Better text contrast of pixel brightness values in Qt window

This offsets text brightness of pixel brightness values
by offsetting it by 127 to the curent pixel value.
The text is now readable even if pixels are black.

modules/highgui/src/window_QT.cpp

@@ -3094,6 +3094,11 @@ void DefaultViewPort::drawImgRegion(QPainter *painter)
             if (nbChannelOriginImage==CV_8UC1)
             {
                 QString val = tr("%1").arg(qRed(rgbValue));
+                int pixel_brightness_value = qRed(rgbValue);
+                int text_brightness_value = 0;
+
+                text_brightness_value = pixel_brightness_value > 127 ? pixel_brightness_value - 127 : 127 + pixel_brightness_value;
+                painter->setPen(QPen(QColor(text_brightness_value, text_brightness_value, text_brightness_value)));
                 painter->drawText(QRect(pos_in_view.x(),pos_in_view.y(),pixel_width,pixel_height),
                     Qt::AlignCenter, val);
             }