Add images.Opacity filter

Fixes #11471

resources/images/opacity.go

@@ -0,0 +1,39 @@
+// Copyright 2023 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package images
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ gift.Filter = (*opacityFilter)(nil)
+
+type opacityFilter struct {
+	opacity float32
+}
+
+func (f opacityFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	// 0 is fully transparent and 255 is opaque.
+	alpha := uint8(f.opacity * 255)
+	mask := image.NewUniform(color.Alpha{alpha})
+	draw.DrawMask(dst, dst.Bounds(), src, image.Point{}, mask, image.Point{}, draw.Over)
+}
+
+func (f opacityFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	return image.Rect(0, 0, srcBounds.Dx(), srcBounds.Dy())
+}