Add Luminance to Color

To sort an image's colors from darkest to lightest, you can then do:

```handlebars
{{ {{ $colorsByLuminance := sort $image.Colors "Luminance" }}
```

This uses the formula defined here: https://www.w3.org/TR/WCAG21/#dfn-relative-luminance

Fixes #10450

12 files changed, 204 insertions, 39 deletions

diff --git a/common/hstrings/strings.go b/common/hstrings/strings.go
index d9426ab5d..1232eee37 100644
--- a/common/hstrings/strings.go
+++ b/common/hstrings/strings.go
@@ -123,6 +123,20 @@ func InSlicEqualFold(arr []string, el string) bool {
 	return false
 }
 
+// ToString converts the given value to a string.
+// Note that this is a more strict version compared to cast.ToString,
+// as it will not try to convert numeric values to strings,
+// but only accept strings or fmt.Stringer.
+func ToString(v any) (string, bool) {
+	switch vv := v.(type) {
+	case string:
+		return vv, true
+	case fmt.Stringer:
+		return vv.String(), true
+	}
+	return "", false
+}
+
 type Tuple struct {
 	First  string
 	Second string
diff --git a/resources/errorResource.go b/resources/errorResource.go
index 220869fc1..582c54f6d 100644
--- a/resources/errorResource.go
+++ b/resources/errorResource.go
@@ -128,7 +128,7 @@ func (e *errorResource) Exif() *exif.ExifInfo {
 	panic(e.ResourceError)
 }
 
-func (e *errorResource) Colors() ([]string, error) {
+func (e *errorResource) Colors() ([]images.Color, error) {
 	panic(e.ResourceError)
 }
 
diff --git a/resources/image.go b/resources/image.go
index 78a57bb53..8f70a665a 100644
--- a/resources/image.go
+++ b/resources/image.go
@@ -67,7 +67,7 @@ type imageResource struct {
 	meta        *imageMeta
 
 	dominantColorInit sync.Once
-	dominantColors    []string
+	dominantColors    []images.Color
 
 	baseResource
 }
@@ -143,7 +143,7 @@ func (i *imageResource) getExif() *exif.ExifInfo {
 
 // Colors returns a slice of the most dominant colors in an image
 // using a simple histogram method.
-func (i *imageResource) Colors() ([]string, error) {
+func (i *imageResource) Colors() ([]images.Color, error) {
 	var err error
 	i.dominantColorInit.Do(func() {
 		var img image.Image
@@ -153,7 +153,7 @@ func (i *imageResource) Colors() ([]string, error) {
 		}
 		colors := color_extractor.ExtractColors(img)
 		for _, c := range colors {
-			i.dominantColors = append(i.dominantColors, images.ColorToHexString(c))
+			i.dominantColors = append(i.dominantColors, images.ColorGoToColor(c))
 		}
 	})
 	return i.dominantColors, nil
diff --git a/resources/image_test.go b/resources/image_test.go
index 231a06453..7e26c1f55 100644
--- a/resources/image_test.go
+++ b/resources/image_test.go
@@ -85,9 +85,16 @@ func TestImageTransformBasic(t *testing.T) {
 		assertWidthHeight(c, img, w, h)
 	}
 
-	colors, err := image.Colors()
+	gotColors, err := image.Colors()
 	c.Assert(err, qt.IsNil)
-	c.Assert(colors, qt.DeepEquals, []string{"#2d2f33", "#a49e93", "#d39e59", "#a76936", "#737a84", "#7c838b"})
+	expectedColors := images.HexStringsToColors("#2d2f33", "#a49e93", "#d39e59", "#a76936", "#737a84", "#7c838b")
+	c.Assert(len(gotColors), qt.Equals, len(expectedColors))
+	for i := range gotColors {
+		c1, c2 := gotColors[i], expectedColors[i]
+		c.Assert(c1.ColorHex(), qt.Equals, c2.ColorHex())
+		c.Assert(c1.ColorGo(), qt.DeepEquals, c2.ColorGo())
+		c.Assert(c1.Luminance(), qt.Equals, c2.Luminance())
+	}
 
 	c.Assert(image.RelPermalink(), qt.Equals, "/a/sunset.jpg")
 	c.Assert(image.ResourceType(), qt.Equals, "image")
@@ -445,6 +452,24 @@ func TestImageExif(t *testing.T) {
 	getAndCheckExif(c, image)
 }
 
