📖 Read the full article on Rubystack News: Imprint: Signed, Expiring Image Rendering with Dynamic Watermarks in Ruby

Signed, expiring image watermark rendering for Ruby

imprint-image is a Ruby library for generating signed, time-limited image renders with dynamic text watermarks. It allows you to securely distribute images using expiring tokens, preventing unauthorized reuse, hotlinking, or permanent access.

The library is framework-agnostic and works as pure Ruby, with optional Rails integration.

Image rendering is powered by the GD graphics library.

• 🔐 Signed tokens with expiration
• 🖼 Dynamic text watermarks rendered on the fly
• ⏱ Time-limited image access
• 🚫 Prevents hotlinking and unauthorized reuse
• 🧩 Pure Ruby core
• 🚆 Optional Rails integration (no forced dependency)

gem install imprint-image

or in your Gemfile:

gem 'imprint-image'

• Ruby ≥ 3.3
• GD graphics library (libgd)
• The ruby-libgd gem (installed automatically)

On Debian / Ubuntu:

sudo apt install libgd-dev

token = Imprint.sign(
  source: '/path/to/image.png',
  watermark: 'CONFIDENTIAL',
  expires_in: 60
)

output_path = Imprint.render_from_token(token)

if output_path
  puts "Rendered image at: #{output_path}"
else
  puts "Token expired or invalid"
end

Imprint does not force routes or controllers.

get '/imprint/:token', to: 'imprint#show'

class ImprintController < ApplicationController
  def show
    path = Imprint.render_from_token(params[:token])
    return head :not_found unless path
    send_file path, type: 'image/png', disposition: 'inline'
  end
end

<img src="/imprint/<%= token.split('/').last %>" />

<%= imprint_image_tag(token, class: "watermarked") %>

• Tokens are signed and tamper-proof
• Tokens include an expiration timestamp
• Expired tokens cannot be rendered

Rendered images are written to Dir.tmpdir. You are responsible for cleaning them if needed.

bundle install
bundle exec rake

MIT License.

Germán Giménez Silva
https://github.com/ggerman