feat: strip alpha channel in vips image processor

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

@@ -66,10 +66,12 @@ def process!
       apply_crop!
       apply_filters!
 
-      return unless @options['output.limit'] && @output_width && @output_height
+      if @options['output.limit'] && @output_width && @output_height
+        scale_factor = [@output_width, @output_height].max.to_f / [@img.width, @img.height].max
+        @img = @img.resize(scale_factor) if scale_factor < 1.0
+      end
 
-      scale_factor = [@output_width, @output_height].max.to_f / [@img.width, @img.height].max
-      @img = @img.resize(scale_factor) if scale_factor < 1.0
+      strip_alpha!
     end
 
     def write_to_png(_write_to, _orientation = :any)
@@ -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