Disable sharpening by default in vips backend

Sharpening by default sometimes produces worse results than without
sharpening. Even though newer versions of libvips improve upon this,
it's still better to prioritize correctness by default. So, we disable
sharpening by default, like we did with MiniMagick backend.

Closes #113

Author: janko

doc/vips.md

@@ -497,8 +497,16 @@ ImageProcessing::Vips
 
 ## Sharpening
 
-All `#resize_*` operations will automatically sharpen the resulting thumbnails
-after resizing, using the following [convolution mask]:
+With `#resize_*` operators, you can additional sharpen the thumbnails via the
+`:sharpen` option:
+
+```rb
+ImageProcessing::Vips
+  .source(image)
+  .resize_to_limit!(400, 400, sharpen: true)
+```
+
+This will use the following [convolution mask] by default:
 
 ```rb
 Vips::Image.new_from_array [
@@ -507,7 +515,7 @@ Vips::Image.new_from_array [
   [-1, -1, -1]], 24
 ```
 
-You can assign a different convolution mask via the `:sharpen` option:
+You can also assign your own convolution mask:
 
 ```rb
 sharpen_mask = Vips::Image.new_from_array [
@@ -520,14 +528,6 @@ ImageProcessing::Vips
   .resize_to_limit!(400, 400, sharpen: sharpen_mask)
 ```
 
-You can disable automatic sharpening by setting `:sharpen` to `false`:
-
-```rb
-ImageProcessing::Vips
-  .source(image)
-  .resize_to_limit!(400, 400, sharpen: false)
-```
-
 [ruby-vips]: https://github.com/libvips/ruby-vips
 [libvips]: https://github.com/libvips/libvips
 [installation instructions]: https://github.com/libvips/libvips/wiki#building-and-installing

lib/image_processing/vips.rb

@@ -155,7 +155,7 @@ def remove(*args)    image.tap { |img| img.remove(*args) }    end
 
       # Resizes the image according to the specified parameters, and sharpens
       # the resulting thumbnail.
-      def thumbnail(width, height, sharpen: SHARPEN_MASK, **options)
+      def thumbnail(width, height, sharpen: nil, **options)
         if self.image.is_a?(String) # path
           # resize on load
           image = ::Vips::Image.thumbnail(self.image, width, height: height, **options)
@@ -167,7 +167,11 @@ def thumbnail(width, height, sharpen: SHARPEN_MASK, **options)
           image = self.image.thumbnail_image(width, height: height, **options)
         end
 
-        image = image.conv(sharpen, precision: :integer) if sharpen
+        if sharpen
+          sharpen_mask = sharpen.is_a?(TrueClass) ? SHARPEN_MASK : sharpen
+          image = image.conv(sharpen_mask, precision: :integer)
+        end
+
         image
       end
 

test/vips_test.rb

@@ -181,7 +181,7 @@
     end
 
     it "sharpening uses integer precision" do
-      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      sharpened = @pipeline.resize_to_limit(400, 400, sharpen: true).call(save: false)
       assert_equal :uchar, sharpened.format
     end
   end
@@ -229,7 +229,7 @@
     end
 
     it "sharpening uses integer precision" do
-      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      sharpened = @pipeline.resize_to_limit(400, 400, sharpen: true).call(save: false)
       assert_equal :uchar, sharpened.format
     end
   end
@@ -265,7 +265,7 @@
     end
 
     it "sharpening uses integer precision" do
-      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      sharpened = @pipeline.resize_to_limit(400, 400, sharpen: true).call(save: false)
       assert_equal :uchar, sharpened.format
     end
   end
@@ -319,7 +319,7 @@
     end
 
     it "sharpening uses integer precision" do
-      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      sharpened = @pipeline.resize_to_limit(400, 400, sharpen: true).call(save: false)
       assert_equal :uchar, sharpened.format
     end
   end
@@ -386,7 +386,7 @@
     end
 
     it "sharpening uses integer precision" do
-      sharpened = @portrait_pipeline.resize_to_cover(400, 400).call(save: false)
+      sharpened = @portrait_pipeline.resize_to_cover(400, 400, sharpen: true).call(save: false)
       assert_equal :uchar, sharpened.format
     end