Module opencv::img_hash

The module brings implementations of different image hashing algorithms.

Provide algorithms to extract the hash of images and fast way to figure out most similar images in huge data set.

Namespace for all functions is cv::img_hash.

Supported Algorithms

• Average hash (also called Different hash)
• PHash (also called Perceptual hash)
• Marr Hildreth Hash
• Radial Variance Hash
• Block Mean Hash (modes 0 and 1)
• Color Moment Hash (this is the one and only hash algorithm resist to rotation attack(-90~90 degree))

You can study more about image hashing from following paper and websites:

• “Implementation and benchmarking of perceptual image hash functions” zauner2010implementation
• “Looks Like It” lookslikeit

Code Example

@include samples/hash_samples.cpp

Performance under different attacks

Speed comparison with PHash library (100 images from ukbench)

As you can see, hash computation speed of img_hash module outperform PHash library a lot.

PS : I do not list out the comparison of Average hash, PHash and Color Moment hash, because I cannot find them in PHash.

Motivation

Collects useful image hash algorithms into opencv, so we do not need to rewrite them by ourselves again and again or rely on another 3rd party library(ex : PHash library). BOVW or correlation matching are good and robust, but they are very slow compare with image hash, if you need to deal with large scale CBIR(content based image retrieval) problem, image hash is a more reasonable solution.

More info

You can learn more about img_hash modules from following links, these links show you how to find similar image from ukbench dataset, provide thorough benchmark of different attacks(contrast, blur, noise(gaussion,pepper and salt), jpeg compression, watermark, resize).

• Introduction to image hash module of opencv
• Speed up image hashing of opencv(img_hash) and introduce color moment hash

Contributors

Tham Ngap Wei, thamngapwei@gmail.com

Modules

• prelude

Structs

• AverageHash
  Computes average hash value of the input image
• BlockMeanHash
  Image hash based on block mean.
• ColorMomentHash
  Image hash based on color moments.
• ImgHashBase
  The base class for image hash algorithms
• MarrHildrethHash
  Marr-Hildreth Operator Based Hash, slowest but more discriminative.
• PHash
  pHash
• RadialVarianceHash
  Image hash based on Radon transform.

Enums

• BlockMeanHashMode

Constants

• BLOCK_MEAN_HASH_MODE_0
  use fewer block and generate 16*16/8 uchar hash value
• BLOCK_MEAN_HASH_MODE_1
  use block blocks(step sizes/2), generate 31*31/8 + 1 uchar hash value

Traits

• AverageHashTrait
  Mutable methods for crate::img_hash::AverageHash
• AverageHashTraitConst
  Constant methods for crate::img_hash::AverageHash
• BlockMeanHashTrait
  Mutable methods for crate::img_hash::BlockMeanHash
• BlockMeanHashTraitConst
  Constant methods for crate::img_hash::BlockMeanHash
• ColorMomentHashTrait
  Mutable methods for crate::img_hash::ColorMomentHash
• ColorMomentHashTraitConst
  Constant methods for crate::img_hash::ColorMomentHash
• ImgHashBaseTrait
  Mutable methods for crate::img_hash::ImgHashBase
• ImgHashBaseTraitConst
  Constant methods for crate::img_hash::ImgHashBase
• MarrHildrethHashTrait
  Mutable methods for crate::img_hash::MarrHildrethHash
• MarrHildrethHashTraitConst
  Constant methods for crate::img_hash::MarrHildrethHash
• PHashTrait
  Mutable methods for crate::img_hash::PHash
• PHashTraitConst
  Constant methods for crate::img_hash::PHash
• RadialVarianceHashTrait
  Mutable methods for crate::img_hash::RadialVarianceHash
• RadialVarianceHashTraitConst
  Constant methods for crate::img_hash::RadialVarianceHash

Functions

• average_hash
  Calculates img_hash::AverageHash in one call
• block_mean_hash
  Computes block mean hash of the input image
• color_moment_hash
  Computes color moment hash of the input, the algorithm is come from the paper “Perceptual Hashing for Color Images Using Invariant Moments”
• marr_hildreth_hash
  Computes average hash value of the input image
• p_hash
  Computes pHash value of the input image
• radial_variance_hash
  Computes radial variance hash of the input image