QImage变暗

   _image = QPixmap::grabWindow(desk->winId()).toImage();
    _darkImage = _image;
    int bytesPerLine = _darkImage.width() * _darkImage.depth() / 8;
    int h = _darkImage.height();
    for (int i = 0; i < h; i++)
    {
        unsigned char *lineBuf = _darkImage.scanLine(i);
        for (int x = 0; x < bytesPerLine; x++)
        {
            lineBuf[x] /= 2;
        }
    }

来源：http://wangjiajun53880.blog.163.com/blog/static/11700139420118661417711/