images: Add images.Overlay filter

This allows for constructs ala:

```
{{ $overlay := $img.Filter (images.Overlay $logo 50 50 )}}
```
Or:

```
{{ $logoFilter := (images.Overlay $logo 50 50 ) }}
{{ $overlay := $img | images.Filter $logoFilter }}
```

Which will overlay the logo in the top left corner (x=50, y=50) of `$img`.

Fixes #8057
Fixes #4595
Updates #6731

11 files changed, 77 insertions, 2 deletions

diff --git a/resources/image.go b/resources/image.go
index ed303613f..0396c2208 100644
--- a/resources/image.go
+++ b/resources/image.go
@@ -242,7 +242,7 @@ func (i *imageResource) doWithImageConfig(conf images.ImageConfig, f func(src im
 		errOp := conf.Action
 		errPath := i.getSourceFilename()
 
-		src, err := i.decodeSource()
+		src, err := i.DecodeImage()
 		if err != nil {
 			return nil, nil, &os.PathError{Op: errOp, Path: errPath, Err: err}
 		}
@@ -324,7 +324,9 @@ func (i *imageResource) decodeImageConfig(action, spec string) (images.ImageConf
 	return conf, nil
 }
 
-func (i *imageResource) decodeSource() (image.Image, error) {
+// DecodeImage decodes the image source into an Image.
+// This an internal method and may change.
+func (i *imageResource) DecodeImage() (image.Image, error) {
 	f, err := i.ReadSeekCloser()
 	if err != nil {
 		return nil, _errors.Wrap(err, "failed to open image for decode")
diff --git a/resources/image_test.go b/resources/image_test.go
index 9c1861960..8c69591cf 100644
--- a/resources/image_test.go
+++ b/resources/image_test.go
@@ -533,6 +533,11 @@ func TestImageOperationsGolden(t *testing.T) {
 		fmt.Println(workDir)
 	}
 
+	gopher := fetchImageForSpec(spec, c, "gopher-hero8.png")
+	var err error
+	gopher, err = gopher.Resize("30x")
+	c.Assert(err, qt.IsNil)
+
 	// Test PNGs with alpha channel.
 	for _, img := range []string{"gopher-hero8.png", "gradient-circle.png"} {
 		orig := fetchImageForSpec(spec, c, img)
@@ -589,6 +594,7 @@ func TestImageOperationsGolden(t *testing.T) {
 			f.Invert(),
 			f.Hue(22),
 			f.Contrast(32.5),
+			f.Overlay(gopher.(images.ImageSource), 20, 30),
 		}
 
 		resized, err := orig.Fill("400x200 center")
diff --git a/resources/images/filters.go b/resources/images/filters.go
index dd7b58345..74c50363e 100644
--- a/resources/images/filters.go
+++ b/resources/images/filters.go
@@ -25,6 +25,14 @@ const filterAPIVersion = 0
 type Filters struct {
 }
 
+// Overlay creates a filter that overlays src at position x y.
+func (*Filters) Overlay(src ImageSource, x, y interface{}) gift.Filter {
+	return filter{
+		Options: newFilterOpts(src.Key(), x, y),
+		Filter:  overlayFilter{src: src, x: cast.ToInt(x), y: cast.ToInt(y)},
+	}
+}
+
 // Brightness creates a filter that changes the brightness of an image.
 // The percentage parameter must be in range (-100, 100).
 func (*Filters) Brightness(percentage interface{}) gift.Filter {
diff --git a/resources/images/image.go b/resources/images/image.go
index 88eed2f7e..3d28263c0 100644
--- a/resources/images/image.go
+++ b/resources/images/image.go
@@ -325,3 +325,9 @@ func IsOpaque(img image.Image) bool {
 
