Package dev.brachtendorf.jimagehash.hashAlgorithms

Class RotAverageHash

java.lang.Object

dev.brachtendorf.jimagehash.hashAlgorithms.HashingAlgorithm

dev.brachtendorf.jimagehash.hashAlgorithms.RotAverageHash

All Implemented Interfaces:

Serializable

public class RotAverageHash
extends HashingAlgorithm

Calculate a hash value based on the average luminosity in an image after mapping each pixel to a rotational bucket.

Each bucket corresponds to a single pixel which means that the rescaled image gets rather large when higher bit resolutions are required which affects the performance for high bit keys lengths.

Note: Unlike the other hashing algorithms this algorithm is only thread safe after it successfully hashed it's first image. If concurrent usage is required perform a single hash pass on an arbitrary image.

Author:

Kilian

See Also:

Serialized Form

Field Summary

Fields inherited from class dev.brachtendorf.jimagehash.hashAlgorithms.HashingAlgorithm

bitResolution, immutableState, keyResolution, opaqueReplacementColor, opaqueReplacementThreshold, preProcessing

Constructor Summary

Constructors

Constructor	Description
RotAverageHash(int bitResolution)

Method Summary

Modifier and Type	Method	Description
int	computePartition(double originalX, double originalY)	Compute the ring partition this specific pixel will fall into.
protected BigInteger	hash(BufferedImage image, HashBuilder hash)	Calculate a hash for the given image.
protected int	precomputeAlgoId()	A unique id identifying the settings and algorithms used to generate the output result.

Methods inherited from class dev.brachtendorf.jimagehash.hashAlgorithms.HashingAlgorithm

addFilter, algorithmId, createAlgorithmSpecificHash, createPixelAccessor, equals, getKeyResolution, getOpaqueReplacementColor, getOpaqueReplacementThreshold, hash, hash, hash, hash, hashCode, removeFilter, setOpaqueHandling, setOpaqueHandling, toString

Methods inherited from class java.lang.Object

clone, finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

RotAverageHash

public RotAverageHash(int bitResolution)

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 key is not always desired and this implementation specifically suffers performance penalties when dealing with huge hash sizes

The average kernel hash will produce a hash with at least the number of bits defined by this argument. In turn this also means that different bit resolutions may be mapped to the same final 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

computePartition

public int computePartition(double originalX,
                            double originalY)

Compute the ring partition this specific pixel will fall into.

Parameters:

originalX - the x pixel index in the picture

originalY - the y pixel index in the picture

Returns:

the bucket index

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