A Paperclip mix-in to dynamically generate re-sized photos
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Attachment on the Fly

Attachment on the Fly is a module that extends Paperclip by allowing dynamic image resizing without going through ActiveRecord first. This gem can only resize images. If you try to resize a PDF or other attachment, nothing will happen.


  • thoughtbot-paperclip gem

  • ImageMagick

Quick Start

gem install attachment_on_the_fly

Setup your models the same way you normally do with Paperclip. See github.com/thoughtbot/paperclip for detailed instructions.

Once your attachments are working with Paperclip, add this line to the top of your models:

require “attachment_on_the_fly”

To use, reference the instance of your attachment like this:

<%= my_image_model.attachment.s_640_480 %>

This will output an image path based on your url interpolation and a resized image named S_640_480_your_image_name.jpg (or gif or png).


Attachment on the Fly can accept three types of inputs: width by height, height proportional, and width proportional.

my_image_model.attachment.s_640_480 will generate a 640x480 pixel image.

my_image_model.attachment.s_640_width will generate an image that is 640 pixels wide and proportionally high based on the original image dimensions.

my_image_model.attachment.s_480_height will generate an image that is 480 pixels high and proportionally wide based on the original image dimensions.

Parameters options

Attachment-on-the-fly uses 'Convert' from image-magick to resize image and you can pass any convert options in hash notation. my_image_model.attachment.s_800_600 :morph => 15


The only exception is :extension parameter my_image_model.attachment.s_400_300 :quality => 75, :extension => 'jpg' will generate an image that is 400x300 with quality 75% and convert jpg if necessary.

Note that the extension parameter will not trigger conversion to a different file type if the original file has an alpha channel (in order to avoid problems with, for example, PNG with alpha converted to JPEG). (Attachment-on-the-fly uses 'Identify' from image-magick to check for alpha before changing file type.)

When an image's instance is first accessed through the s_ method, a new image will be generated according to the image's path interpolation. Then, every time the image is accessed with the same dimensions, the previously generated image will be used. This allows the flexibility to resize images at will but still be able to cache previously generated images.

Currently, convert's sharpen is set to 1. If you need to change the sharpness or any other of convert's command line options, you will need to do so in the final lines of lib/attachment_on_the_fly.rb. We are working on a feature to do this dynamically, but it is not available yet.

Paperclip options

Attachment-on-the-fly refers to the following optional entries in Paperclip.options:

  • command_path (use to specify where to find “convert” binary)

  • extension (define a global default extension, can be overridden by parameters)

  • identify_path (use to specify where to find “identify” binary)

  • missing_image_path (default image to serve when original file is missing)

  • quality (define a global default quality, can be overridden by parameters)

  • version_prefix (inserted in name of generated files)

  • whiny (boolean, if true, raises an error when original file is missing instead of serving up image from missing_image_path)

Example: Paperclip.options = true


If you find this gem useful, please post your comments, bugs, patches and suggestions on our Github site at github.com/drpentode/Attachment-on-the-Fly.