1_original.bmp");
   img.invertPixels();
   img.save("2_original_inverted.bmp");

   // rotate image
   QMatrix m;
   m.rotate(45);
   img = img.transformed(m);
   img.save("3_rotated_inverted.bmp");

   img.invertPixels();
   img.save("4_rotated_normal.bmp");

   return 0;
}

VRonin

img = img.transformed(m); will convert the format() to QImage::Format_ARGB32_Premultiplied (you can check by adding qDebug() << img.format(); before img.save("3_rotated_inverted.bmp");) so you have to convert it back to Mono

#include <QImage>
#include <QPainter>
int main()
{
   // size of square Image
   const int nSize = 100;

   // create monochrome Image
   QImage img = QImage(nSize, nSize, 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("1_original.bmp");
   img.invertPixels();
   img.save("2_original_inverted.bmp");

   // rotate image
   QTransform m;
   m.rotate(45);
   img = img.transformed(m);
   img.save("3_rotated_inverted.bmp");
   img=img.convertToFormat(QImage::Format_Mono);
   img.invertPixels();
   img.save("4_rotated_normal.bmp");

   return 0;
}

I had a quick look at the sources and I couldn't see any way of setting the "background" colour nor to prevent the conversion to ARGB32.