Merge pull request #2666 from tvdeyen/preserve-transparent-bg

Preserve transparent backgrounds for PNGs

app/models/alchemy/picture_variant.rb

@@ -100,6 +100,7 @@ def encoded_image(image, options = {})
       end
 
       if options[:flatten]
+        encoding_options << "-background transparent" if render_format == "png"
         encoding_options << "-flatten"
       end
 

config/initializers/dragonfly.rb

@@ -16,6 +16,7 @@
 
 # Dragonfly 1.4.0 only allows `quality` as argument to `encode`
 Dragonfly::ImageMagick::Processors::Encode::WHITELISTED_ARGS << "flatten"
+Dragonfly::ImageMagick::Processors::Encode::WHITELISTED_ARGS << "background"
 
 Rails.application.config.after_initialize do
   Dragonfly.app(:alchemy_pictures).add_processor(:crop_resize, Alchemy::Dragonfly::Processors::CropResize.new)

spec/models/alchemy/picture_variant_spec.rb

@@ -178,7 +178,7 @@
       it "flattens the image." do
         step = subject.steps[0]
         expect(step.name).to eq(:encode)
-        expect(step.arguments).to eq(["png", "-flatten"])
+        expect(step.arguments).to eq(["png", "-background transparent -flatten"])
       end
 
       context "converted to webp" do