active_storage_watermark_image_with_scale.rb

module ImageProcessing
  module MiniMagick
    module Processing
      extend ActiveSupport::Concern

      included do
        def watermark(scale: 0.15)
          # ActiveStorage custom variant reference:
          # https://stackoverflow.com/questions/71832729/how-do-i-create-a-non-trivial-activestorage-variant-for-watermarking-images
          # https://round-tractor.medium.com/custom-image-variants-with-rails-active-storage-88d84ea007bc
          #
          # IM7 watermark solution: https://stackoverflow.com/questions/71599820/imagemagick-add-watermark-proportionally
          #
          # Watermark using IM6 syntax
          # (We can only use IM6 syntax because the image_processing gem does not support mini_magick 5 yet)
          # (https://github.com/janko/image_processing/issues/125)
          # https://imagemagick.org/discourse-server/viewtopic.php?t=34883
          #
          # Couldn't find a way to offset the watermark in the original image by percentage.
          # The geometry option can only accept pixels.
          #
          # Padding the watermark image instead would work, since the paddings will scale together with the
          # watermark image during "distort".
          #
          # the watermark3x-padded image is the watermark3x image with padding 40
          # https://onlinepngtools.com/add-padding-to-png
          watermark_path = Rails.root.join('public', 'watermark3x-padded.png')

          # magick convert car-sample.jpg public/watermark3x-padded.png \
          # +distort affine "0,0 0,0 %[w],%[h] %[fx:t?v.w*(u.h/v.h*0.15):s.w],%[fx:t?v.h*(u.h/v.h*0.15):s.h]" \
          # -shave 1 -gravity northeast -geometry +15+15 -composite output.png
          magick << watermark_path
          magick << "+distort"
          magick << "affine"
          magick << "0,0 0,0 %[w],%[h] %[fx:t?v.w*(u.h/v.h*#{scale}):s.w],%[fx:t?v.h*(u.h/v.h*#{scale}):s.h]"

          magick.shave(1)
          magick.gravity('northeast')
          magick.geometry('+15+15')
          magick.composite
        end
      end
    end
  end
end

ImageProcessing::MiniMagick::Processor.include(
  ImageProcessing::MiniMagick::Processing
)

# in model
class Listing < ApplicationRecord
  has_one_attached :image do |img|
    img.variant :web, watermark: { scale: 0.1 }
  end
  
  after_commit :process_image

  def process_image
    if self.image.attached?
      self.image.variant(:web).processed
    end
  end
end