Question

I’ve written some code that should make a new image. My background image has black areas, when the for loop comes on a black pixel then it should draw in the new image a blue one instead otherwise it should just draw the original pixel. Thought I could do that like this but the program keeps running.

   QApplication a(argc, argv);
int c, m, y, k, al;
QColor color;
QColor drawColor;
QImage background;
QImage world(1500, 768, QImage::Format_RGB32);
QSize sizeImage;
int height, width;
background.load("Background.jpg");
world.fill(1);
QPainter painter(&background);
sizeImage = background.size();
width = sizeImage.width();
height = sizeImage.height();

for(int i = 0; i < height; i++)
{
    for(int z = 0; z < width; z++)
    {
        color = QColor::fromRgb (background.pixel(i,z) );
        color.getCmyk(&c,&m,&y,&k,&al);

        if(c == 0 && m == 0 && y == 0 && k == 0) //then we have black as color and then we draw the color blue
        {
            drawColor.setBlue(255);
            painter.setPen(drawColor);
            painter.drawPoint(i,z);
        }
    }

}


//adding new image to the graphicsScene
QGraphicsPixmapItem item( QPixmap::fromImage(background));
QGraphicsScene* scene = new QGraphicsScene;
scene->addItem(&item);

QGraphicsView view(scene);
view.show();

Is my for loop wrong or is it my painter? It sais QImage::pixel: coordinate (292,981) out of range but for soo many pixels, it is also not fast enough to use.

Answer 1

As noted in the comments, drawing pixels one by one can be incredibly slow. Even pixel-by-pixel access can be quite slow. E.g. the following is probably faster, but still not very good:

  const QRgb black = 0;
  const QRgb blue = 255;
  for(int y = 0; y < height; y++) {
    for(int x = 0; x < width; x++) {
      if (background.pixel(x,y) == black) {
         background.SetPixel(blue);
      }
    }
  }

The quicker solution involves direct bitoperations via scanline(). You might want to call convertToFormat() first, so you don’t need to deal with the different possible scanline formats.

As a creative hack, call createMaskFromColor to make all black pixels transparent, and then paint over a blue background.