A small Ruby class for generating the identicons that GitHub uses. Requires the `color`, `chunky_png`, and `oily_png` gems.

identicon.rb

# frozen_string_literal: true

require 'color'
require 'oily_png'

class Identicon
  # How big each "pixel" is
  PIXEL_SIZE = 70
  # How many rows and columns
  SPRITE_SIZE = 5

  def initialize(gravatar_id)
    @hash = gravatar_id
    @sprite_color = nil
    generate_color
  end

  def generate_color
    # Use values at the end of the hash to determine HSL values
    hue = map(@hash[25, 3].to_i(16), 0, 4095, 0, 360)
    sat = map(@hash[28, 2].to_i(16), 0, 255, 0, 20)
    lum = map(@hash[30, 2].to_i(16), 0, 255, 0, 20)

    color = Color::HSL.new(hue, 65 - sat, 75 - lum)
    rgb = color.to_rgb
    r = (rgb.r * 255).round
    g = (rgb.g * 255).round
    b = (rgb.b * 255).round

    @bg_color = ChunkyPNG::Color.rgb(240, 240, 240)
    @sprite_color = ChunkyPNG::Color.rgb(r, g, b)
  end

  def generate_image
    half_axis = (SPRITE_SIZE - 1) / 2
    margin_size = PIXEL_SIZE / 2

    png = ChunkyPNG::Image.new(420, 420, @bg_color)

    i = 0 # Overall count
    x = half_axis * PIXEL_SIZE
    while x >= 0
      y = 0
      while y < SPRITE_SIZE * PIXEL_SIZE
        if @hash[i].to_i(16).even?
          png.rect(x + margin_size, y + margin_size, x + PIXEL_SIZE + margin_size, y + PIXEL_SIZE + margin_size,
                   @sprite_color, @sprite_color)
          if x != half_axis * PIXEL_SIZE
            x_start = (2 * half_axis * PIXEL_SIZE) - x
            png.rect(x_start + margin_size, y + margin_size, x_start + PIXEL_SIZE + margin_size,
                     y + PIXEL_SIZE + margin_size, @sprite_color, @sprite_color)
          end
        end
        i += 1
        y += PIXEL_SIZE
      end
      x -= PIXEL_SIZE
    end

    png.to_blob(:fast_rgb)
  end

  # A Ruby implementation of Processing's `map` function
  # https://processing.org/reference/map_.html
  def map(value, v_min, v_max, d_min, d_max)
    v_value = value.to_f

    v_range = v_max - v_min
    d_range = d_max - d_min
    d_value = ((v_value - v_min) * d_range / v_range) + d_min
  end
end

you would use it like this:

require 'digest'

hash = Digest::MD5.hexdigest('email')

identicon = Identicon.new(hash)
icon = identicon.generate_image
icon # you could return this to a browser with the header `Content-Type: image/png` or save it to a file or something

which would produce the following image: