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: