Extracting PNG splitter strategy from IIIF Print

Related to:

- scientist-softserv/iiif_print#194
- samvera/bulkrax#760
- scientist-softserv/utk-hyku#343
- https://github.com/scientist-softserv/adventist-dl/issues/330

lib/space_stone/pdf_splitter/strategies.rb

@@ -10,4 +10,5 @@ module Strategies
 
 require 'space_stone/pdf_splitter/strategies/base'
 require 'space_stone/pdf_splitter/strategies/jpg'
+require 'space_stone/pdf_splitter/strategies/png'
 require 'space_stone/pdf_splitter/strategies/tiff'

lib/space_stone/pdf_splitter/strategies/png.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+module SpaceStone
+  module PdfSplitter
+    module Strategies
+      # The purpose of this class is to split the PDF into constituent png files.
+      class Png < Strategies::Base
+        self.image_extension = 'png'
+
+        def gsdevice
+          return @gsdevice if defined?(@gsdevice)
+
+          color = pdf_pages_summary.color_description
+          bits_per_channel = pdf_pages_summary.bits_per_channel
+          if color == 'gray'
+            # 1 Bit Grayscale, if applicable:
+            return @gsdevice = 'pngmonod' if bits_per_channel == 1
+            return @gsdevice = 'pnggray' if bits_per_channel > 1
+          end
+
+          @gsdevice = 'png16m'
+        end
+      end
+    end
+  end
+end

spec/space_stone/pdf_splitter/strategies/png_spec.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+require 'spec_helper'
+
+RSpec.describe SpaceStone::PdfSplitter::Strategies::Png do
+  let(:path) { __FILE__ }
+  let(:pdf_pages_summary) { double(SpaceStone::PdfSplitter::PdfPagesSummary) }
+
+  let(:splitter) { described_class.new(path, pdf_pages_summary: pdf_pages_summary) }
+
+  describe '.compression' do
+    subject { described_class.compression }
+    it { is_expected.to be_nil }
+  end
+
+  describe '.compression?' do
+    subject { described_class.compression? }
+    it { is_expected.to be_falsey }
+  end
+
+  describe '.image_extension' do
+    subject { described_class.image_extension }
+    it { is_expected.to eq('png') }
+  end
+
+  describe '#gsdevice' do
+    DEFAULT_SUMMARY_ATTRIBUTES = {
+      color_description: 'rgb',
+      bits_per_channel: 0
+    }.freeze
+
+    [
+      [{ color_description: 'gray', bits_per_channel: 2 }, 'pnggray'],
+      [{ color_description: 'gray', bits_per_channel: 1 }, 'pngmonod'],
+      [{ color_description: 'rgb', bits_per_channel: 1 }, 'png16m']
+    ].each do |attributes, expected_value|
+      context 'with #{attributes.inspect}' do
+        it "is expected to be #{expected_value.inspect}" do
+          summary = SpaceStone::PdfSplitter::PdfPagesSummary.new(**DEFAULT_SUMMARY_ATTRIBUTES.merge(attributes))
+          expect(described_class.new(__FILE__, pdf_pages_summary: summary).gsdevice).to eq(expected_value)
+        end
+      end
+    end
+  end
+end