Fix: add alpha premultiplication steps before and after resize (#302)

* Add alpha premultiplication steps before and after resize * Only premultiply if image has alpha * Fix transparent images not being resized, add test
rbuchberger · Feb 19, 2024 · f393067 · f393067
1 parent d3efe34
commit f393067
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diff --git a/lib/jekyll_picture_tag/images/image_file.rb b/lib/jekyll_picture_tag/images/image_file.rb
@@ -86,7 +86,13 @@ def notify
     end
 
     def resize(image)
-      image.resize(scale_value)
+      if image.has_alpha?
+        image = image.premultiply
+        image = image.resize(scale_value)
+        image.unpremultiply
+      else
+        image.resize(scale_value)
+      end
     end
 
     def crop(image)

diff --git a/test/image_files/pestka_transparent.png b/test/image_files/pestka_transparent.png
diff --git a/test/unit/images/test_image_file.rb b/test/unit/images/test_image_file.rb
@@ -129,6 +129,15 @@ def test_resize
     assert_equal(10, width)
   end
 
+  def test_resize_with_alpha
+    restub_source('pestka_transparent', 'png')
+    tested
+
+    width = Vips::Image.new_from_file(filename).width
+
+    assert_equal(10, width)
+  end
+
   def test_quality
     # We have to use slightly larger images for the quality to make a difference
     # in file size.