code: public static List<double[]> histogramGray( List<File> images ) {
  		List<double[]> points = new ArrayList<>();
  
  		GrayU8 gray = new GrayU8(1,1);
  		for( File f : images ) {
  			BufferedImage buffered = UtilImageIO.loadImage(f.getPath());
  			if( buffered == null ) throw new RuntimeException("Can't load image!");
  
  			gray.reshape(buffered.getWidth(), buffered.getHeight());
  			ConvertBufferedImage.convertFrom(buffered, gray, true);
  
  			TupleDesc_F64 imageHist = new TupleDesc_F64(150);
  			HistogramFeatureOps.histogram(gray, 255, imageHist);
  
  			UtilFeature.normalizeL2(imageHist); // normalize so that image size doesn't matter
  
  			points.add(imageHist.value);
  		}
  
  		return points;
  	}
docs: Computes a histogram from the gray scale intensity image alone.  Probably the least effective at looking up
  similar images.