When a paperclip attachment gains a new style and you have many attachments, reprocessing can take ages. This is because all styles are being recomputed.
To create only missing images, patch Paperclip like this in your script that does the reprocessing:
Paperclip <2.3.2
class Paperclip::Attachment
  private
  def post_process_styles_with_extreme_lazyness
    @old_styles = @styles
    @styles = @styles.reject do |name, _|
      exists?(name)
    end
    post_process_styles_without_extreme_lazyness
  ensure
    @styles = @old_styles
  end
  alias_method_chain :post_process_styles, :extreme_lazyness
end
Paperclip >= 2.3.2 (untested):
class Paperclip::Attachment
  private
  def post_process_styles_with_extreme_lazyness
    styles
    @old_styles = @normalized_styles
    @normalized_styles = @normalized_styles.reject do |name, _|
      exists?(name)
    end
    post_process_styles_without_extreme_lazyness
  ensure
    @normalized_styles = @old_styles
  end
  alias_method_chain :post_process_styles, :extreme_lazyness
end
Posted by Tobias Kraze to makandra dev (2010-08-25 14:03)