Fix and rewrite contrast color calculation, fix project-related bugs (#30237)

1. The previous color contrast calculation function was incorrect at
least for the `#84b6eb` where it output low-contrast white instead of
black. I've rewritten these functions now to accept hex colors and to
match GitHub's calculation and to output pure white/black for maximum
contrast. Before and after:
<img width="94" alt="Screenshot 2024-04-02 at 01 53 46"
src="https://github.com/go-gitea/gitea/assets/115237/00b39e15-a377-4458-95cf-ceec74b78228"><img
width="90" alt="Screenshot 2024-04-02 at 01 51 30"
src="https://github.com/go-gitea/gitea/assets/115237/1677067a-8d8f-47eb-82c0-76330deeb775">

2. Fix project-related issues:

- Expose the new `ContrastColor` function as template helper and use it
for project cards, replacing the previous JS solution which eliminates a
flash of wrong color on page load.
- Fix a bug where if editing a project title, the counter would get
lost.
- Move `rgbToHex` function to color utils.

@HesterG fyi

---------

Co-authored-by: delvh <dev.lh@web.de>
Co-authored-by: Giteabot <teabot@gitea.io>

4 files changed, 45 insertions, 60 deletions

diff --git a/modules/templates/helper.go b/modules/templates/helper.go
index 9e770a2606..5d2fa79bc5 100644
--- a/modules/templates/helper.go
+++ b/modules/templates/helper.go
@@ -53,13 +53,13 @@ func NewFuncMap() template.FuncMap {
 		"JsonUtils":   NewJsonUtils,
 
 		// -----------------------------------------------------------------
-		// svg / avatar / icon
+		// svg / avatar / icon / color
 		"svg":           svg.RenderHTML,
 		"EntryIcon":     base.EntryIcon,
 		"MigrationIcon": MigrationIcon,
 		"ActionIcon":    ActionIcon,
-
-		"SortArrow": SortArrow,
+		"SortArrow":     SortArrow,
+		"ContrastColor": util.ContrastColor,
 
 		// -----------------------------------------------------------------
 		// time / number / format
diff --git a/modules/templates/util_render.go b/modules/templates/util_render.go
index d1c9b082fa..0b53965f25 100644
--- a/modules/templates/util_render.go
+++ b/modules/templates/util_render.go
@@ -123,16 +123,10 @@ func RenderIssueTitle(ctx context.Context, text string, metas map[string]string)
 func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_model.Label) template.HTML {
 	var (
 		archivedCSSClass string
-		textColor        = "#111"
+		textColor        = util.ContrastColor(label.Color)
 		labelScope       = label.ExclusiveScope()
 	)
 
-	r, g, b := util.HexToRBGColor(label.Color)
-	// Determine if label text should be light or dark to be readable on background color
-	if util.UseLightTextOnBackground(r, g, b) {
-		textColor = "#eee"
-	}
-
 	description := emoji.ReplaceAliases(template.HTMLEscapeString(label.Description))
 
 	if label.IsArchived() {
@@ -153,7 +147,7 @@ func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_m
 
 	// Make scope and item background colors slightly darker and lighter respectively.
 	// More contrast needed with higher luminance, empirically tweaked.
-	luminance := util.GetLuminance(r, g, b)
+	luminance := util.GetRelativeLuminance(label.Color)
 	contrast := 0.01 + luminance*0.03
 	// Ensure we add the same amount of contrast also near 0 and 1.
 	darken := contrast + math.Max(luminance+contrast-1.0, 0.0)
@@ -162,6 +156,7 @@ func RenderLabel(ctx context.Context, locale translation.Locale, label *issues_m
 	darkenFactor := math.Max(luminance-darken, 0.0) / math.Max(luminance, 1.0/255.0)
 	lightenFactor := math.Min(luminance+lighten, 1.0) / math.Max(luminance, 1.0/255.0)
 
+	r, g, b := util.HexToRBGColor(label.Color)
 	scopeBytes := []byte{
 		uint8(math.Min(math.Round(r*darkenFactor), 255)),
 		uint8(math.Min(math.Round(g*darkenFactor), 255)),
diff --git a/modules/util/color.go b/modules/util/color.go
index 240b045c28..9c520dce78 100644
--- a/modules/util/color.go
+++ b/modules/util/color.go
@@ -4,22 +4,10 @@ package util
 
 import (
 	"fmt"
-	"math"
 	"strconv"
 	"strings"
 )
 
