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Common use cases for thumbnails are pretty easy with Paperclip. You define your preferred sizes in your initial has_attached_file call:

class User < ActiveRecord::Base
  # Paperclip
  has_attached_file :photo,
    :styles => {
      :thumb => "100x100#",
      :small  => "150x150>",
      :medium => "200x200" }
end

Then you use them like this:

<%= image_tag @user.photo.url %>
<%= image_tag @user.photo.url(:thumb) %>
<%= image_tag @user.photo.url(:small) %>

:styles also accepts a lambda, enabling you to access the instance being operated on:

class User < ActiveRecord::Base
  # Paperclip
  has_attached_file :photo,
    :styles => lambda { |a|
                        { :thumb => "100x100#",
                          :user_defined => "#{a.instance.width}x#{a.instance.height}#" }
                      }
end

Resizing options

Default behavior is to resize the image and maintain aspect ratio (i.e. the :medium version of a 300×150 image will be 200×100). Some commonly used options are:

  • trailing #, thumbnail will be centrally cropped, ensuring the requested dimensions.
  • trailing >, thumbnail will only be modified if it is currently larger requested dimensions. (i.e. the :small thumb for a 120×80 original image will be unchanged)

You can see a complete list of ImageMagick options for more.

Optimizing thumbnails for the web

If you’re creating thumbnails for heavy images, you can greatly reduce sizes for your thumbnails by stripping EXIF data and controlling compression options.

Check out the -strip, -quality and -thumbnail options from ImageMagick’s options. You can expect thumbnails to be 5-10x lighter in size.

Here’s an example on how to apply these flags to your thumbnails:

# Paperclip
has_attached_file :photo,
  :styles => {
    :thumb => "100x100#" },
  :convert_options => {
    :thumb => "-quality 75 -strip" }

Generating/regenerating your thumbnails

You can (re)generate your thumbnails en masse with Paperclip’s rake tasks. Using our example class above:

rake paperclip:refresh:thumbnails CLASS=User

or to refresh all of your defined styles in one go (:thumb, :small, :medium from the above example)

rake paperclip:refresh CLASS=User

and to refresh only missing styles:
a list of styles will be defined or updated in a file “/public/system/paperclip_attachments.yml”

rake paperclip:refresh:missing_styles

If you are using the gem required version of paperclip the rake tasks may not be auto-loaded, but you can copy them into your lib/tasks directory if needed.

If you need more manual control or have a lot of thumbnails and only want to process a few, you can use #reprocess! like so:

users_to_reprocess.each do |user|
  user.photo.reprocess!
end

Additionally, if you want to only reprocess a single style you may do so like:

users_to_reprocess.each do |user|
  user.photo.reprocess! :medium
end

Prevent thumbnail generation for invalid uploads

If you upload a file that is too large or otherwise doesn’t fit the requirements, paperclip still processes all styles during assignment before the model object is recognized as invalid during #save. This can be a huge waste of time if you’re working with big files and/or many styles.

You can use a before_post_process filter to avoid post processing. For example, if your attribute is named image, and you have a restriction on the file size, the filter could look like this:

before_post_process :check_file_size
def check_file_size
  valid?
  errors[:image_file_size].blank?
end
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