Package dev.brachtendorf.jimagehash.hashAlgorithms

Class DifferenceHash

  • All Implemented Interfaces:
    Serializable
    public class DifferenceHash
extends HashingAlgorithm
    Calculates a hash based on gradient tracking. This hash is cheap to compute and provides a high degree of accuracy. Robust to a huge range of color transformation
    Since:
    1.0.0
    Author:
    Kilian
    See Also:
    Serialized Form

    • Constructor Detail

      • DifferenceHash

        public DifferenceHash​(int bitResolution,
                      DifferenceHash.Precision precision)
        Create a difference hasher with the given settings. The bit resolution always corresponds to the simple precision value and will increase accordingly depending on the precision chosen.

        Tests have shown that a 64 bit simple precision hash usually performs better than a 32 bit double precision hash.

        Parameters:
        bitResolution - The bit resolution specifies the final length of the generated hash. A higher resolution will increase computation time and space requirement while being able to track finer detail in the image. Be aware that a high resolution is not always desired. The bit resolution is only an approximation of the final hash length.
        precision - Algorithm precision. Allowed Values:
        Simple:
        Calculates top - bottom gradient
        Double:
        Additionally computes left - right gradient (doubles key length)
        Tripple:
        Additionally computes diagonal gradient (triples key length)

    • Method Detail

      • hash

        protected BigInteger hash​(BufferedImage image,
                          HashBuilder hash)
        Description copied from class: HashingAlgorithm
        Calculate a hash for the given image. Invoking the hash function on the same image has to return the same hash value. A comparison of the hashes relates to the similarity of the images. The lower the value the more similar the images are. Equal images will produce a similarity of 0.

        This method is intended to be overwritten by implementations and takes a baseHash argument to allow concatenating multiple hashes as well to be able to compute the effective hash length in HashingAlgorithm.getKeyResolution(). Preceding 0's are omitted in big integer objects, while the usual hamming distance can be calculated due to xoring without issue the normalized distance requires the potential length of the key to be known.

        Specified by:
        hash in class HashingAlgorithm
        Parameters:
        image - Image whose hash will be calculated
        hash - a hash builder used to construct the hash
        Returns:
        the hash encoded as a big integer

      • precomputeAlgoId

        protected int precomputeAlgoId()
        Description copied from class: HashingAlgorithm
        A unique id identifying the settings and algorithms used to generate the output result. This method shall contain a hash code for the object which
        • Stays consistent throughout restart of the jvm
        • Value does not change after the constructor finished
        • Must return the same value if two instances compute the same hashes for identical input

        Even if different bitResolutions are used in the constructor HashingAlgorithm(int) the algorithId MUST return the same id for two instances if the returned hashes for the same input will always be equal. Therefore instead of checking against the bitResolution the actual resolution as returned by HashingAlgorithm.getKeyResolution() should be used. This method algorithm id's as information available to the child class and will be extended by the hashcode of the kernels.

        Specified by:
        precomputeAlgoId in class HashingAlgorithm
        Returns:
        the preliminary algorithm id identifying this hashing algorithm

      • hash

        public Hash hash​(BufferedImage image)
        Description copied from class: HashingAlgorithm
        Calculate a hash for the given image. Invoking the hash function on the same image has to return the same hash value. A comparison of the hashes relates to the similarity of the images. The lower the value the more similar the images are. Equal images will produce a similarity of 0.
        Overrides:
        hash in class HashingAlgorithm
        Parameters:
        image - Image whose hash will be calculated
        Returns:
        The hash representing the image
        See Also:
        Hash

      • createAlgorithmSpecificHash

        public Hash createAlgorithmSpecificHash​(Hash original)
        Description copied from class: HashingAlgorithm
        Wraps the values supplied in the argument hash into a hash object as it would be produced by this algorithm.

        Some algorithms may choose to return an extended hash class to overwrite certain behavior, in particular the Hash.toImage(int) is likely to differ.

        If the algorithm does not utilize a special hash sub class this method returns the supplied argument.

        Overrides:
        createAlgorithmSpecificHash in class HashingAlgorithm
        Parameters:
        original - the hash to transform
        Returns:
        a hash as it would be created by this algorithm.