Use Int Precision w/Vips During Sharpen (#56)

Work in #22 added automatic image sharpening for Vips. However, discovered in #55 `conv()` defaults to float precision and can leave to visual artifacts in some scenarios. Per @jcupitt recommendation, we can add `precision: :integer` to conv and this should also give us a speed-up, since int convolutions will use SIMD. #55

Fixes #55

lib/image_processing/vips.rb

@@ -140,7 +140,7 @@ def thumbnail(width, height, sharpen: SHARPEN_MASK, **options)
         else
           image = self.image.thumbnail_image(width, height: height, **options)
         end
-        image = image.conv(sharpen) if sharpen
+        image = image.conv(sharpen, precision: :integer) if sharpen
         image
       end
 

test/vips_test.rb

@@ -174,6 +174,11 @@
       normal    = @pipeline.resize_to_limit!(400, 400, sharpen: false)
       assert sharpened.size > normal.size, "Expected sharpened thumbnail to have bigger filesize than not sharpened thumbnail"
     end
+
+    it "sharpening uses integer precision" do
+      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      assert_equal :uchar, sharpened.format
+    end
   end
 
   describe "#resize_to_fit" do
@@ -217,6 +222,11 @@
       normal    = @pipeline.resize_to_fit!(400, 400, sharpen: false)
       assert sharpened.size > normal.size, "Expected sharpened thumbnail to have bigger filesize than not sharpened thumbnail"
     end
+
+    it "sharpening uses integer precision" do
+      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      assert_equal :uchar, sharpened.format
+    end
   end
 
   describe "#resize_to_fill" do
@@ -248,6 +258,11 @@
       normal   = @pipeline.resize_to_fill!(400, 400, sharpen: false)
       assert sharpened.size > normal.size, "Expected sharpened thumbnail to have bigger filesize than not sharpened thumbnail"
     end
+
+    it "sharpening uses integer precision" do
+      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      assert_equal :uchar, sharpened.format
+    end
   end
 
   describe "#resize_and_pad" do
@@ -297,6 +312,11 @@
       normal    = @pipeline.resize_and_pad!(400, 400, sharpen: false)
       assert sharpened.size > normal.size, "Expected sharpened thumbnail to have bigger filesize than not sharpened thumbnail"
     end
+
+    it "sharpening uses integer precision" do
+      sharpened = @pipeline.resize_to_limit(400, 400).call(save: false)
+      assert_equal :uchar, sharpened.format
+    end
   end
 
   describe "#rotate" do