Converting pdf to image using rmagick produces black background image

Question

I am using Active Storage to upload the pdf files and i need to convert it to image and save it as a attachment. Below is the code i used for this purpose, but it gives me the converted image with black background.

project_file.rb
class ProjectFile < ApplicationRecord
  has_many_attached: files
end

converted_image.rb
class ConvertedImage < ApplicationRecord
  has_one_attached: image
end

some_controller.rb
def show
  pdf = url_for(ProjectFile.last.files.first)
  PdfToImage.new(pdf).perform
end

pdf_to_image.rb
class PdfToImage
  require 'rmagick'
  require 'open-uri'

  attr_reader :pdf

  def initialize(pdf)
    @pdf = pdf
  end

  def perform
    jpg = Magick::ImageList.new(URI.open(pdf).path)

    file = Tempfile.new(['converted', '.jpg'])

    jpg.write(file.path)
    image = File.open(file)
    ConvertedImage.last.image.attach(io: image, filename: "converted.jpg", content_type: 'application/jpg')

    image.close
  end
end

Anyone has any solution to this? I am new to Rmagick.

ruby '2.6.5'

rails '6.0.1'

gem 'rmagick'

Answer 1

RMagick renders the alpha channel as black. If the alpha channel is removed the image background will render in white.

The alpha channel can be removed with the following snippet:

jpg = Magick::ImageList.new(URI.open(pdf).path)
jpg.each { |j| j.alpha(Magick::RemoveAlphaChannel) }