eliminating a color in image

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Hi i wanted to eliminate gray color from my image, i just need black or white

So i used this code,but in beagle bone black and in some computers, code crashes but in my laptop it works fine what is happening?

code:

typedef cv::Point3_<uint8_t> Pixel;
 for (Pixel &p : cv::Mat_<Pixel>(frame))
   {
         if(p.x<127 ||p.y<127 ||p.z<127 )
             {
                p.x = 0;
                p.y = 0;
                p.z = 0;
              }
   }

gives me this error: OpenCV Error: Bad argument (The total number of matrix elements is not divisible by the new number of rows) in reshape