OpenCV Python

Building an NFT Search Engine in 3 steps using Python and OpenCV

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In this tutorial, you’ll learn how to build your own NFT Search Engine using Python and OpenCV. We’ll first walk you through the concept behind image search engines on how they work, and finally, a guild on how to build your own algorithm using any dataset.

Then in the following tutorial, you’ll learn how to integrate this current solution into a Flask Web Application. Without further ado, let’s start by understanding what is an Image Search Engine.

Today’s tutorial is part one in our two-part series of building an NFT Search Engine using Python, OpenCV and Flask:

  1. Building an NFT Search Engine in 3 steps using Python and OpenCV (this tutorial)
  2. Adding a web interface to our NFT Search Engine with Flask (next tutorial)

The Inspiration

I remember when the NFT business started booming back in 2021. Hearing about how people were flipping a single piece of digital art that looked worthless to me (at that time) – making millions of dollars out of these digital paintings was ridiculous.

One afternoon, while scrolling through Instagram, I spotted a painting that sold for a huge amount of money. I’m talking about millions of dollars. Since I was moving at a fast pace, I couldn’t remember the same profile avatar I saw. However, the only memory I had left of it was a screenshot I had taken months ago.

I would have shown you; however, I had already de-cluttered my gallery months ago.

During that time, I searched through the Openseas website to see if I could search for the name of the image by similarity uploading a look-alike image. I guess I wasn’t lucky to find that feature embedded within their system.

As they didn’t have that feature at that time and until now, I decided to implement a simple solution to help me solve the problem I faced months ago; that’s why I decided to work on this project and implement the best solution with little dataset I got.

A few of the issues I had while implementing this project was getting the dataset I needed. Unfortunately, I tried applying for their API keys to let me scrape the image, yet until now, I haven’t heard from them. Then, I decided to download those images manually. What a painful experience. Isn’t it?

By the end of this tutorial, you’ll learn how I built this Search Engine from the ground to the top. No hidden details.

Before we dive into the explanation of the procedure of this algorithm, let’s first get a good understanding of what I meant by an Image Search Engine.

You can think of this application as a process that takes one query image as an input and retrieves a collection of images similar to the query image. Like the same way Google search works. Once you ask a question and get multiple results based on what you search. It’s the same procedure, but this is in the form of using images.

Dataset

For this tutorial, we will utilize the images I manually scraped from the OpenSeas website (a market place where you could purchase NFTs) which contains a bunch of NFT profile avatars. The downloaded ones are:

  1. Bored Apes
  2. Cryptopunk, and
  3. Alien Frens

Let’s have a look at what these downloaded samples look like:

Figure 1: Sample images from different NFTs collections.

Configuring your Development Environment

Before we get started programming, to follow this tutorial successfully, you’ll need to have OpenCV installed on your system environment or in a virtual environment.

Good thing you don’t have to worry too much about OpenCV’s installation, as I’ve covered them in this tutorial here. Mostly for Linux users. You can easily pip install them, and you’re ready to go.

Project Structure

Before we get started implementing our Python script for our NFT search engine with Python and Opencv, let’s first review our project directory structure:

Inside the neurapsike module, we have four Python scripts we’ll be implementing:

  1. descriptor.py: Our color histogram descriptor.
  2. searcher.py: Holds the functions for comparing indexing images.
  3. __init__.py: Lets the Python interpreter know the directory contains code for a Python module and acts as a constructor for the neuraspike package.
  4. utils.py: Helps loads the images from the directory and extract the filename from each document.

The extract_features.py file is our training script. It will:

  1. Load all the images in the folder into the computer’s memory
  2. Extract relevant features from each image
  3. Then store the extracted features into a database (CSV file)

The search.py file is our query script. It will:

  1. Load the query image
  2. Perform the same features extraction done during the training step on the query image.
  3. Index all the images similar to the query image.
  4. Then return the list of ranked results in order of importance.

Now we have all these details cleared out, let’s start coding!

