“…Several systems convert the RGB images to grayscale by averaging the color channels [33, 58, 59, 63, 64, 67-74, 92, 94, 95, 117, 124]. Weighted averages such as the NTSC standard [135], brightness [126,127], luminance [62,114], based on the pipe material [116], or a linear transformation based on Fischer's linear discriminant classifier [96,100,102,105,109,110] have also been utilized when emphasis on specific color combinations was necessary. Color conversion has also been used in conjunction with image enhancement [111][112][113].…”