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 MiniMagick for the
actual transformations of the file, so you must add
"mini_magick" to your Gemfile if you wish to use variants.
Note that to create a variant it's necessary to download the entire blob file from the service and load it into memory. The larger the image, the more memory is used. 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::VariantsController.
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 url_for(Current.user.avatar.variant(resize: "100x100")) %>
This will create a URL for that specific blob with that specific variant, which the ActiveStorage::VariantsController 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:
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.
A list of all possible transformations is available at www.imagemagick.org/script/mogrify.php. You can combine as many as you like freely:
avatar.variant(resize: "100x100", monochrome: true, flip: "-90")