Working with the MNIST Dataset in JavaScript

Today, I wanted to work with the MNIST dataset in the browser, so I needed to first put it in a workable format. The process involved a few steps:

Download the MNIST dataset with labels. I used tensorflow.keras.datasets for convenience.
Reduce the dataset size and convert the images to vectors.
Export the dataset as JSON.
Use HTML5 Canvas to visualize the JSON array as images again.

Download the MNIST Dataset with Labels

Code to grab the dataset:

import numpy as np
import json
from tensorflow.keras.datasets import mnist

# Load the MNIST dataset
(train_images, train_labels), (test_images, test_labels) = mnist.load_data()

Reduce the Dataset and Convert Images to Vectors

For my experiments, I only needed a subset of the data so I limited the dataset to the first 10,000 images.

limit = 10000

# Limit the dataset
train_images = train_images[:limit]
train_labels = train_labels[:limit]
test_images = test_images[:int(limit * 0.01)]
test_labels = test_labels[:int(limit * 0.01)]

# Convert to vector arrays
train_images = train_images.reshape((limit, 28 * 28))
test_images = test_images.reshape((int(limit * 0.01), 28 * 28))

# Normalize pixel values
train_images = train_images.astype('float32') / 255
test_images = test_images.astype('float32') / 255

# Combine images and labels and export to JSON
train_data = [{'image': train_images[i].tolist(), 'label': int(train_labels[i])} for i in range(len(train_images))]
test_data = [{'image': test_images[i].tolist(), 'label': int(test_labels[i])} for i in range(len(test_images))]

# Export to JSON
with open('train_data.json', 'w') as f:
  json.dump(train_data, f)
with open('test_data.json', 'w') as f:
  json.dump(test_data, f)

This code snippet does the following:

Limits the dataset: Reduces the train_images, train_labels, test_images, and test_labels arrays to the first 10,000 and 100 images, respectively.
Reshapes the image data: Transforms the 2D arrays representing the images into 1D vectors, a nice format for inputs into a neural network.
Normalizes pixel values: Scales the pixel values to be between 0 and 1, which can help with model training.
Combines and exports as JSON: Merges the image vectors and their corresponding labels into a list of dictionaries and exports them as JSON files (train_data.json and test_data.json).

HTML and JavaScript for Visualizing JSON Data

We can convert the training data back into images using HTML5 Canvas. The key function required is outlined below with explanations:

const drawImage = (data, canvasId) => {
  const canvas = document.getElementById(canvasId)
  const ctx = canvas.getContext('2d')

  // Create a new ImageData object with a width and height
  // of 28 pixels (the size of the MNIST images).
  const imageData = ctx.createImageData(28, 28)

  // Loop through the data.image array:
  data.image.forEach((pixel, index) => {

    // Scale the pixel value (which is originally between 0 and 1) to
    //  between 0 and 255, the range for color values in the ImageData object
    let value = pixel * 255

    // Set the pixel values in the imageData object starting with
    // the red channel of the pixel.
    imageData.data[index * 4 + 0] = value

    // Set the green channel of the pixel.
    imageData.data[index * 4 + 1] = value

    // Set the blue channel of the pixel.
    imageData.data[index * 4 + 2] = value

    // Set the alpha channel of the pixel (255 means fully opaque)
    imageData.data[index * 4 + 3] = 255
  })

  ctx.putImageData(imageData, 0, 0)
}

Creates a new ImageData object representing a 28x28 pixel area (the size of MNIST images).
Iterates through the data.image array (the image vector), scaling each pixel value from the normalized range (0-1) back to the standard color range (0-255).
Sets the red, green, blue, and alpha values for each pixel in the imageData object.
Finally, draws the created imageData onto the canvas at position (0, 0).

Demo

Live demo of the images: https://jsbin.com/mowoham/edit?js,output

javascript ml

2024-05-29