Add images.Process filter

This allows for constructs like:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg q30 resize 200x") }}
{{ $img = $img | images.Filter $filters }}
```

Note that the `action` option in `images.Process` is optional (`resize` in the example above), so you can use the above to just set the target format, e.g.:

```
{{ $filters := slice (images.GaussianBlur 8) (images.Grayscale) (images.Process "jpg") }}
{{ $img = $img | images.Filter $filters }}
```

Fixes gohugoio#8439

resources/image.go

@@ -206,15 +206,7 @@ var imageActions = []string{images.ActionResize, images.ActionCrop, images.Actio
 // This makes this method a more flexible version that covers all of Resize, Crop, Fit and Fill,
 // but it also supports e.g. format conversions without any resize action.
 func (i *imageResource) Process(spec string) (images.ImageResource, error) {
-	var action string
-	options := strings.Fields(spec)
-	for i, p := range options {
-		if hstrings.InSlicEqualFold(imageActions, p) {
-			action = p
-			options = append(options[:i], options[i+1:]...)
-			break
-		}
-	}
+	action, options := i.resolveActionOptions(spec)
 	return i.processActionOptions(action, options)
 }
 
@@ -245,22 +237,85 @@ func (i *imageResource) Fill(spec string) (images.ImageResource, error) {
 }
 
 func (i *imageResource) Filter(filters ...any) (images.ImageResource, error) {
-	conf := images.GetDefaultImageConfig("filter", i.Proc.Cfg)
+	var conf images.ImageConfig
 
 	var gfilters []gift.Filter
 
 	for _, f := range filters {
 		gfilters = append(gfilters, images.ToFilters(f)...)
 	}
 
+	var (
+		targetFormat images.Format
+		configSet    bool
+	)
+	for _, f := range gfilters {
+		f = images.UnwrapFilter(f)
+		if specProvider, ok := f.(images.ImageProcessSpecProvider); ok {
+			action, options := i.resolveActionOptions(specProvider.ImageProcessSpec())
+			var err error
+			conf, err = images.DecodeImageConfig(action, options, i.Proc.Cfg, i.Format)
+			if err != nil {
+				return nil, err
+			}
+			configSet = true
+			if conf.TargetFormat != 0 {
+				targetFormat = conf.TargetFormat
+				// We only support one target format, but prefer the last one,
+				// so we keep going.
+			}
+		}
+	}
+
+	if !configSet {
+		conf = images.GetDefaultImageConfig("filter", i.Proc.Cfg)
+	}
+
+	conf.Action = "filter"
 	conf.Key = identity.HashString(gfilters)
-	conf.TargetFormat = i.Format
+	conf.TargetFormat = targetFormat
+	if conf.TargetFormat == 0 {
+		conf.TargetFormat = i.Format
+	}
 
 	return i.doWithImageConfig(conf, func(src image.Image) (image.Image, error) {
-		return i.Proc.Filter(src, gfilters...)
+		filters := gfilters
+		for j, f := range gfilters {
+			f = images.UnwrapFilter(f)
+			if specProvider, ok := f.(images.ImageProcessSpecProvider); ok {
+				processSpec := specProvider.ImageProcessSpec()
+				action, options := i.resolveActionOptions(processSpec)
+				conf, err := images.DecodeImageConfig(action, options, i.Proc.Cfg, i.Format)
+				if err != nil {
+					return nil, err
+				}
+				pFilters, err := i.Proc.FiltersFromConfig(src, conf)
+				if err != nil {
+					return nil, err
+				}
+				// Replace the filter with the new filters.
+				// This slice will be empty if this is just a format conversion.
+				filters = append(filters[:j], append(pFilters, filters[j+1:]...)...)
+
+			}
+		}
+		return i.Proc.Filter(src, filters...)
 	})
 }
 
+func (i *imageResource) resolveActionOptions(spec string) (string, []string) {
+	var action string
+	options := strings.Fields(spec)
+	for i, p := range options {
+		if hstrings.InSlicEqualFold(imageActions, p) {
+			action = p
+			options = append(options[:i], options[i+1:]...)
