Generate PDF preview with Shrine

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I'm converting our backend file upload to work with Shrine. I managed to get the image upload and thumbnailing up and running pretty easily but I've been struggling to do the same with PDF files.

The upload itself works, however, I can't manage to generate a thumbnail/preview for the file. I'm using Shrine alongside ImageProcessing and vipslib.

I tried using the thumbnail method provided by vips but that seems to work just with image files and I also tried to follow this SO with no success.

Let me give you now some context:

This is the Shrine initializer

require "shrine"
require "shrine/storage/file_system"
require "shrine/storage/google_cloud_storage"


Shrine.storages = {
  cache: Shrine::Storage::GoogleCloudStorage.new(bucket: ENV['CACHE_BUCKET']),
  store: Shrine::Storage::GoogleCloudStorage.new(bucket: ENV['STORE_BUCKET'])
}

Shrine.plugin :activerecord
Shrine.plugin :cached_attachment_data # for retaining the cached file across form redisplays
Shrine.plugin :restore_cached_data # re-extract metadata when attaching a cached file
Shrine.plugin :determine_mime_type

And this is the Uploader

class DocumentUploader < Shrine
  require 'vips'

  def generate_location(io, context)
    "documents/#{Time.now.to_i}/#{super}"
  end


  plugin :processing
  # plugin :processing # allows hooking into promoting
  # plugin :versions   # enable Shrine to handle a hash of files
  # plugin :delete_raw # delete processed files after uploading
  # plugin :determine_mime_type
  #
  process(:store) do |io, context|
    preview = Tempfile.new(["shrine-pdf-preview", ".pdf"], binmode: true)
    begin
      IO.popen *%W[mutool draw -F png -o - #{io.path} 1], "rb" do |command|
        IO.copy_stream(command, preview)
      end
    rescue Errno::ENOENT
      fail "mutool is not installed"
    end

    preview.open # flush & rewind

    versions = { original: io }
    versions[:preview] = preview if preview && preview.size > 0
    versions
  end
end

As mentioned, the uploader, at the moment is what breaks and doesn't generate the preview. The previous version of the file looked like this:

class DocumentUploader < Shrine
  require 'vips'

  def generate_location(io, context)
    "documents/#{Time.now.to_i}/#{super}"
  end


  plugin :processing
  # plugin :processing # allows hooking into promoting
  # plugin :versions   # enable Shrine to handle a hash of files
  # plugin :delete_raw # delete processed files after uploading
  # plugin :determine_mime_type
  #
  process(:store) do |io, context|
    thumb = Vips::Image.thumbnail(io.metadata["filename"], 300)
    thumb
  end
end

I have seen very little documentation online about this topic.

Update: Answering questions

The command vips pdfload spits out the usage information and it indeed says that PDF would be loaded using libpoppler.

I installed the tar file straight from their download page and the version is 8.7.0 running on a Debian system.

Thanks about the license info - will look into that as well!

1 Answers

1
ilrock On

After hours of struggle, yesterday I finally got the thing to work.

The solution at the end was pretty simple. I used the versioning plugin offered by shrine and kept the original version in there.

class DocumentUploader < Shrine
  include ImageProcessing::Vips

  def generate_location(io, context)
    "documents/#{Time.now.to_i}/#{super}"
  end


  plugin :processing # allows hooking into promoting
  plugin :versions   # enable Shrine to handle a hash of files
  plugin :delete_raw # delete processed files after uploading


  process(:store) do |io, context|
    versions = { original: io } # retain original

    io.download do |original|
      pipeline = ImageProcessing::Vips.source(original)
      pipeline = pipeline.convert("jpeg").saver(interlace: true)
      versions[:large]  = pipeline.resize_to_limit!(800, 800)
      versions[:medium] = pipeline.resize_to_limit!(500, 500)
      versions[:small]  = pipeline.resize_to_limit!(300, 300)
    end

    versions # return the hash of processed files
  end
end