Rough sketch of Active Storage calculated variants

active_storage.rb

Rails.application.config.to_prepare do
  ActiveStorage::Attachment.class_eval do
    private

    def transform_variants_later
      if record.respond_to?(:calculated_variants)
        record.calculated_variants[name.to_sym]&.values.each do |transformations|
          blob.preprocessed(transformations)
        end
      end

      named_variants.each do |_name, named_variant|
        blob.preprocessed(named_variant.transformations) if named_variant.preprocessed?(record)
      end
    end
  end
end

custom_processing.rb

require 'image_processing'

module ImageProcessing::Vips::CustomProcessing
  extend ActiveSupport::Concern

  included do
    def lighten(percentage)
      overlay = image.new_from_image([255, 255, 255, (255 * percentage).to_i])
      image.composite(overlay, :over)
    end
  end
end

document.rb

class Document < ApplicationRecord
  has_one_attached :watermark
  
  after_save_commit :transform_watermark_print_variant_later, if: ->(document) { document.watermark_opacity_previously_changed? }
  
  def calculated_variants
    {
      watermark: {
        print: { resize_to_limit: [1_000, 1_000], lighten: lighten_value }
      }
    }
  end

  def lighten_value
    return 0 if watermark_opacity < 0

    [watermark_opacity.to_f / 100.0, 1].min
  end

  def transform_watermark_print_variant_later
    ActiveStorage::TransformJob.perform_later(watermark.blob, calculated_variants.dig(:watermark, :print))
  end
end

Part of discussion here: https://x.com/jeremysmithco/status/1821642513356300514