+func TestImageColorsLuminance(t *testing.T) {
+	c := qt.New(t)
+
+	_, image := fetchSunset(c)
+	c.Assert(image, qt.Not(qt.IsNil))
+	colors, err := image.Colors()
+	c.Assert(err, qt.IsNil)
+	c.Assert(len(colors), qt.Equals, 6)
+	var prevLuminance float64
+	for i, color := range colors {
+		luminance := color.Luminance()
+		c.Assert(err, qt.IsNil)
+		c.Assert(luminance > 0, qt.IsTrue)
+		c.Assert(luminance, qt.Not(qt.Equals), prevLuminance, qt.Commentf("i=%d", i))
+		prevLuminance = luminance
+	}
+}
+
 func BenchmarkImageExif(b *testing.B) {
 	getImages := func(c *qt.C, b *testing.B, fs afero.Fs) []images.ImageResource {
 		spec := newTestResourceSpec(specDescriptor{fs: fs, c: c})
diff --git a/resources/images/color.go b/resources/images/color.go
index 71872a30e..e2ff2377f 100644
--- a/resources/images/color.go
+++ b/resources/images/color.go
@@ -16,10 +16,76 @@ package images
 import (
 	"encoding/hex"
 	"fmt"
+	"hash/fnv"
 	"image/color"
+	"math"
 	"strings"
+
+	"github.com/gohugoio/hugo/common/hstrings"
 )
 
+type colorGoProvider interface {
+	ColorGo() color.Color
+}
+
+type Color struct {
+	// The color.
+	color color.Color
+
+	// The color prefixed with a #.
+	hex string
+
+	// The relative luminance of the color.
+	luminance float64
+}
+
+// Luminance as defined by w3.org.
+// See https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
+func (c Color) Luminance() float64 {
+	return c.luminance
+}
+
+// ColorGo returns the color as a color.Color.
+// For internal use only.
+func (c Color) ColorGo() color.Color {
+	return c.color
+}
+
+// ColorHex returns the color as a hex string  prefixed with a #.
+func (c Color) ColorHex() string {
+	return c.hex
+}
+
+// String returns the color as a hex string prefixed with a #.
+func (c Color) String() string {
+	return c.hex
+}
+
+// For hashstructure. This struct is used in template func options
+// that needs to be able to hash a Color.
+// For internal use only.
+func (c Color) Hash() (uint64, error) {
+	h := fnv.New64a()
+	h.Write([]byte(c.hex))
+	return h.Sum64(), nil
+}
+
+func (c *Color) init() error {
+	c.hex = ColorGoToHexString(c.color)
+	r, g, b, _ := c.color.RGBA()
+	c.luminance = 0.2126*c.toSRGB(uint8(r)) + 0.7152*c.toSRGB(uint8(g)) + 0.0722*c.toSRGB(uint8(b))
+	return nil
+}
+
+func (c Color) toSRGB(i uint8) float64 {
+	v := float64(i) / 255
+	if v <= 0.04045 {
+		return v / 12.92
+	} else {
+		return math.Pow((v+0.055)/1.055, 2.4)
+	}
+}
+
 // AddColorToPalette adds c as the first color in p if not already there.
 // Note that it does no additional checks, so callers must make sure
 // that the palette is valid for the relevant format.
@@ -45,14 +111,60 @@ 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 {
+// ColorGoToHexString converts a color.Color to a hex string.
+func ColorGoToHexString(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)
+	if rgba.A == 0xff {
+		return fmt.Sprintf("#%.2x%.2x%.2x", rgba.R, rgba.G, rgba.B)
+	}
+	return fmt.Sprintf("#%.2x%.2x%.2x%.2x", rgba.R, rgba.G, rgba.B, rgba.A)
+}
+
+// ColorGoToColor converts a color.Color to a Color.
+func ColorGoToColor(c color.Color) Color {
+	cc := Color{color: c}
+	if err := cc.init(); err != nil {
+		panic(err)
+	}
+	return cc
+}
+
+func hexStringToColor(s string) Color {
+	c, err := hexStringToColorGo(s)
+	if err != nil {
+		panic(err)
+	}
+	return ColorGoToColor(c)
+}
+
+// HexStringsToColors converts a slice of hex strings to a slice of Colors.
+func HexStringsToColors(s ...string) []Color {
+	var colors []Color
+	for _, v := range s {
+		colors = append(colors, hexStringToColor(v))
+	}
+	return colors
+}
+
+func toColorGo(v any) (color.Color, bool, error) {
+	switch vv := v.(type) {
+	case colorGoProvider:
+		return vv.ColorGo(), true, nil
+	default:
+		s, ok := hstrings.ToString(v)
+		if !ok {
+			return nil, false, nil
+		}
+		c, err := hexStringToColorGo(s)
+		if err != nil {
+			return nil, false, err
+		}
+		return c, true, nil
+	}
 }
 
