I would also suggest working with a QImage, which allows pixel-precise manipulation. You can always convert it to a QPixmap later.
i.e.
Qt Code:
  1. QImage image(20,20, QImage::Format_ARGB32);
  2. for(int i=0;i<400;i++)
  3. {
  4. QRgb rgb = string[i]=='b' ? qRgb(0,0,0) : qRgb(255,255,255);
  5. image.setPixel(i%20, i/20, rgb);
  6. }
  7. QPixmap pixmap = QPixmap::fromImage(image);
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