feat: add support for stripping alpha channel for vips image processor

That's how most operations of the GdkPixbuf-based image processor work when saving to jpeg.

- match-multiple-operations - MAE (Mean Average Error): 0.000500558
- match-multiple-operations-and-straighten - MAE (Mean Average Error): 0.0400593
MAE 0.04 comes from improving consistency in vips implementation. Pixbuf handled transparency
differently depending on whether straighten option was used (flattening alpha) or not (discarding alpha),
now it's always discarding. Long-term it feels that flattening is more desired, but I'm trying to avoid
changing too much at once.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Vips rotate
 - Vips straighten
 - Vips gamma
+- Vips stripping alpha
 
 ### Removed
 - [BREAKING] dropped support for a broken 'dominant' border colour

lib/morandi/vips_image_processor.rb

@@ -61,6 +61,8 @@ def process!
         @scale = 1.0
       end
 
+      strip_alpha!
+
       apply_gamma!
       apply_rotate!
       apply_crop!
@@ -88,6 +90,15 @@ def write_to_jpeg(target_path, quality = nil)
 
     private
 
+    # Remove the alpha channel if present. Vips supports alpha, but the current Pixbuf processor happens to strip it in
+    # most cases (straighten and cropping beyond image bounds are exceptions)
+    #
+    # Alternatively, alpha can be left intact for more accurate processing and transparent output or merged into an
+    # image using Vips::Image#flatten for less resource-intensive processing
+    def strip_alpha!
+      @img = @img.extract_band(0, n: @img.bands - 1) if @img.has_alpha?
+    end
+
     def apply_gamma!
       return unless @options['gamma'] && not_equal_to_one(@options['gamma'])
 

spec/morandi_spec.rb

@@ -433,7 +433,7 @@
       end
     end
 
-    context 'with transparent png input', vips_wip: processor_name == 'vips' do
+    context 'with transparent png input' do
       let(:file_in) { 'spec/fixtures/match-with-transparency.png' }
       let(:options) do
         {
@@ -448,7 +448,7 @@
 
         expect(File).to exist(file_out)
         expect(processed_image_type).to eq('jpeg')
-        expect(file_out).to match_reference_image('match-multiple-operations')
+        expect(file_out).to match_reference_image(reference_image_prefix, 'match-multiple-operations')
       end
 
       context 'with straighten option' do
@@ -460,7 +460,7 @@
 
           expect(File).to exist(file_out)
           expect(processed_image_type).to eq('jpeg')
-          expect(file_out).to match_reference_image('match-multiple-operations-and-straighten')
+          expect(file_out).to match_reference_image(reference_image_prefix, 'match-multiple-operations-and-straighten')
         end
       end
     end