 	return false
 }
+
+// ImageSource identifies and decodes an image.
+type ImageSource interface {
+	DecodeImage() (image.Image, error)
+	Key() string
+}
diff --git a/resources/images/overlay.go b/resources/images/overlay.go
new file mode 100644
index 000000000..780e28fd1
--- /dev/null
+++ b/resources/images/overlay.go
@@ -0,0 +1,43 @@
+// Copyright 2020 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 (
+	"fmt"
+	"image"
+	"image/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ gift.Filter = (*overlayFilter)(nil)
+
+type overlayFilter struct {
+	src  ImageSource
+	x, y int
+}
+
+func (f overlayFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	overlaySrc, err := f.src.DecodeImage()
+	if err != nil {
+		panic(fmt.Sprintf("failed to decode image: %s", err))
+	}
+
+	gift.New().Draw(dst, src)
+	gift.New().DrawAt(dst, overlaySrc, image.Pt(f.x, f.y), gift.OverOperator)
+}
+
+func (f overlayFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	return image.Rect(0, 0, srcBounds.Dx(), srcBounds.Dy())
+}
diff --git a/resources/resource/resourcetypes.go b/resources/resource/resourcetypes.go
index f42372fa3..206ce8de8 100644
--- a/resources/resource/resourcetypes.go
+++ b/resources/resource/resourcetypes.go
@@ -14,6 +14,8 @@
 package resource
 
 import (
+	"image"
+
 	"github.com/gohugoio/hugo/common/maps"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/media"
@@ -59,6 +61,9 @@ type ImageOps interface {
 	Resize(spec string) (Image, error)
 	Filter(filters ...interface{}) (Image, error)
 	Exif() *exif.Exif
+
+	// Internal
+	DecodeImage() (image.Image, error)
 }
 
 type ResourceTypeProvider interface {
diff --git a/resources/testdata/golden/gohugoio24_huc57dd738f4724f4b341121e66fd85555_267952_3ad578dd67cd055b4382e4062918d0a2.png b/resources/testdata/golden/gohugoio24_huc57dd738f4724f4b341121e66fd85555_267952_3ad578dd67cd055b4382e4062918d0a2.png
new file mode 100644
index 000000000..3a267fbdb
--- /dev/null
+++ b/resources/testdata/golden/gohugoio24_huc57dd738f4724f4b341121e66fd85555_267952_3ad578dd67cd055b4382e4062918d0a2.png
diff --git a/resources/testdata/golden/gohugoio8_hu7f72c00afdf7634587afaa5eff2a25b2_73538_b4afd8d32218a87ed1f7e351368501c3.png b/resources/testdata/golden/gohugoio8_hu7f72c00afdf7634587afaa5eff2a25b2_73538_b4afd8d32218a87ed1f7e351368501c3.png
new file mode 100644
index 000000000..86896ef01
--- /dev/null
+++ b/resources/testdata/golden/gohugoio8_hu7f72c00afdf7634587afaa5eff2a25b2_73538_b4afd8d32218a87ed1f7e351368501c3.png
diff --git a/resources/testdata/golden/gopher-hero8_huaa0cd7d2cfc14ff32a57f171896f2285_13327_30x0_resize_box_2.png b/resources/testdata/golden/gopher-hero8_huaa0cd7d2cfc14ff32a57f171896f2285_13327_30x0_resize_box_2.png
new file mode 100644
index 000000000..5b8748351
--- /dev/null
+++ b/resources/testdata/golden/gopher-hero8_huaa0cd7d2cfc14ff32a57f171896f2285_13327_30x0_resize_box_2.png
diff --git a/resources/testdata/golden/sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_da536a2a5436e387d0d482675e08ad48.jpg b/resources/testdata/golden/sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_da536a2a5436e387d0d482675e08ad48.jpg
new file mode 100644
index 000000000..15f2ef1c5
--- /dev/null
+++ b/resources/testdata/golden/sunset_hu59e56ffff1bc1d8d122b1403d34e039f_90587_da536a2a5436e387d0d482675e08ad48.jpg
diff --git a/resources/transform.go b/resources/transform.go
index a9ec84671..9007ead18 100644
--- a/resources/transform.go
+++ b/resources/transform.go
@@ -16,6 +16,7 @@ package resources
 import (
 	"bytes"
 	"fmt"
+	"image"
 	"io"
 	"path"
 	"strings"
@@ -264,6 +265,10 @@ func (r *resourceAdapter) Width() int {
 	return r.getImageOps().Width()
 }
 
+func (r *resourceAdapter) DecodeImage() (image.Image, error) {
+	return r.getImageOps().DecodeImage()
+}
+
 func (r *resourceAdapter) getImageOps() resource.ImageOps {
 	img, ok := r.target.(resource.ImageOps)
 	if !ok {