Use ImageProcessing gem for ActiveStorage variants

ImageProcessing gem is a wrapper around MiniMagick and ruby-vips, and
implements an interface for common image resizing and processing. This
is the canonical image processing gem recommended in [Shrine], and
that's where it developed from. The initial implementation was extracted
from Refile, which also implements on-the-fly transformations.

Some features that ImageProcessing gem adds on top of MiniMagick:

  * resizing macros
    - #resize_to_limit
    - #resize_to_fit
    - #resize_to_fill
    - #resize_and_pad
  * automatic orientation
  * automatic thumbnail sharpening
  * avoids the complex and inefficient MiniMagick::Image class
  * will use "magick" instead of "convert" on ImageMagick 7

However, the biggest feature of the ImageProcessing gem is that it has
an alternative implementation that uses libvips. Libvips is an
alternative to ImageMagick that can process images very rapidly (we've
seen up 10x faster than ImageMagick).

What's great is that the ImageProcessing gem provides the same interface
for both implementations. The macros are named the same, and the libvips
implementation does auto orientation and thumbnail sharpening as well;
only the operations/options specific to ImageMagick/libvips differ. The
integration provided by this PR should work for both implementations.

The plan is to introduce the ImageProcessing backend in Rails 6.0 as the
default backend and deprecate the MiniMagick backend, then in Rails 6.1
remove the MiniMagick backend.

1 files changed, 15 insertions, 6 deletions

diff --git a/guides/source/active_storage_overview.md b/guides/source/active_storage_overview.md
index d67f65e88a..f56db61538 100644
--- a/guides/source/active_storage_overview.md
+++ b/guides/source/active_storage_overview.md
@@ -337,14 +337,15 @@ rails_blob_path(user.avatar, disposition: "attachment")
 Transforming Images
 -------------------
 
-To create variation of the image, call `variant` on the Blob.
-You can pass any [MiniMagick](https://github.com/minimagick/minimagick)
-supported transformation to the method.
+To create variation of the image, call `variant` on the Blob. You can pass
+any transformation to the method supported by the procecssor. The default
+processor is [MiniMagick](https://github.com/minimagick/minimagick), but you
+can also use [Vips](http://www.rubydoc.info/gems/ruby-vips/Vips/Image).
 
-To enable variants, add `mini_magick` to your `Gemfile`:
+To enable variants, add `image_processing` gem to your `Gemfile`:
 
 ```ruby
-gem 'mini_magick'
+gem 'image_processing', '~> 1.2'
 ```
 
 When the browser hits the variant URL, Active Storage will lazy transform the
@@ -352,7 +353,15 @@ original blob into the format you specified and redirect to its new service
 location.
 
 ```erb
-<%= image_tag user.avatar.variant(resize: "100x100") %>
+<%= image_tag user.avatar.variant(resize_to_fit: [100, 100]) %>
+```
+
+To switch to the Vips processor, you would add the following to
+`config/application.rb`:
+
+```ruby
+# Use Vips for processing variants.
+config.active_storage.processor = :vips
 ```
 
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