Apply watermark over image using Paperclip

model.rb

has_attached_file :file,
	styles: { 
		detail: {
			geometry: "__width__x__height__#",
			watermark_path: Rails.root.join("app/assets/images/__watermark_image__.png"),
			position: 'Center' # SouthEast, NorthWest, etc
		} 
	}, processors: [:watermark]

watermark.rb

# /lib/paperclip_processors/watermark.rb
module Paperclip
	class Watermark < Processor

		def initialize file, options = {}, attachment = nil
			super
			@file             = file
			@attachment       = attachment
			@whiny            = options[:whiny].nil? ? true : options[:whiny]
			@format           = options[:format]
			@current_format   = File.extname(@file.path)
			@basename         = File.basename(@file.path, @current_format)
		end
		 
		def make
			dst = Tempfile.new([@basename, @format].compact.join("."))
			dst.binmode
	 		outfile = File.expand_path(dst.path)
		
			if options[:watermark_path]
				command = "convert"
				params  = %W['#{fromfile}']
				params += transformation_command
				params += %W['#{options[:watermark_path]}' -gravity #{options[:position]} -composite]
				params << "'#{tofile(dst)}'"
			else
				command = "convert"
				params = ["'#{fromfile}'"]
				params += transformation_command
				params << "'#{tofile(dst)}'"
			end
			begin
				Paperclip.run(command, params.join(' '))
			rescue ArgumentError, Cocaine::CommandLineError
				raise Exception, "There was an error processing the watermark for #{@basename}" if @whiny
			end
			dst
		end
	 
		def fromfile
		  "#{ File.expand_path(@file.path) }[0]"
		end

		def tofile(destination)
			File.expand_path(destination.path)
		end

		def transformation_command
	      		scale, crop = (Geometry.from_file file).transformation_to((Geometry.parse options[:geometry]), crop?)
	      		trans = %W[-resize '#{scale}']
	      		trans += %W[-crop '#{crop}' +repage] if crop
	      		trans << options[:convert_options] if options[:convert_options]
	      		trans
    		end

	    	# Returns true if the +target_geometry+ is meant to crop.
		def crop?
			options[:geometry][-1,1] == '#'
		end
	
		# Returns true if the image is meant to make use of additional convert options.
		def convert_options?
			not options[:convert_options].blank?
		end

	end
end