Use this snippet to add a watermark to an image, after the image is uploaded:

   1  class Image
   2    .
   3    has_attachment ...
   4    .
   5    .
   6    after_attachment_saved do |record|    
   7      # Don't add watermarks to thumbnails
   8      if record.thumbnail.nil?
   9        full_path = File.join(RAILS_ROOT, 'public/', record.public_filename)
  10        
  11        img = MiniMagick::Image.from_file(full_path)
  12        
  13        width = img[:width]
  14        height = img[:height]
  15        
  16        if width > 150 && height > 150
  17          img.combine_options do |c|
  18            c.gravity 'SouthWest'
  19            # This is RAILS_ROOT/images/watermark.gif
  20            c.draw "image Over 0,0 0,0 \"images/watermark.gif\""
  21          end
  22  
  23          img.write(full_path)
  24          
  25        end
  26      end
  27      
  28    end

Note that after_attachment_saved is a callback added by attachment_fu, use after_save if you’re not using attachment_fu.

I had no success installing ImageMagick and mini-magick with the instructions I found on this page but after some googling I found this blog post, which had the magic commands that worked for me:

   1  sudo port install tiff -macosx  #disables the linkage with Apple's open gl
   2  sudo port install ImageMagick
   3  
   4  sudo gem install rmagick
   5  sudo gem install mini_magick

To test mini-magick, open an irb console and paste in the following code:

   1  require 'rubygems'
   2  require 'mini_magick'
   3  
   4  path = "public/images/0000/0003/logo.jpg"
   5  image = MiniMagick::Image.new(path)
   6  
   7  #print width and height
   8  puts image[:width]
   9  puts image[:height]