+			break
+		}
+	}
+	return action, options
+}
+
 func (i *imageResource) processActionSpec(action, spec string) (images.ImageResource, error) {
 	return i.processActionOptions(action, strings.Fields(spec))
 }

resources/images/filters.go

@@ -30,6 +30,15 @@ const filterAPIVersion = 0
 
 type Filters struct{}
 
+func (*Filters) Process(spec any) gift.Filter {
+	return filter{
+		Options: newFilterOpts(spec),
+		Filter: processFilter{
+			spec: cast.ToString(spec),
+		},
+	}
+}
+
 // Overlay creates a filter that overlays src at position x y.
 func (*Filters) Overlay(src ImageSource, x, y any) gift.Filter {
 	return filter{

resources/images/image.go

@@ -201,7 +201,7 @@ func (p *ImageProcessor) DecodeExif(r io.Reader) (*exif.ExifInfo, error) {
 	return p.exifDecoder.Decode(r)
 }
 
-func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfig) (image.Image, error) {
+func (p *ImageProcessor) FiltersFromConfig(src image.Image, conf ImageConfig) ([]gift.Filter, error) {
 	var filters []gift.Filter
 
 	if conf.Rotate != 0 {
@@ -246,6 +246,14 @@ func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfi
 	default:
 
 	}
+	return filters, nil
+}
+
+func (p *ImageProcessor) ApplyFiltersFromConfig(src image.Image, conf ImageConfig) (image.Image, error) {
+	filters, err := p.FiltersFromConfig(src, conf)
+	if err != nil {
+		return nil, err
+	}
 
 	if len(filters) == 0 {
 		return p.resolveSrc(src, conf.TargetFormat), nil
@@ -396,6 +404,15 @@ func imageConfigFromImage(img image.Image) image.Config {
 	return image.Config{Width: b.Max.X, Height: b.Max.Y}
 }
 
+// UnwrapFilter unwraps the given filter if it is a filter wrapper.
+func UnwrapFilter(in gift.Filter) gift.Filter {
+	if f, ok := in.(filter); ok {
+		return f.Filter
+	}
+	return in
+}
+
+// ToFilters converts the given input to a slice of gift.Filter.
 func ToFilters(in any) []gift.Filter {
 	switch v := in.(type) {
 	case []gift.Filter:

resources/images/process.go

@@ -0,0 +1,43 @@
+// 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/draw"
+
+	"github.com/disintegration/gift"
+)
+
+var _ ImageProcessSpecProvider = (*processFilter)(nil)
+
+type ImageProcessSpecProvider interface {
+	ImageProcessSpec() string
+}
+
+type processFilter struct {
+	spec string
+}
+
+func (f processFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
+	panic("not supported")
+}
+
+func (f processFilter) Bounds(srcBounds image.Rectangle) image.Rectangle {
+	panic("not supported")
+}
+
+func (f processFilter) ImageProcessSpec() string {
+	return f.spec
+}

resources/integration_test.go

@@ -73,7 +73,6 @@ anigif: {{ $anigif.RelPermalink }}|{{ $anigif.Width }}|{{ $anigif.Height }}|{{ $
 	b.Build()
 
 	assertImages()
-
 }
 
 func TestSVGError(t *testing.T) {
@@ -98,7 +97,6 @@ Width: {{ $svg.Width }}
 
 	b.Assert(err, qt.IsNotNil)
 	b.Assert(err.Error(), qt.Contains, `error calling Width: this method is only available for raster images. To determine if an image is SVG, you can do {{ if eq .MediaType.SubType "svg" }}{{ end }}`)
-
 }
 
 // Issue 10255.
@@ -137,5 +135,33 @@ iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAA
 		b.AssertFileCount("public/images", 1)
 		b.Build()
 	}
+}
+
+func TestProcessFilter(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- assets/images/pixel.png --
+iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==
+-- layouts/index.html --
+{{ $pixel := resources.Get "images/pixel.png" }}
+{{ $filters := slice (images.GaussianBlur 6) (images.Pixelate 8) (images.Process "jpg") }}
+{{ $image := $pixel.Filter $filters }}
+jpg|RelPermalink: {{ $image.RelPermalink }}|MediaType: {{ $image.MediaType }}|Width: {{ $image.Width }}|Height: {{ $image.Height }}|
+{{ $filters := slice (images.GaussianBlur 6) (images.Pixelate 8) (images.Process "jpg resize 20x30") }}
+{{ $image := $pixel.Filter $filters }}
+resize|RelPermalink: {{ $image.RelPermalink }}|MediaType: {{ $image.MediaType }}|Width: {{ $image.Width }}|Height: {{ $image.Height }}|
+
+`
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		}).Build()
 
+	b.AssertFileContent("public/index.html",
+		"jpg|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_17010532266664966692.jpg|MediaType: image/jpeg|Width: 1|Height: 1|",
+		"resize|RelPermalink: /images/pixel_hu8aa3346827e49d756ff4e630147c42b5_70_filter_6707036659822075562.jpg|MediaType: image/jpeg|Width: 20|Height: 30|",
+	)
 }