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, 54 insertions, 18 deletions

diff --git a/activestorage/app/models/active_storage/variation.rb b/activestorage/app/models/active_storage/variation.rb
index 12e7f9f0b5..3bdbc5bacb 100644
--- a/activestorage/app/models/active_storage/variation.rb
+++ b/activestorage/app/models/active_storage/variation.rb
@@ -8,15 +8,7 @@
 #
 #   ActiveStorage::Variation.new(resize: "100x100", monochrome: true, trim: true, rotate: "-90")
 #
-# You can also combine multiple transformations in one step, e.g. for center-weighted cropping:
-#
-#   ActiveStorage::Variation.new(combine_options: {
-#     resize: "100x100^",
-#     gravity: "center",
-#     crop: "100x100+0+0",
-#   })
-#
-# A list of all possible transformations is available at https://www.imagemagick.org/script/mogrify.php.
+# The options map directly to {ImageProcessing}[https://github.com/janko-m/image_processing] commands.
 class ActiveStorage::Variation
   attr_reader :transformations
 
@@ -51,10 +43,49 @@ class ActiveStorage::Variation
     @transformations = transformations
   end
 
-  # Accepts an open MiniMagick image instance, like what's returned by <tt>MiniMagick::Image.read(io)</tt>,
-  # and performs the +transformations+ against it. The transformed image instance is then returned.
-  def transform(image)
+  # Accepts a File object, performs the +transformations+ against it, and
+  # saves the transformed image into a temporary file. If +format+ is specified
+  # it will be the format of the result image, otherwise the result image
+  # retains the source format.
+  def transform(file, format: nil)
     ActiveSupport::Notifications.instrument("transform.active_storage") do
+      if processor
+        image_processing_transform(file, format)
+      else
+        mini_magick_transform(file, format)
+      end
+    end
+  end
+
+  # Returns a signed key for all the +transformations+ that this variation was instantiated with.
+  def key
+    self.class.encode(transformations)
+  end
+
+  private
+    # Applies image transformations using the ImageProcessing gem.
+    def image_processing_transform(file, format)
+      operations = transformations.inject([]) do |list, (name, argument)|
+        if name.to_s == "combine_options"
+          ActiveSupport::Deprecation.warn("The ImageProcessing ActiveStorage variant backend doesn't need :combine_options, as it already generates a single MiniMagick command. In Rails 6.1 :combine_options will not be supported anymore.")
+          list.concat argument.to_a
+        else
+          list << [name, argument]
+        end
+      end
+
+      processor
+        .source(file)
+        .loader(page: 0)
+        .convert(format)
+        .apply(operations)
+        .call
+    end
+
+    # Applies image transformations using the MiniMagick gem.
+    def mini_magick_transform(file, format)
+      image = MiniMagick::Image.new(file.path, file)
+
       transformations.each do |name, argument_or_subtransformations|
         image.mogrify do |command|
           if name.to_s == "combine_options"
@@ -66,15 +97,20 @@ class ActiveStorage::Variation
           end
         end
       end
+
+      image.format(format) if format
+
+      image.tempfile.tap(&:open)
     end
-  end
 
-  # Returns a signed key for all the +transformations+ that this variation was instantiated with.
-  def key
-    self.class.encode(transformations)
-  end
+    # Returns the ImageProcessing processor class specified by `ActiveStorage.processor`.
+    def processor
+      require "image_processing"
+      ImageProcessing.const_get(ActiveStorage.processor.to_s.camelize) if ActiveStorage.processor
+    rescue LoadError
+      ActiveSupport::Deprecation.warn("Using mini_magick gem directly is deprecated and will be removed in Rails 6.1. Please add `gem 'image_processing', '~> 1.2'` to your Gemfile.")
+    end
 
-  private
     def pass_transform_argument(command, method, argument)
       if eligible_argument?(argument)
         command.public_send(method, argument)