Paperclip Processor that finds the background color and replaces it with a transparent background. Also simulates anti-aliasing using '-fuzz' to get rid of pixelated artifacts.

ttransparency.rb

# lib/paperclip_processors/transparency.rb

module Paperclip
  class Transparency < Thumbnail
    # Find the background and replace with transparency.
    # -fuzz 20% simulates antialiasing
    CONVERT_OPTIONS = [
      '-alpha', 'set',
      '-fill', 'white',
      '-draw', "'color 0,0 replace'",
      '-fuzz', '20%',
      '-transparent', 'white'
    ]

    # Append the convert options to existing options 
    # defined via style and convert_options
    def transformation_command
      super + CONVERT_OPTIONS
    end
  end
end

# Usage

# has_attached_file :logo,
#   styles: { small: ['150x50', :png] },
#   processors: [:transparency]