Implementing the NFT Search Engine Script

First, make a new script inside the neuraspike folder, naming it utils.py, and insert the following code:

We begin by importing our required Python packages os which provides functions for interacting with the operating systems (Line 2).

The load_images_from_folder function accepts a single parameter which is the path to the directory our images are stored. This iterates through all the images found within the specified folder, builds and return all relative path to the images (Lines 4 – 11).

Also the extract_filename function accepts one parameter as an input. As an input, it receives the full path to each image and for the return, it provides the filename, excluding the outer paths (Lines 14 – 16).

Take for example, you have a relative path "./dataset/test_cryptopunk.png" the output will be "test_cryptopunk.png".

__init__.py

Within the __init__.py file, copy and paste the following code into your script to enable you to utilize the functions built within the neuraspike package.

Defining our Image Descriptor

To successfully build our NFTs Search Engine, we’ll rely mostly on the color histograms (i.e., the color distribution of an image) to help quantify each image. Since with the assumption that images with similar color distribution, even though the contents are different, will be considered the same.

Let’s code our descriptor.py file in the neuraspike module:

We begin by importing our required Python packages, cv2, for our OpenCV binding (Line 1).

Moving on, we convert the default color space of our image (that’s the standard RGB – BGR in this case) to the L*A*B space as it has more perceptual meaning compared to the RGB color space (Line 7).

Figure 2: A 3D diagram of the LAB* color space.

Besides another valuable piece of information about the LAB space, it has three components and doesn’t only encode the brightness compared to the RGB space.

The three components it contains are:

  1. L – Lightness (Intensity): It’s independent of color information and only encodes brightness.
  2. A – color component that ranges from Green to Magenta.
  3. B – color components that range from Blue to Yellow.

Based on this information, we can then rely on the euclidean distance to compute the difference between the query image and other image samples.

Next, we calculate the image’s histogram by making the call to cv2.calcHist using the supplied bins from the default parameter settings (Lines 7 – 8).

Then on Line 12, our color histogram is normalized and converted into a feature vector (Line 15).

Note: One great thing about using the color histogram is that your images can vary in size since your output feature vector will always be the number of bins in the histogram.

Feature Extraction from Our Dataset

After implementing our image descriptor, the next steps are to extract features (i.e., color histograms) of every available image and store the result in a database. That’s the features.csv file.

The format of the output should look like this:

>>> [filename, feature vector] -> [102256.jpg, 0.8, 0.2, 0.5, ….., 0.21].

Here, we have the filename, followed by the flattened color histogram each row.

To extract features from the NFT profile avatars we’ve scraped from the internet, open up a new file naming it extract_features.py and insert the following code:

We begin by importing our required Python packages, namely argparse, cv2(Opencv binding), and neuraspike module.

Next, our script accepts two command-line arguments:

  • --dataset: The path to the folder filled with photos of different NFT avatars.
  • --index: The path to store the output of the CSV file contains the filename and its associated features in each image.

Now that we’ve handled the imports in our command line, let’s perform indexing on all images in the dataset folder:

On line 14, we will generate a list of paths to all the images in the dataset folder and initialize an empty list of features and labels; that’s where each extracted feature from the image and filename will be stored (Lines 19 – 20).

Then for each image path stored in the list of image_paths, we will:

  1. Load the image into memory (Line 27).
  2. Extract the filename, using it as a universally unique identifier (UUID) (Line 28).
  3. Apply our image descriptor to the loaded image, then extract the features from the image, converting the histogram to a list (Lines 32 – 33).
  4. Finally, we will then populate the empty features and labels lists which we defined earlier (Lines 36 – 37).

After the histograms are computed for all images, we saved the image filename and its associated feature vector into a CSV file (Lines 41 – 51).

Once you have this implemented, open up your terminal, navigate to the directory where your code lives, and issue the following command:

The Searcher

Now that we have all the features extracted from the images in our database, we need to implement a method that allows us to search through our dataset folder.

