Image blobs can have variants that are the result of a set of transformations applied to the original. These variants are used to create thumbnails, fixed-size avatars, or any other derivative image from the original.

Variants rely on ImageProcessing gem for the actual transformations of the file, so you must add gem "image_processing" to your Gemfile if you wish to use variants. By default, images will be processed with ImageMagick using the MiniMagick gem, but you can also switch to the libvips processor operated by the ruby-vips gem).

Rails.application.config.active_storage.variant_processor
# => :mini_magick

Rails.application.config.active_storage.variant_processor = :vips
# => :vips

Note that to create a variant it's necessary to download the entire blob file from the service. Because of this process, you also want to be considerate about when the variant is actually processed. You shouldn't be processing variants inline in a template, for example. Delay the processing to an on-demand controller, like the one provided in ActiveStorage::RepresentationsController.

To refer to such a delayed on-demand variant, simply link to the variant through the resolved route provided by Active Storage like so:

<%= image_tag Current.user.avatar.variant(resize_to_limit: [100, 100]) %>

This will create a URL for that specific blob with that specific variant, which the ActiveStorage::RepresentationsController can then produce on-demand.

When you do want to actually produce the variant needed, call processed. This will check that the variant has already been processed and uploaded to the service, and, if so, just return that. Otherwise it will perform the transformations, upload the variant to the service, and return itself again. Example:

avatar.variant(resize_to_limit: [100, 100]).processed.url

This will create and process a variant of the avatar blob that's constrained to a height and width of 100. Then it'll upload said variant to the service according to a derivative key of the blob and the transformations.

You can combine any number of ImageMagick/libvips operations into a variant, as well as any macros provided by the ImageProcessing gem (such as resize_to_limit):

avatar.variant(resize_to_limit: [800, 800], monochrome: true, rotate: "-90")

Visit the following links for a list of available ImageProcessing commands and ImageMagick/libvips operations:

Methods

Attributes

[R] blob
[R] variation

Class Public methods

new(blob, variation_or_variation_key)

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 58
  def initialize(blob, variation_or_variation_key)
    @blob, @variation = blob, ActiveStorage::Variation.wrap(variation_or_variation_key)
  end
🔎 See on GitHub

Instance Public methods

download(&block)

Downloads the file associated with this variant. If no block is given, the entire file is read into memory and returned. That'll use a lot of RAM for very large files. If a block is given, then the download is streamed and yielded in chunks.

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 87
  def download(&block)
    service.download key, &block
  end
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filename()

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 91
  def filename
    ActiveStorage::Filename.new "#{blob.filename.base}.#{variation.format.downcase}"
  end
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image()

Returns the receiving variant. Allows ActiveStorage::Variant and ActiveStorage::Preview instances to be used interchangeably.

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 102
  def image
    self
  end
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key()

Returns a combination key of the blob and the variation that together identifies a specific variant.

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 69
  def key
    "variants/#{blob.key}/#{Digest::SHA256.hexdigest(variation.key)}"
  end
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processed()

Returns the variant instance itself after it's been processed or an existing processing has been found on the service.

📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 63
  def processed
    process unless processed?
    self
  end
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service_url(expires_in: ActiveStorage.service_urls_expire_in, disposition: :inline)

Alias for: url

url(expires_in: ActiveStorage.service_urls_expire_in, disposition: :inline)

Returns the URL of the blob variant on the service. See {ActiveStorage::Blob#url} for details.

Use url_for(variant) (or the implied form, like +link_to variant+ or +redirect_to variant+) to get the stable URL for a variant that points to the ActiveStorage::RepresentationsController, which in turn will use this service_call method for its redirection.

Also aliased as: service_url
📝 Source code 
# File activestorage/app/models/active_storage/variant.rb, line 78
  def url(expires_in: ActiveStorage.service_urls_expire_in, disposition: :inline)
    service.url key, expires_in: expires_in, disposition: disposition, filename: filename, content_type: content_type
  end
🔎 See on GitHub