Here is what I changed it after skoa's advice:

In maincontrol.h I declared a QGraphicsPixmapItem pointer to use with the QGraphicsScene:

Qt Code: 
  1. QGraphicsPixmapItem *control_pixmap_item;
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Then in the constructor I used:

Qt Code: 
  1. control_pixmap_item->setPixmap(*control)
  2. control_scene->addItem(control_pixmap_item);
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Then on maincontrol.cpp I changed the update part in the process_canvas function:

Qt Code: 
  1. void MainControl::process_canvas()
  2. {
  3.  QImage control_image(image_w,image_h,QImage::Format_RGB32);
  4.  control_image.fill(set_pixel_value(0,0,0));
  5.  int cy=0;
  6.  int cx=0;
  7.  while(cy<control_image.height())
  8.  {
  9.      while(cx<control_image.width())
  10.      {
  11.          if(cx%(10*SCALE)==0 || cy%(10*SCALE)==0)
  12.          {
  13.              control_image.setPixel(cx,cy,set_pixel_value(150,150,150));
  14.          }
  15.          cx++;
  16.      }
  17.      cx=0;
  18.      cy++;
  19.  }
  20.  control_pixmap->convertFromImage(control_image.scaled(image_w,image_h));
  21. control_pixmap_item->setPixmap(*control_pixmap);
  22.  control_view->update();
  23. }
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