Rounded corners with Paperclip

问题

How do you create rounded corners with Paperclip? I have found this solution which creates rounded corners with paperclip using convert_options, but it does not work with Rails 3 and Paperclip 2.4.5. The generated convert command works only if I use the ImageMagick alpha parameter instead of the threshold paramter:

  convert example.jpg \
     \( +clone  -alpha extract \
        -draw 'fill black polygon 0,0 0,15 15,0 fill white circle 15,15 15,0' \
        \( +clone -flip \) -compose Multiply -composite \
        \( +clone -flop \) -compose Multiply -composite \
     \) -alpha off -compose CopyOpacity -composite  rounded_corners.png

which corresponds to

has_attached_file :avatar,
                  :styles => { :medium => ["918x483#", :png] },
                  :convert_options => {:medium => Proc.new{self.convert_options}}

def self.convert_options(px = 15)
  trans = ""
  trans << " \\( +clone  -alpha extract "
  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 << "\\) +alpha off -compose CopyOpacity -composite "
end

This code snippet seems to produce the right convert command, but gives an "error processing the thumbnail for stream-xyz".

Command :: convert '/tmp/stream20120109-15817-1lju7p6-0.jpg[0]' -resize "918x" 
  -crop "918x483+0+105" +repage \( +clone -alpha extract -draw 'fill black polygon 
  0,0 0,15 15,0 fill white circle 15,15 15,0' \( +clone -flip \) -compose 
  Multiply -composite \( +clone -flop \) -compose Multiply -composite \) +alpha off
  -compose CopyOpacity -composite '/tmp/stream20120109-15817-1lju7p6-....png'
[paperclip] An error was received while processing: #<Paperclip::PaperclipError: 
There was an error processing the thumbnail for stream20120109-15817-1lju7p6-0>

回答1:

Finally I found it out, it was a simple typo. You have to use -alpha off instead of +alpha off, then the code above works without error. Sometimes a single sign (- instead of + or vice versa) makes the difference.

来源：https://stackoverflow.com/questions/8789820/rounded-corners-with-paperclip