docs: public static void savePGM(GrayU8 gray , String fileName ) throws IOException {
  		File out = new File(fileName);
  		DataOutputStream os = new DataOutputStream(new FileOutputStream(out));
  
  		String header = String.format("P5\n%d %d\n255\n", gray.width, gray.height);
  		os.write(header.getBytes());
  
  		os.write(gray.data,0,gray.width*gray.height);
  
  		os.close();
  	}