## no-std blockhash

A perceptual hashing algorithm for detecting similar images

### 6 releases(1 stable)

 1.0.0 Apr 8, 2024 Sep 17, 2022 Mar 29, 2022 Aug 12, 2021 May 14, 2020

#109 in Images

Used in 3 crates

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# blockhash

This is an implementation of the Blockhash algorithm for detecting similar images, and can produce 16-, 64-, 144-, and 256-bit perceptual hashes.

Support for the `image` crate is provided by default, but support for any image type can be easily added.

Documentation

## Usage

``````use blockhash::blockhash64;

let img = image::open("images/example.png").unwrap();
let hash = blockhash64(&img);

assert_eq!(hash.to_string(), "c7c48f8989c77e0c");
``````

## The Blockhash algorithm

This is a basic outline of how the algorithm works. Note that this explanation uses floating-point numbers, but the library itself only uses integer operations in order to avoid any rounding errors.

To demonstrate the algorithm, we will calculate the 64-bit hash of the following image:

First we convert it to a grayscale image by taking the average of the red, green, and blue components:

Then we divide the image into 64 (8×8) blocks and calculate the average brightness for all the pixels in each block:

Next, we divide the blocks into 4 horizontal bands and find the median value for each band:

Blocks brighter than the median represent a 1 and blocks darker than the median represent a 0. This gives us the 64 bits of the hash, read from left to right, top to bottom:

We can represent the hash as a hexadecimal string:

``````c7c48f8989c77e0c
``````