feat: add gamma support to vips image processor

The visual tests difference:
- plasma-gamma - MAE (Mean Average Error): 0.0105821
The vips image appears to have minimally higher contrast, best visible on purple pixels

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Vips crop
 - Vips rotate
 - Vips straighten
+- Vips gamma
 
 ### 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
 
+
+      apply_gamma!
       apply_rotate!
       apply_crop!
       apply_filters!
@@ -87,6 +89,12 @@ def write_to_jpeg(target_path, quality = nil)
 
     private
 
+    def apply_gamma!
+      return unless @options['gamma'] && not_equal_to_one(@options['gamma'])
+
+      @img = @img.gamma(exponent: @options['gamma'])
+    end
+
     def angle
       @options['angle'].to_i % 360
     end

spec/morandi_spec.rb

@@ -279,7 +279,7 @@
       end
     end
 
-    context 'when given a gamma option', vips_wip: processor_name == 'vips' do
+    context 'when given a gamma option' do
       let(:options) { { 'gamma' => 2.0 } }
 
       it 'should apply the gamma to the image' do
@@ -288,7 +288,7 @@
         expect(File).to exist(file_out)
         expect(processed_image_type).to eq('jpeg')
 
-        expect(file_out).to match_reference_image('plasma-gamma')
+        expect(file_out).to match_reference_image(reference_image_prefix, 'plasma-gamma')
       end
     end