-func hexStringToColor(s string) (color.Color, error) {
+func hexStringToColorGo(s string) (color.Color, error) {
 	s = strings.TrimPrefix(s, "#")
 
 	if len(s) != 3 && len(s) != 4 && len(s) != 6 && len(s) != 8 {
diff --git a/resources/images/color_test.go b/resources/images/color_test.go
index c3860a82c..cbbc76cf9 100644
--- a/resources/images/color_test.go
+++ b/resources/images/color_test.go
@@ -46,7 +46,7 @@ func TestHexStringToColor(t *testing.T) {
 		c.Run(test.arg, func(c *qt.C) {
 			c.Parallel()
 
-			result, err := hexStringToColor(test.arg)
+			result, err := hexStringToColorGo(test.arg)
 
 			if b, ok := test.expect.(bool); ok && !b {
 				c.Assert(err, qt.Not(qt.IsNil))
@@ -70,13 +70,18 @@ func TestColorToHexString(t *testing.T) {
 		{color.White, "#ffffff"},
 		{color.Black, "#000000"},
 		{color.RGBA{R: 0x42, G: 0x87, B: 0xf5, A: 0xff}, "#4287f5"},
+
+		// 50% opacity.
+		// Note that the .Colors (dominant colors) received from the Image resource
+		// will always have an alpha value of 0xff.
+		{color.RGBA{R: 0x42, G: 0x87, B: 0xf5, A: 0x80}, "#4287f580"},
 	} {
 
 		test := test
 		c.Run(test.expect, func(c *qt.C) {
 			c.Parallel()
 
-			result := ColorToHexString(test.arg)
+			result := ColorGoToHexString(test.arg)
 
 			c.Assert(result, qt.Equals, test.expect)
 		})
@@ -91,9 +96,9 @@ func TestAddColorToPalette(t *testing.T) {
 
 	c.Assert(AddColorToPalette(color.White, palette), qt.HasLen, 2)
 
-	blue1, _ := hexStringToColor("34c3eb")
-	blue2, _ := hexStringToColor("34c3eb")
-	white, _ := hexStringToColor("fff")
+	blue1, _ := hexStringToColorGo("34c3eb")
+	blue2, _ := hexStringToColorGo("34c3eb")
+	white, _ := hexStringToColorGo("fff")
 
 	c.Assert(AddColorToPalette(white, palette), qt.HasLen, 2)
 	c.Assert(AddColorToPalette(blue1, palette), qt.HasLen, 3)
@@ -104,10 +109,18 @@ func TestReplaceColorInPalette(t *testing.T) {
 	c := qt.New(t)
 
 	palette := color.Palette{color.White, color.Black}
-	offWhite, _ := hexStringToColor("fcfcfc")
+	offWhite, _ := hexStringToColorGo("fcfcfc")
 
