Simple rails task that will take a `template.png` image and generate all the favicons you need for your app. It also generates a view partial that you can include. Uses Image Magick under the hood. Would recommend later polishing up each image but this gets you set up pretty quickly.

favicon.rake

namespace :favicons do
  desc "Build favicons for all platforms"
  task build: :environment do
    if /Version/m =~ (`convert -version`)
      # Main Icon Sizes
      ico_sizes = %w(16 32)
      apple_sizes = %w(57 60 72 76 114 120 129 144 152)
      apple_precomposed_sizes = %w(120 129 152)
      ms_tile_sizes = %w(144)
      
      # It will use template.png as the main image
      # If you provide template-small.png, any image smaller than this break point will use that
      # You can do the same with template-large.png
      small_break = 50
      large_break = 150

      favico_dir = "favicons"
      file_favico_dir = "#{Rails.root}/app/assets/images/#{favico_dir}"

      unless File.exists?(file_favico_dir)
        Dir.mkdir file_favico_dir
      end

      favico_template = "#{Rails.root}/app/views/layouts/shared/_favicon.html.erb"
      if File.file?("#{Rails.root}/app/assets/stylesheets/application/variables.scss")

        /\$favico-color:\s*(?<icocolor>#([a-zA-Z0-9]{6}|[a-zA-Z0-9]{3}));/ =~ File.read("#{Rails.root}/app/assets/stylesheets/application/variables.scss")

        unless icocolor
          /\$primary-color:\s*(?<icocolor>#([a-zA-Z0-9]{6}|[a-zA-Z0-9]{3}));/ =~ File.read("#{Rails.root}/app/assets/stylesheets/application/variables.scss")
        end
      end
      icocolor ||= "#333333"


      if File.file?("#{file_favico_dir}/template.png")
        print "Converting template image to favicons..."
        template_name = "#{file_favico_dir}/template.png"
        template_small_name = "#{file_favico_dir}/template-small.png"
        template_large_name = "#{file_favico_dir}/template-large.png"

        template_small_name = template_name unless File.file?(template_small_name)
        template_large_name = template_name unless File.file?(template_large_name)

        ico_sizes.each do |size|
          ico_template = template_name
          ico_template = template_small_name if size.to_i <= small_break
          ico_template = template_small_name if size.to_i >= large_break
          (`convert #{ico_template} -resize #{size}x#{size} #{file_favico_dir}/favicon-#{size}x#{size}.ico`)
        end

        apple_sizes.each do |size|
          ico_template = template_name
          ico_template = template_small_name if size.to_i <= small_break
          ico_template = template_small_name if size.to_i >= large_break
          (`convert #{ico_template} -resize #{size}x#{size} #{file_favico_dir}/apple-touch-icon-#{size}x#{size}.png`)
        end

        apple_precomposed_sizes.each do |size|
          ico_template = template_name
          ico_template = template_small_name if size.to_i <= small_break
          ico_template = template_small_name if size.to_i >= large_break
          (`convert #{ico_template} -resize #{size}x#{size} #{file_favico_dir}/apple-touch-icon-#{size}x#{size}-precomposed.png`)
        end

        ms_tile_sizes.each do |size|
          ico_template = template_name
          ico_template = template_small_name if size.to_i <= small_break
          ico_template = template_small_name if size.to_i >= large_break
          (`convert #{ico_template} -resize #{size}x#{size} #{file_favico_dir}/mstile-#{size}x#{size}.png`)
        end


        ico_template = template_name
        ico_template = template_small_name if 152 <= small_break
        ico_template = template_small_name if 152 >= large_break
        (`convert #{ico_template} -resize 152x152 #{file_favico_dir}/apple-touch-icon.png`)
        (`convert #{ico_template} -resize 152x152 #{file_favico_dir}/apple-touch-icon-precomposed.png`)
      else
        print "Creating favicons from application..."
        favico_letter = Rails.application.class.parent_name.first

        ico_sizes.each do |size|
          (`convert -background "#{icocolor}" -fill white -size #{size}x#{size} -gravity center label:#{favico_letter} #{file_favico_dir}/favicon-#{size}x#{size}.ico`)
        end

        apple_sizes.each do |size|
          (`convert -background "#{icocolor}" -fill white -size #{size}x#{size} -gravity center label:#{favico_letter} #{file_favico_dir}/apple-touch-icon-#{size}x#{size}.png`)
        end

        apple_precomposed_sizes.each do |size|
          (`convert -background "#{icocolor}" -fill white -size #{size}x#{size} -gravity center label:#{favico_letter} #{file_favico_dir}/apple-touch-icon-#{size}x#{size}-precomposed.png`)
        end

        ms_tile_sizes.each do |size|
          (`convert -background "#{icocolor}" -fill white -size #{size}x#{size} -gravity center label:#{favico_letter} #{file_favico_dir}/mstile-#{size}x#{size}.png`)
        end

        (`convert -background "#{icocolor}" -fill white -size 152x152 -gravity center label:#{favico_letter} #{file_favico_dir}/apple-touch-icon.png`)
        (`convert -background "#{icocolor}" -fill white -size 152x152 -gravity center label:#{favico_letter} #{file_favico_dir}/apple-touch-icon-precomposed.png`)
      end

      puts "done"
      print "Composing layout..."

      File.open(favico_template, 'w') do |f|
        ico_sizes.each do |size|
          f.puts "<%= favicon_link_tag \"#{favico_dir}/favicon-#{size}x#{size}.ico\", rel: 'shortcut icon', type: 'image/x-icon' %>"
        end

        apple_sizes.each do |size|
          f.puts "<%= favicon_link_tag \"#{favico_dir}/apple-touch-icon-#{size}x#{size}.png\", rel: 'apple-touch-icon', type: 'image/png', sizes: \"#{size}x#{size}\" %>"
        end
        f.puts "<%= favicon_link_tag \"#{favico_dir}/apple-touch-icon.png\", rel: 'apple-touch-icon', type: 'image/png' %>"

        apple_precomposed_sizes.each do |size|
          f.puts "<%= favicon_link_tag \"#{favico_dir}/apple-touch-icon-#{size}x#{size}-precomposed.png\", rel: 'apple-touch-icon-precomposed', type: 'image/png', sizes: \"#{size}x#{size}\" %>"
        end
        f.puts "<%= favicon_link_tag \"#{favico_dir}/apple-touch-icon-precomposed.png\", rel: 'apple-touch-icon-precomposed', type: 'image/png' %>"

        ms_tile_sizes.each do |size|
          f.puts "<meta name=\"msapplication-TileImage\" content=\"<%= image_path(\"#{favico_dir}/mstile-#{size}x#{size}\") %>\" />"
        end


        f.puts "<meta name=\"msapplication-TileColor\" content=\"#{icocolor}\" />"
        f.puts "<meta name=\"application-name\" content=\"#{Rails.application.class.parent_name}\">"
      end

      puts "done"
      puts "Add to your views with: <%= render \"layouts/shared/favicon\" %>"
    else
      puts "You do not have ImageMagick installed.  Command 'convert' was not found."
    end
  end
end