Thumbnail com crop circular

user.rb

    validates_attachment_presence :avatar
    validates_attachment_content_type :avatar, :content_type => ["image/png", "image/jpeg", "image/jpg"]
    validates_attachment_size :avatar, :less_than => 10.megabytes  
    has_attached_file :avatar, 
                      :styles => { 
                          :medium => "446x310>", 
#importante definir png como formato do thumb senão ele não aplica o crop se for jpg
                          :thumb => ["100x105#", :png],
                          :dashboard => "150X200>" },
                     :convert_options => {
                          :thumb => Proc.new{ self.convert_options }
                      }

#http://loo.no/2009/08/09/rounded-corners-with-paperclip
    def self.convert_options
        trans = ""
        px = 50
        trans << " \\( +clone  -threshold -1 "
        trans << "-draw 'fill black polygon 0,0 0,#{px} #{px},0 fill white circle #{px},#{px} #{px},0' "
        trans << "\\( +clone -flip \\) -compose Multiply -composite "
        trans << "\\( +clone -flop \\) -compose Multiply -composite "
        trans << "\\) +matte -compose CopyOpacity -composite "
    end