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.

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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