 	ReplaceColorInPalette(offWhite, palette)
 
 	c.Assert(palette, qt.HasLen, 2)
 	c.Assert(palette[0], qt.Equals, offWhite)
 }
+
+func TestColorLuminance(t *testing.T) {
+	c := qt.New(t)
+	c.Assert(hexStringToColor("#000000").Luminance(), qt.Equals, 0.0)
+	c.Assert(hexStringToColor("#768a9a").Luminance(), qt.Equals, 0.24361603589088263)
+	c.Assert(hexStringToColor("#d5bc9f").Luminance(), qt.Equals, 0.5261577672685374)
+	c.Assert(hexStringToColor("#ffffff").Luminance(), qt.Equals, 1.0)
+}
diff --git a/resources/images/config.go b/resources/images/config.go
index 186f8fa6b..9655e9a51 100644
--- a/resources/images/config.go
+++ b/resources/images/config.go
@@ -171,7 +171,7 @@ func DecodeConfig(in map[string]any) (*config.ConfigNamespace[ImagingConfig, Ima
 			return i, nil, err
 		}
 
-		i.BgColor, err = hexStringToColor(i.Imaging.BgColor)
+		i.BgColor, err = hexStringToColorGo(i.Imaging.BgColor)
 		if err != nil {
 			return i, nil, err
 		}
@@ -230,7 +230,7 @@ func DecodeImageConfig(action string, options []string, defaults *config.ConfigN
 			c.Hint = hint
 		} else if part[0] == '#' {
 			c.BgColorStr = part[1:]
-			c.BgColor, err = hexStringToColor(c.BgColorStr)
+			c.BgColor, err = hexStringToColorGo(c.BgColorStr)
 			if err != nil {
 				return c, err
 			}
@@ -424,7 +424,7 @@ type ImagingConfigInternal struct {
 
 func (i *ImagingConfigInternal) Compile(externalCfg *ImagingConfig) error {
 	var err error
-	i.BgColor, err = hexStringToColor(externalCfg.BgColor)
+	i.BgColor, err = hexStringToColorGo(externalCfg.BgColor)
 	if err != nil {
 		return err
 	}
diff --git a/resources/images/config_test.go b/resources/images/config_test.go
index 86f70c1bf..6dd545f2c 100644
--- a/resources/images/config_test.go
+++ b/resources/images/config_test.go
@@ -132,7 +132,7 @@ func newImageConfig(action string, width, height, quality, rotate int, filter, a
 	c.qualitySetForImage = quality != 75
 	c.Rotate = rotate
 	c.BgColorStr = bgColor
-	c.BgColor, _ = hexStringToColor(bgColor)
+	c.BgColor, _ = hexStringToColorGo(bgColor)
 
 	if filter != "" {
 		filter = strings.ToLower(filter)
diff --git a/resources/images/filters.go b/resources/images/filters.go
index 53818c97d..0a620716d 100644
--- a/resources/images/filters.go
+++ b/resources/images/filters.go
@@ -1,4 +1,4 @@
-// Copyright 2019 The Hugo Authors. All rights reserved.
+// Copyright 2024 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.
@@ -65,7 +65,7 @@ func (*Filters) Opacity(opacity any) gift.Filter {
 func (*Filters) Text(text string, options ...any) gift.Filter {
 	tf := textFilter{
 		text:        text,
-		color:       "#ffffff",
+		color:       color.White,
 		size:        20,
 		x:           10,
 		y:           10,
@@ -78,7 +78,9 @@ func (*Filters) Text(text string, options ...any) gift.Filter {
 		for option, v := range opt {
 			switch option {
 			case "color":
-				tf.color = cast.ToString(v)
+				if color, ok, _ := toColorGo(v); ok {
+					tf.color = color
+				}
 			case "size":
 				tf.size = cast.ToFloat64(v)
 			case "x":
@@ -128,15 +130,14 @@ func (*Filters) Padding(args ...any) gift.Filter {
 
 	var top, right, bottom, left int
 	var ccolor color.Color = color.White // canvas color
-	var err error
 
