Active Storage image variants can run on two different engines: MiniMagic/Imagemagic or ruby-vips/libvips which variant options have different syntax. The following will work for MiniMagic/Imagemagic setup:

tested on Rails Ruby 2.7.1 on 2020-09-29

class Medium < ApplicationRecord
  has_one_attached :image

Let say we upload an image

medium = Medium.create(params.require(:medium).permit(:image))

Here is the original image. Size is 1024x4246

medium.image.variant(resize: "400x300^", crop: '400x300+0+0')


Order of combine options does matter !!! If I swap resize and crop this will happen

medium.image.variant(crop: '400x300+0+0', resize: "400x300^")


If you want to center

medium.image.variant(:gravity=>"Center", resize: "400x300^", crop: '400x300+0+0')


If you need to rotate/auto-orient image:

medium.image.variant(auto_orient: true, rotate: 0, resize: "400x300^", crop: '400x300+0+0')

Need some blur ?

medium.image.variant(auto_orient: true, rotate: 0, resize: "400x300^", crop: '400x300+0+0', gaussian_blur: ['0x1.5'])


As For libvips yes you can do the same thing but I don’t have code to paste here. I’ll update this article once I have time to test it out.

combine_options is depreciated and will be removed in Rails 6.1

In Rails 6.1 :combine_options will not be supported anymore

Rails warn message:

DEPRECATION WARNING: Active Storage's ImageProcessing transformer doesn't support :combine_options, as it always generates a single ImageMagick command. Passing :combine_options will not be supported in Rails 6.1.

So this will no longer work in future:

# this is now depricated !!
medium.image.variant({:combine_options=>{:gravity=>"Center", :resize=>"400x300^", :crop=>"400x300+0+0" }})
# this is now depricated !!

How to debug this ?

One thing to realize is that it takes quite some time to get the options right. It’s a pain to always reload web-browser with the Active Storage blob variant endpoint.

What I do instead: I just look up how the images are generated in test environment looking them up with file manager.

note: no this NOT how to test generated images! This is how to debug right variant options

# app/models/medium.rb
class Medium < ApplicationRecord
  has_one_attached :image

  def thumb
    image.variant(auto_orient: true, rotate: 0, resize: "400x300^", crop: '400x300+0+0')
# spec/model/medium_spec.rb
require 'rails_helper'
RSpec.describe Medium, type: :model do
  it 'should not throw error' do
    medium = create :medium, :with_typed_image  # How to do FactoryBot traits with file attachement, check out:

    if medium.image.attached?
      puts "\n"

      k = medium.image.key.slice(0..3)
      folders = [k[0..1], k[2..4]].join('/')
      puts Rails.root.join('tmp', 'storage', folders).to_s
      # => "/home/myuser/name_of_my_rails_project/tmp/storage/ks/cl"

      variant = medium.thumb.processed  # `.processed` is important!
      ## ...or if you want to play with options inside test:
      # variant = medium.variant(auto_orient: true, rotate: 0, resize: "400x300^", crop: '400x300+0+0').processed

      puts Rails.root.join('tmp', 'storage', 'va', 'ri', variant.key).to_s
      # => /home/myuser/name_of_my_rails_project/tmp/storage/va/ri/variants/y0u5bhq109zzi1h3n8pa5v7p5f7u/500b88c14897d627f68fdd949ec32081f48593b82bdff4b4efed2fb1194d10d4

      binding.irb # to stop the execution as your project may be set up to wipe out images after test finish
      raise 'image was not attached?'

This way I’m able to open up the image easily with my file manager. E.g. in ubuntu I open up nautilus with

nautilus /home/myuser/name_of_my_rails_project/tmp/storage/va/ri/variants/y0u5bhq109zzi1h3n8pa5v7p5f7u/500b88c14897d627f68fdd949ec32081f48593b82bdff4b4efed2fb1194d10d4

I just check what was generated by my eye.

Once I’m done I just comment out this test.