resources/images: Add $image.Colors

Which returns the most dominant colors of an image using a simple histogram method.

Fixes #10307

resources/errorResource.go

@@ -123,6 +123,10 @@ func (e *errorResource) Exif() *exif.ExifInfo {
 	panic(e.ResourceError)
 }
 
+func (e *errorResource) Colors() ([]string, error) {
+	panic(e.ResourceError)
+}
+
 func (e *errorResource) DecodeImage() (image.Image, error) {
 	panic(e.ResourceError)
 }

resources/image.go

@@ -30,6 +30,8 @@ import (
 	"strings"
 	"sync"
 
+	color_extractor "github.com/marekm4/color-extractor"
+
 	"github.com/gohugoio/hugo/common/paths"
 
 	"github.com/disintegration/gift"
@@ -64,6 +66,9 @@ type imageResource struct {
 	metaInitErr error
 	meta        *imageMeta
 
+	dominantColorInit sync.Once
+	dominantColors    []string
+
 	baseResource
 }
 
@@ -135,6 +140,24 @@ func (i *imageResource) getExif() *exif.ExifInfo {
 	return i.meta.Exif
 }
 
+// Colors returns a slice of the most dominant colors in an image
+// using a simple histogram method.
+func (i *imageResource) Colors() ([]string, error) {
+	var err error
+	i.dominantColorInit.Do(func() {
+		var img image.Image
+		img, err = i.DecodeImage()
+		if err != nil {
+			return
+		}
+		colors := color_extractor.ExtractColors(img)
+		for _, c := range colors {
+			i.dominantColors = append(i.dominantColors, images.ColorToHexString(c))
+		}
+	})
+	return i.dominantColors, nil
+}
+
 // Clone is for internal use.
 func (i *imageResource) Clone() resource.Resource {
 	gr := i.baseResource.Clone().(baseResource)

resources/image_test.go

@@ -84,6 +84,10 @@ func TestImageTransformBasic(t *testing.T) {
 		c.Assert(img.Height(), qt.Equals, h)
 	}
 
+	colors, err := image.Colors()
+	c.Assert(err, qt.IsNil)
+	c.Assert(colors, qt.DeepEquals, []string{"#2d2f33", "#a49e93", "#d39e59", "#a76936", "#737a84", "#7c838b"})
+
 	c.Assert(image.RelPermalink(), qt.Equals, "/a/sunset.jpg")
 	c.Assert(image.ResourceType(), qt.Equals, "image")
 	assertWidthHeight(image, 900, 562)

resources/images/color.go

@@ -45,6 +45,14 @@ func ReplaceColorInPalette(c color.Color, p color.Palette) {
 	p[p.Index(c)] = c
 }
 
+// ColorToHexString converts a color to a hex string.
+func ColorToHexString(c color.Color) string {
+	r, g, b, a := c.RGBA()
+	rgba := color.RGBA{uint8(r), uint8(g), uint8(b), uint8(a)}
+	return fmt.Sprintf("#%.2x%.2x%.2x", rgba.R, rgba.G, rgba.B)
+
+}
+
 func hexStringToColor(s string) (color.Color, error) {
 	s = strings.TrimPrefix(s, "#")
 

resources/images/color_test.go

@@ -60,6 +60,30 @@ func TestHexStringToColor(t *testing.T) {
 	}
 }
 
+func TestColorToHexString(t *testing.T) {
+	c := qt.New(t)
+
+	for _, test := range []struct {
+		arg    color.Color
+		expect string
+	}{
+		{color.White, "#ffffff"},
+		{color.Black, "#000000"},
+		{color.RGBA{R: 0x42, G: 0x87, B: 0xf5, A: 0xff}, "#4287f5"},
+	} {
+
+		test := test
+		c.Run(test.expect, func(c *qt.C) {
+			c.Parallel()
+
+			result := ColorToHexString(test.arg)
+
+			c.Assert(result, qt.Equals, test.expect)
+		})
+
+	}
+}
+
 func TestAddColorToPalette(t *testing.T) {
 	c := qt.New(t)
 

resources/images/image_resource.go

@@ -48,6 +48,10 @@ type ImageResourceOps interface {
 	// Exif returns an ExifInfo object containing Image metadata.
 	Exif() *exif.ExifInfo
 
+	// Colors returns a slice of the most dominant colors in an image
+	// using a simple histogram method.
+	Colors() ([]string, error)
+
 	// Internal
 	DecodeImage() (image.Image, error)
 }

resources/transform.go

@@ -213,6 +213,10 @@ func (r *resourceAdapter) Exif() *exif.ExifInfo {
 	return r.getImageOps().Exif()
 }
 
+func (r *resourceAdapter) Colors() ([]string, error) {
+	return r.getImageOps().Colors()
+}
+
 func (r *resourceAdapter) Key() string {
 	r.init(false, false)
 	return r.target.(resource.Identifier).Key()