-// Check similar implementation in web_src/js/utils/color.js and keep synchronization
-
-// Return R, G, B values defined in reletive luminance
-func getLuminanceRGB(channel float64) float64 {
-	sRGB := channel / 255
-	if sRGB <= 0.03928 {
-		return sRGB / 12.92
-	}
-	return math.Pow((sRGB+0.055)/1.055, 2.4)
-}
-
 // Get color as RGB values in 0..255 range from the hex color string (with or without #)
 func HexToRBGColor(colorString string) (float64, float64, float64) {
 	hexString := colorString
@@ -47,19 +35,23 @@ func HexToRBGColor(colorString string) (float64, float64, float64) {
 	return r, g, b
 }
 
-// return luminance given RGB channels
-// Reference from: https://www.w3.org/WAI/GL/wiki/Relative_luminance
-func GetLuminance(r, g, b float64) float64 {
-	R := getLuminanceRGB(r)
-	G := getLuminanceRGB(g)
-	B := getLuminanceRGB(b)
-	luminance := 0.2126*R + 0.7152*G + 0.0722*B
-	return luminance
+// Returns relative luminance for a SRGB color - https://en.wikipedia.org/wiki/Relative_luminance
+// Keep this in sync with web_src/js/utils/color.js
+func GetRelativeLuminance(color string) float64 {
+	r, g, b := HexToRBGColor(color)
+	return (0.2126729*r + 0.7151522*g + 0.0721750*b) / 255
 }
 
-// Reference from: https://firsching.ch/github_labels.html
-// In the future WCAG 3 APCA may be a better solution.
-// Check if text should use light color based on RGB of background
-func UseLightTextOnBackground(r, g, b float64) bool {
-	return GetLuminance(r, g, b) < 0.453
+func UseLightText(backgroundColor string) bool {
+	return GetRelativeLuminance(backgroundColor) < 0.453
+}
+
+// Given a background color, returns a black or white foreground color that the highest
+// contrast ratio. In the future, the APCA contrast function, or CSS `contrast-color` will be better.
+// https://github.com/color-js/color.js/blob/eb7b53f7a13bb716ec8b28c7a56f052cd599acd9/src/contrast/APCA.js#L42
+func ContrastColor(backgroundColor string) string {
+	if UseLightText(backgroundColor) {
+		return "#fff"
+	}
+	return "#000"
 }
diff --git a/modules/util/color_test.go b/modules/util/color_test.go
index d96ac36730..be6e6b122a 100644
--- a/modules/util/color_test.go
+++ b/modules/util/color_test.go
@@ -33,33 +33,31 @@ func Test_HexToRBGColor(t *testing.T) {
 	}
 }
 
-func Test_UseLightTextOnBackground(t *testing.T) {
+func Test_UseLightText(t *testing.T) {
 	cases := []struct {
-		r        float64
-		g        float64
-		b        float64
-		expected bool
+		color    string
+		expected string
 	}{
-		{215, 58, 74, true},
-		{0, 117, 202, true},
-		{207, 211, 215, false},
-		{162, 238, 239, false},
-		{112, 87, 255, true},
-		{0, 134, 114, true},
-		{228, 230, 105, false},
-		{216, 118, 227, true},
-		{255, 255, 255, false},
-		{43, 134, 133, true},
-		{43, 135, 134, true},
-		{44, 135, 134, true},
-		{59, 182, 179, true},
-		{124, 114, 104, true},
-		{126, 113, 108, true},
-		{129, 112, 109, true},
-		{128, 112, 112, true},
+		{"#d73a4a", "#fff"},
+		{"#0075ca", "#fff"},
+		{"#cfd3d7", "#000"},
+		{"#a2eeef", "#000"},
+		{"#7057ff", "#fff"},
+		{"#008672", "#fff"},
+		{"#e4e669", "#000"},
+		{"#d876e3", "#000"},
+		{"#ffffff", "#000"},
+		{"#2b8684", "#fff"},
+		{"#2b8786", "#fff"},
+		{"#2c8786", "#000"},
+		{"#3bb6b3", "#000"},
+		{"#7c7268", "#fff"},
+		{"#7e716c", "#fff"},
+		{"#81706d", "#fff"},
+		{"#807070", "#fff"},
+		{"#84b6eb", "#000"},
 	}
 	for n, c := range cases {
-		result := UseLightTextOnBackground(c.r, c.g, c.b)
-		assert.Equal(t, c.expected, result, "case %d: error should match", n)
+		assert.Equal(t, c.expected, ContrastColor(c.color), "case %d: error should match", n)
 	}
 }