With that said, open up a new file, name it searcher.py, and insert the following:

After importing our packages, let’s define a function perform_image_search to compare our query image feature vector among all the other vectors saved in the CSV file populated earlier.

On line 8, a query_result dictionary is initialized to store the output results.

From lines 11 – 18, we loaded the CSV file, iterated through each row while keeping track of the filename and its associated feature vector. Then compute the distance between the query image, and the saved feature vector (from the CSV file) using the euclidean distance.

Then, Lines 23 – 24 sorts the populated dictionary based on the most relevant image – the first item on the list being the closest image to the query image. And using the limit parameter, we control the number of outputs to be displayed to the user. 

Performing the Search

The time has come to put our NFT Search Engine to the test. So let’s open a script called search.py and insert the following code below:

We began importing our required Python packages (Lines 2 – 4). Then, we parsed our command-line arguments (Lines 7 – 13).

The command-line arguments in our search.py script requires three arguments:

  • --dataset:  The path to the query image.
  • --index: The path to where the features are stored. That’s the CSV file.
  • --dataset: The path to the download NFTs database.

Okay, time to perform the actual search:

First, we load the query image from the disk (line 20) and then grab the image’s color histogram (line 21).

Then to find similar images (i.e., color-wise) among other images within the datasets folder, a call to the perform_image_search function is made (line 24), which returns a limited number of images based on the set limit of the parameter.

Lastly, the query image is displayed on our screen (line 27), along with the return of similar results (Lines 31 – 37).

Up and Running Color-Based Search Engine

Let’s put our NFTs (Color-Based) Search Engine to the test. Open up a terminal and launch the search.py script with the following commands:

Figure 3: Query Image of the bored ape
Figure 4: Using our Color-Based Image Search Engine built using OpenCV to find images of bored apes in our dataset.
Figure 5: Cryptopunk Query Image
Figure 6: Using our Color-Based NFT Search Engine built using OpenCV to find images of crypto punks in our dataset.

 

Figure 7: Query Image of the alien frens
Figure 8: Using our Color-Based NFT Search Engine built using OpenCV to find images of alien frens in our dataset.

 

Guild to Building our Color-Based Image Search Engine.

For this NFT (color-based) Image Search Engine we built, it boiled down into these four distinct steps:

  1. Defining your image descriptor: At this stage, we need to select the aspect of the image we want to describe, which is the image’s color. It could be a different portion of the image or the entire image itself.
  2. Indexing your dataset: As we have our image descriptor defined, our next task is to apply this procedure to each image in our datasets, extract all the essential features from these images, then write the features to data storage. For this tutorial, we’ve stored them using CSV files so we can use them for similarity comparison.
  3. Defining your similarity metric: Once all the features vectors are extracted, we need to determine how we will compare these vectors using a defined metric. However, the similarity metrics must be chosen carefully based on the dataset’s type and extraction process. Some options are Euclidean distance, Cosine distance, chi-square distance, e.t.c. It depends on the algorithm you’re using to quantify your collection of images.
  4. Searching: The step where we perform is the actual step required, and an individual will submit a query image which:
    • Extract the features from the query image
    • Apply the similarity function,” the euclidean distance,” which will compare all the features extracted from the query image and all the features already indexed.
    • Then finally, return all the most important results based on the similarity function chosen.

Summary

In this tutorial, you learnt how to build your own NFT Search Engine using Python and OpenCV. We learnt about:

  1. Defining our image descriptor which helps quantify each image
  2. How to extract features from each image in our database.
  3. And how to perform a quick search to get all images which are similiar to the query image.

Credits

The article was inspired by Adrian Rosebrock at PyImageSearch. I used his idea as a reference guild. Then changed the approach of search, using the LAB* color-space and the euclidean distance function as he worked with HSV color-space and Chi-squared distance function. Check out Adrian article if you feel so inclined.

What’s Next?

Now, what’s next? in the following tutorial, we will learn how to build a web application for our NFTs Search Engine to make it easier to upload an image and return all the images

Further Reading

We have listed some useful resources below if you thirst for more reading.

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