feat: add colour filters support for vips image processor

Notes:
- the filter matrices are designed for RGB, so the image needs to use that colourspace
- the recomb call effectively converts image to grayscale; there are potentially more elegant
  and efficient ways to achieve similar result (eg img.colourspace('b-w')); this solution was chosen to preserve
  the same grayscale formula as used in GdkPixbuf-based processor

CHANGELOG.md

@@ -6,7 +6,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## UNRELEASED
 ### Added
-- Vips image processor
+- Vips image processor (resizing)
+- Vips colour filters
 
 ### Removed
 - [BREAKING] dropped support for a broken 'dominant' border colour

lib/morandi/vips_image_processor.rb

@@ -7,6 +7,16 @@
 module Morandi
   # An alternative to ImageProcessor which is based on libvips for concurrent and less memory-intensive processing
   class VipsImageProcessor
+    # Colour filter related constants
+    RGB_LUMINANCE_EXTRACTION_FACTORS = [0.3086, 0.6094, 0.0820].freeze
+    SEPIA_MODIFIER = [25, 5, -25].freeze
+    BLUETONE_MODIFIER = [-10, 5, 25].freeze
+    COLOUR_FILTER_MODIFIERS = {
+      'sepia' => SEPIA_MODIFIER,
+      'bluetone' => BLUETONE_MODIFIER
+    }.freeze
+    SUPPORTED_FILTERS = COLOUR_FILTER_MODIFIERS.keys + ['greyscale']
+
     # Vips options are global, this method sets them for yielding, then restores to original
     def self.with_global_options(cache_max:, concurrency:)
       previous_cache_max = Vips.cache_max
@@ -50,6 +60,8 @@ def process!
         @scale = 1.0
       end
 
+      apply_filters!
+
       return unless @options['output.limit'] && @output_width && @output_height
 
       scale_factor = [@output_width, @output_height].max.to_f / [@img.width, @img.height].max
@@ -70,6 +82,32 @@ def write_to_jpeg(write_to, quality = nil)
 
     private
 
+    def apply_filters!
+      filter_name = @options['fx']
+      return unless SUPPORTED_FILTERS.include?(filter_name)
+
+      # The filter-related constants assume RGB colourspace, so it requires conversion
+      @img = @img.colourspace(:srgb) unless @img.interpretation == :srgb
+
+      # Convert to greyscale using weights
+      rgb_factors = RGB_LUMINANCE_EXTRACTION_FACTORS
+      recombination_matrix = [rgb_factors, rgb_factors, rgb_factors]
+      if @img.has_alpha?
+        # Add "0" multiplier for alpha to ignore it for luminance calculation
+        recombination_matrix = recombination_matrix.map { |channel_multipliers| channel_multipliers + [0] }
+        # Add fourth row in the matrix to preserve unchanged alpha channel
+        recombination_matrix << [0, 0, 0, 1]
+      end
+      @img = @img.recomb(recombination_matrix)
+
+      return unless COLOUR_FILTER_MODIFIERS[filter_name]
+
+      # Apply colour adjustment based on the modifiers setup
+      colour_filter_modifier = COLOUR_FILTER_MODIFIERS[filter_name]
+      colour_filter_modifier += [0] if @img.has_alpha?
+      @img = @img.linear(1.0, colour_filter_modifier)
+    end
+
     def not_equal_to_one(float)
       (float - 1.0).abs >= Float::EPSILON
     end

spec/morandi_spec.rb

@@ -292,7 +292,7 @@
       end
     end
 
-    describe 'when given an fx option', vips_wip: processor_name == 'vips' do
+    describe 'when given an fx option' do
       let(:options) { { 'fx' => filter_name } }
 
       context 'with sepia' do
@@ -303,7 +303,7 @@
 
           expect(File).to exist(file_out)
           expect(processed_image_type).to eq('jpeg')
-          expect(file_out).to match_reference_image('plasma-sepia')
+          expect(file_out).to match_reference_image(reference_image_prefix, 'plasma-sepia')
         end
       end
 
@@ -315,7 +315,7 @@
 
           expect(File).to exist(file_out)
           expect(processed_image_type).to eq('jpeg')
-          expect(file_out).to match_reference_image('plasma-bluetone')
+          expect(file_out).to match_reference_image(reference_image_prefix, 'plasma-bluetone')
         end
       end
 
@@ -327,7 +327,7 @@
 
           expect(File).to exist(file_out)
           expect(processed_image_type).to eq('jpeg')
-          expect(file_out).to match_reference_image('plasma-greyscale')
+          expect(file_out).to match_reference_image(reference_image_prefix, 'plasma-greyscale')
         end
       end
     end
@@ -465,12 +465,12 @@
       end
     end
 
-    context 'with a non-rgb image', vips_wip: processor_name == 'vips' do
+    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
+      it 'changes greyscale image to srgb', vips_wip: processor_name == 'vips' do
         expect(file_in).to match_colourspace('gray') # Testing a setup to protect from a hidden regression
         process_image
 
@@ -484,7 +484,7 @@
         it 'creates a valid, srgb image' do
           process_image
 
-          expect(file_out).to match_reference_image('greyscale-with-sepia')
+          expect(file_out).to match_reference_image(reference_image_prefix, 'greyscale-with-sepia')
           expect(file_out).to match_colourspace('srgb')
         end
       end