Actually compute proper foreground color for labels (#16729)

models/issue_label.go

@@ -8,6 +8,7 @@ package models
 import (
 	"fmt"
 	"html/template"
+	"math"
 	"regexp"
 	"strconv"
 	"strings"
@@ -138,19 +139,44 @@ func (label *Label) BelongsToRepo() bool {
 	return label.RepoID > 0
 }
 
+// SrgbToLinear converts a component of an sRGB color to its linear intensity
+// See: https://en.wikipedia.org/wiki/SRGB#The_reverse_transformation_(sRGB_to_CIE_XYZ)
+func SrgbToLinear(color uint8) float64 {
+	flt := float64(color) / 255
+	if flt <= 0.04045 {
+		return flt / 12.92
+	}
+	return math.Pow((flt+0.055)/1.055, 2.4)
+}
+
+// Luminance returns the luminance of an sRGB color
+func Luminance(color uint32) float64 {
+	r := SrgbToLinear(uint8(0xFF & (color >> 16)))
+	g := SrgbToLinear(uint8(0xFF & (color >> 8)))
+	b := SrgbToLinear(uint8(0xFF & color))
+
+	// luminance ratios for sRGB
+	return 0.2126*r + 0.7152*g + 0.0722*b
+}
+
+// LuminanceThreshold is the luminance at which white and black appear to have the same contrast
+// i.e. x such that 1.05 / (x + 0.05) = (x + 0.05) / 0.05
+// i.e. math.Sqrt(1.05*0.05) - 0.05
+const LuminanceThreshold float64 = 0.179
+
 // ForegroundColor calculates the text color for labels based
 // on their background color.
 func (label *Label) ForegroundColor() template.CSS {
 	if strings.HasPrefix(label.Color, "#") {
 		if color, err := strconv.ParseUint(label.Color[1:], 16, 64); err == nil {
-			r := float32(0xFF & (color >> 16))
-			g := float32(0xFF & (color >> 8))
-			b := float32(0xFF & color)
-			luminance := (0.2126*r + 0.7152*g + 0.0722*b) / 255
+			// NOTE: see web_src/js/components/ContextPopup.vue for similar implementation
+			luminance := Luminance(uint32(color))
 
-			if luminance < 0.66 {
+			// prefer white or black based upon contrast
+			if luminance < LuminanceThreshold {
 				return template.CSS("#fff")
 			}
+			return template.CSS("#000")
 		}
 	}