test: add specs for testing greyscale processing

Pixbuf happens to convert everything to RGB, but other processors are not guaranteed to have such property,
thus the additional specs.

I've also added a new matcher to make colourspace check easy.

spec/colour_helper.rb

@@ -9,6 +9,10 @@ def generate_test_image_plasma_checkers(at_file_path, width: 600, height: 300)
     generate_test_image(at_file_path, fill: fill, width: width, height: height)
   end
 
+  def generate_test_image_greyscale(at_file_path, width: 600, height: 300)
+    generate_test_image(at_file_path, fill: 'gradient:white-black', width: width, height: height)
+  end
+
   def generate_test_image(at_file_path, fill:, width: 600, height: 300)
     fill = Array(fill).flatten
 

spec/morandi_spec.rb

@@ -17,6 +17,9 @@
   let(:processed_image_type) { processed_image_info[0].name }
   let(:processed_image_width) { processed_image_info[1] }
   let(:processed_image_height) { processed_image_info[2] }
+  let(:generate_image) do
+    generate_test_image_plasma_checkers(file_in, width: original_image_width, height: original_image_height)
+  end
 
   before(:all) do
     FileUtils.mkdir_p('sample')
@@ -29,7 +32,7 @@
   before do
     next if File.exist?(file_in)
 
-    generate_test_image_plasma_checkers(file_in, width: original_image_width, height: original_image_height)
+    generate_image
   end
 
   after do |ex|
@@ -456,6 +459,31 @@
         end
       end
     end
+
+    context 'with a non-rgb image' do
+      let(:generate_image) do
+        generate_test_image_greyscale(file_in, width: original_image_width, height: original_image_height)
+      end
+
+      it 'changes greyscale image to srgb' do
+        expect(file_in).to match_colourspace('gray') # Testing a setup to protect from a hidden regression
+        process_image
+
+        expect(file_out).to match_colourspace('srgb')
+      end
+
+      # Colour filters implementation operates on RGB-based constants, thus a dedicated test
+      context 'with colour filter' do
+        let(:options) { super().merge('fx' => 'sepia') }
+
+        it 'creates a valid, srgb image' do
+          process_image
+
+          expect(file_out).to match_reference_image('greyscale-with-sepia')
+          expect(file_out).to match_colourspace('srgb')
+        end
+      end
+    end
   end
 
   context 'pixbuf processor' do

spec/spec_helper.rb

@@ -19,7 +19,8 @@
 require 'super_diff/rspec'
 require 'pry'
 
-require 'support/match_reference_image'
+Dir['spec/support/**/*.rb'].each { |f| require "./#{f}" }
+
 require_relative 'visual_report_helper'
 require_relative 'colour_helper'
 

spec/support/match_colourspace.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require 'open3'
+
+# For testing that under a given path resides an image with desired colourspace.
+# The colourspace is currently extracted using imagemagick's `identify`, resulting in values like 'gray' or 'srgb'
+# According to docs (https://www.imagemagick.org/script/escape.php), it may include number of channels and meta channels
+RSpec::Matchers.define :match_colourspace do |expected_colourspace|
+  match do |tested_path|
+    raise(ArgumentError, "path #{tested_path} is not a file") unless File.file?(tested_path)
+
+    @colourspace, status = Open3.capture2('identify', '-format', '%[channels]', tested_path)
+    raise "Failed to read colorspace of #{tested_path}" unless status.success?
+
+    @colourspace == expected_colourspace
+  end
+
+  failure_message do
+    "Colourspaces don't match. Expected: #{expected_colourspace}, got: #{@colourspace}"
+  end
+end