feat: ensure that output image is in srgb colourspace

That is a default behaviour for GdkPixbuf, so mirroring it in vips for better compatibility

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Vips gamma
 - Vips stripping alpha
 - Explicit error when trying to use VipsProcessor with unsupported options
+- Vips cast to srgb when image uses a different colourspace
 
 ### Removed
 - [BREAKING] dropped support for a broken 'dominant' border colour

lib/morandi/vips_image_processor.rb

@@ -84,6 +84,7 @@ def process!
       end
 
       strip_alpha!
+      ensure_srgb!
     end
 
     def write_to_png(_write_to, _orientation = :any)
@@ -160,8 +161,8 @@ 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
+      # The filter-related constants assume RGB colourspace, so it requires early conversion
+      ensure_srgb!
 
       # Convert to greyscale using weights
       rgb_factors = RGB_LUMINANCE_EXTRACTION_FACTORS
@@ -185,5 +186,9 @@ def apply_filters!
     def not_equal_to_one(float)
       (float - 1.0).abs >= Float::EPSILON
     end
+
+    def ensure_srgb!
+      @img = @img.colourspace(:srgb) unless @img.interpretation == :srgb
+    end
   end
 end

spec/morandi_spec.rb

@@ -470,7 +470,7 @@
         generate_test_image_greyscale(file_in, width: original_image_width, height: original_image_height)
       end
 
-      it 'changes greyscale image to srgb', vips_wip: processor_name == 'vips' do
+      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