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.

   1  require 'rubygems'
   2  require 'mini_magick'
   3  
   4  img = MiniMagick::Image.from_file("jpeg.jpg")
   5  
   6  img.combine_options do |c|
   7    c.gravity 'Southwest'
   8    c.draw 'text 10,10 "whatever"'
   9    c.font '-*-helvetica-*-r-*-*-18-*-*-*-*-*-*-2'
  10    c.fill("#FFFFFF")
  11  end
  12  
  13  img.write("new.jpg")

This code can be used to add a watermark to pictures:

   1  require 'rubygems'
   2  require 'mini_magick'
   3  
   4  # Read the image
   5  img = MiniMagick::Image.from_file("the_picture.jpg")
   6  
   7  # 0,0 0,0 = add the watermark at coordinates: x, y, set watermark size to auto with 0,0
   8  img.draw 'image Over 0,0 0,0 "the_watermark.gif"'
   9  
  10  img.write("watermarked_image.jpg")

How to add a watermark to the bottom-left corner?

To add a watermark to the bottom-left corner, you need to use the combine_options method to pass more than one command plus parameters to MiniMagick (gravity and draw commands):

   1  require 'rubygems'
   2  require 'mini_magick'
   3  
   4  img = MiniMagick::Image.from_file("the_image.jpg")
   5  
   6  img.combine_options do |c|
   7    c.gravity 'SouthWest'
   8    c.draw 'image Over 0,0 0,0 "the_watermark.gif"'
   9  end
  10  
  11  img.write("new.jpg")

Troubleshooting

See mogrify’s documentation for details on how to use the draw command.

If you get this error it might mean that mogrify can’t find the watermark image, or that the syntax is incorrect:

   1  mogrify: Non-conforming drawing primitive definition `image'.