 	_args := args // preserve original args for most stable hash
 
-	if vcs, ok := (args[len(args)-1]).(string); ok {
-		ccolor, err = hexStringToColor(vcs)
+	if vcs, ok, err := toColorGo(args[len(args)-1]); ok || err != nil {
 		if err != nil {
 			panic("invalid canvas color: specify RGB or RGBA using hex notation")
 		}
+		ccolor = vcs
 		args = args[:len(args)-1]
 		if len(args) == 0 {
 			panic("not enough arguments: provide one or more padding values using the CSS shorthand property syntax")
@@ -180,12 +181,11 @@ func (*Filters) Padding(args ...any) gift.Filter {
 // Dither creates a filter that dithers an image.
 func (*Filters) Dither(options ...any) gift.Filter {
 	ditherOptions := struct {
-		Colors     []string
+		Colors     []any
 		Method     string
 		Serpentine bool
 		Strength   float32
 	}{
-		Colors:     []string{"000000ff", "ffffffff"},
 		Method:     "floydsteinberg",
 		Serpentine: true,
 		Strength:   1.0,
@@ -198,14 +198,18 @@ func (*Filters) Dither(options ...any) gift.Filter {
 		}
 	}
 
+	if len(ditherOptions.Colors) == 0 {
+		ditherOptions.Colors = []any{"000000ff", "ffffffff"}
+	}
+
 	if len(ditherOptions.Colors) < 2 {
 		panic("palette must have at least two colors")
 	}
 
 	var palette []color.Color
 	for _, c := range ditherOptions.Colors {
-		cc, err := hexStringToColor(c)
-		if err != nil {
+		cc, ok, err := toColorGo(c)
+		if !ok || err != nil {
 			panic(fmt.Sprintf("%q is an invalid color: specify RGB or RGBA using hexadecimal notation", c))
 		}
 		palette = append(palette, cc)
diff --git a/resources/images/image_resource.go b/resources/images/image_resource.go
index e6be757c2..7cede07dd 100644
--- a/resources/images/image_resource.go
+++ b/resources/images/image_resource.go
@@ -63,7 +63,7 @@ type ImageResourceOps interface {
 
 	// Colors returns a slice of the most dominant colors in an image
 	// using a simple histogram method.
-	Colors() ([]string, error)
+	Colors() ([]Color, error)
 
 	// For internal use.
 	DecodeImage() (image.Image, error)
diff --git a/resources/images/text.go b/resources/images/text.go
index 2d3370c61..c1abc60bd 100644
--- a/resources/images/text.go
+++ b/resources/images/text.go
@@ -15,6 +15,7 @@ package images
 
 import (
 	"image"
+	"image/color"
 	"image/draw"
 	"io"
 	"strings"
@@ -31,7 +32,8 @@ import (
 var _ gift.Filter = (*textFilter)(nil)
 
 type textFilter struct {
-	text, color string
+	text        string
+	color       color.Color
 	x, y        int
 	size        float64
 	linespacing int
@@ -39,11 +41,6 @@ type textFilter struct {
 }
 
 func (f textFilter) Draw(dst draw.Image, src image.Image, options *gift.Options) {
-	color, err := hexStringToColor(f.color)
-	if err != nil {
-		panic(err)
-	}
-
 	// Load and parse font
 	ttf := goregular.TTF
 	if f.fontSource != nil {
@@ -74,7 +71,7 @@ func (f textFilter) Draw(dst draw.Image, src image.Image, options *gift.Options)
 
 	d := font.Drawer{
 		Dst:  dst,
-		Src:  image.NewUniform(color),
+		Src:  image.NewUniform(f.color),
 		Face: face,
 	}
 
diff --git a/resources/transform.go b/resources/transform.go
index 39a8aaccc..d9084b178 100644
--- a/resources/transform.go
+++ b/resources/transform.go
@@ -263,7 +263,7 @@ func (r *resourceAdapter) Exif() *exif.ExifInfo {
 	return r.getImageOps().Exif()
 }
 
-func (r *resourceAdapter) Colors() ([]string, error) {
+func (r *resourceAdapter) Colors() ([]images.Color, error) {
 	return r.getImageOps().Colors()
 }