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Utility for generating fuzzy hashes from images.
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package com.asymmetrik.utils.hash;

import org.bytedeco.javacpp.indexer.Indexer;
import org.bytedeco.javacpp.opencv_core;

import java.io.IOException;
import java.io.InputStream;

import static org.bytedeco.javacpp.opencv_core.mean;
import static org.bytedeco.javacpp.opencv_imgcodecs.CV_LOAD_IMAGE_UNCHANGED;
import static org.bytedeco.javacpp.opencv_imgcodecs.imdecode;
import static org.bytedeco.javacpp.opencv_imgproc.COLOR_BGR2GRAY;
import static org.bytedeco.javacpp.opencv_imgproc.cvtColor;
import static org.bytedeco.javacpp.opencv_imgproc.resize;

/**
 * Generates a 64-bit fuzzy image hash by first computing an image's average pixel value, and then setting a bit
 * for each pixel according to whether that pixel is greater than or equal to the average.
 */
public class AverageHash extends HashGen {

    /**
     * Square dimensions to shrink image into
     */
    private static final int SHRINK_SIZE = 24;

	@Override
	public String getHash(byte[] byteImage) throws IOException {
		// Load original image
		opencv_core.Mat image = imdecode(new opencv_core.Mat(byteImage), CV_LOAD_IMAGE_UNCHANGED);
		
		// openCV returns an empty matrix if the image could not be read (e.g. unsupported or invalid format)
		if (image.size().area() == 0) {
			return null;
		}
		
		// Convert to grayscale and resize
		opencv_core.Mat grayImage;
		if (image.channels() != 1) {
			grayImage = new opencv_core.Mat();
			cvtColor(image, grayImage, COLOR_BGR2GRAY);
		} else {
			// Already single channel
			grayImage = image;
		}
		
		opencv_core.Mat smallImage;
		if (image.rows() != SHRINK_SIZE || image.cols() != SHRINK_SIZE) {
			smallImage = new opencv_core.Mat();
			resize(grayImage, smallImage, new opencv_core.Size(SHRINK_SIZE, SHRINK_SIZE));
		} else {
			smallImage = grayImage;
		}
		
		// Get mean pixel value
		double pixelMean = mean(smallImage).get(0);
		
		// Compute and return hash
		StringBuilder hash = new StringBuilder();
		Indexer idx = smallImage.createIndexer();
		for (int row = 0; row < smallImage.rows(); row++) {
			for (int col = 0; col < smallImage.cols(); col++) {
				int pixel = new Double(idx.getDouble(row, col)).intValue();
				hash.append((pixel > pixelMean) ? 1 : 0);
			}
		}
		return hash.toString();
	}
}