Improve contrast ratio for label text coloring

commands/issue.go

@@ -600,13 +600,44 @@ func formatLabel(label github.IssueLabel, colorize bool) string {
 
 func colorizeLabel(label github.IssueLabel, color *utils.Color) string {
 	bgColorCode := utils.RgbToTermColorCode(color)
-	return fmt.Sprintf("\033[38;5;%d;48;%sm %s \033[m",
-		getSuitableLabelTextColor(color), bgColorCode, label.Name)
+	fgColor := pickHighContrastTextColor(color)
+	fgColorCode := utils.RgbToTermColorCode(fgColor)
+	return fmt.Sprintf("\033[38;%s;48;%sm %s \033[m",
+		fgColorCode, bgColorCode, label.Name)
 }
 
-func getSuitableLabelTextColor(color *utils.Color) int {
-	if color.Brightness() < 0.65 {
-		return 15 // white text
+func pickHighContrastTextColor(color *utils.Color) *utils.Color {
+	var candidates [12]*utils.Color
+	hsl := color.ToHsl()
+	a := hsl.ScaleLightness(0.5)
+	candidates[0] = a.ToRgb()
+	candidates[1] = a.ScaleLightness(0.25).ToRgb()
+	candidates[2] = a.ScaleSaturation(-0.25).ToRgb()
+	b := hsl.ScaleSaturation(0.75)
+	candidates[3] = b.ToRgb()
+	candidates[4] = b.ScaleLightness(0.25).ToRgb()
+	candidates[5] = b.ScaleSaturation(-0.25).ToRgb()
+	c := hsl.ScaleLightness(-0.5)
+	candidates[6] = c.ToRgb()
+	candidates[7] = c.ScaleLightness(-0.25).ToRgb()
+	candidates[8] = c.ScaleSaturation(-0.25).ToRgb()
+	d := hsl.ScaleSaturation(-0.75)
+	candidates[9] = d.ToRgb()
+	candidates[10] = d.ScaleLightness(-0.25).ToRgb()
+	candidates[11] = d.ScaleSaturation(-0.25).ToRgb()
+
+	foundContrastRatio := -999.0
+	ix := -1
+	for i := 0; i < 12; i++ {
+		contrastRatio := color.ContrastRatio(candidates[i])
+		if contrastRatio > foundContrastRatio {
+			ix = i
+			foundContrastRatio = contrastRatio
+		}
+	}
+	if foundContrastRatio >= 7.0 {
+		return candidates[ix]
+	} else {
+		return candidates[11]
 	}
-	return 16 // black text
 }

utils/color.go

@@ -50,6 +50,106 @@ func (c *Color) Distance(other *Color) float64 {
 		math.Pow(float64(c.Blue-other.Blue), 2)))
 }
 
+func rgbComponentToBoldValue(component int64) float64 {
+	srgb := float64(component) / 255
+	if srgb <= 0.03928 {
+		return srgb / 12.92
+	} else {
+		return math.Pow(((srgb + 0.055) / 1.055), 2.4)
+	}
+}
+
+func (c *Color) Luminance() float64 {
+	return 0.2126*rgbComponentToBoldValue(c.Red) +
+		0.7152*rgbComponentToBoldValue(c.Green) +
+		0.0722*rgbComponentToBoldValue(c.Blue)
+}
+
+func (c *Color) ContrastRatio(other *Color) float64 {
+	L := c.Luminance()
+	otherL := other.Luminance()
+	var L1, L2 float64
+	if L > otherL {
+		L1, L2 = L, otherL
+	} else {
+		L1, L2 = otherL, L
+	}
+	ratio := (L1 + 0.05) / (L2 + 0.05)
+	return ratio
+}
+
+type HslColor struct {
+	Hue        float64
+	Saturation float64
+	Lightness  float64
+}
+
+func (c *Color) ToHsl() *HslColor {
+	rPrime := float64(c.Red) / 255
+	gPrime := float64(c.Green) / 255
+	bPrime := float64(c.Blue) / 255
+	cMax := math.Max(rPrime, math.Max(gPrime, bPrime))
+	cMin := math.Min(rPrime, math.Min(gPrime, bPrime))
+	delta := cMax - cMin
+
+	var H float64
+	if delta == 0 {
+		H = 0
+	} else if cMax == rPrime {
+		H = 60 * math.Mod((gPrime-bPrime)/delta, 6)
+	} else if cMax == gPrime {
+		H = 60 * (((bPrime - rPrime) / delta) + 2)
+	} else {
+		H = 60 * (((rPrime - gPrime) / delta) + 4)
+	}
+	var L float64
+	L = (cMax + cMin) / 2
+	var S float64
+	if delta == 0 {
+		S = 0
+	} else {
+		S = delta / (1 - math.Abs((2*L)-1))
+	}
+
+	return &HslColor{H, S, L}
+}
+
+func (c *HslColor) ToRgb() *Color {
+	C := (1 - math.Abs(2*c.Lightness-1)) * c.Saturation
+	X := C * (1 - math.Abs(math.Mod(c.Hue/60, 2)-1))
+	m := c.Lightness - (C / 2)
+	var rPrime, gPrime, bPrime float64
+	switch {
+	case c.Hue < 60.0:
+		rPrime, gPrime, bPrime = C, X, 0
+	case c.Hue < 120.0:
+		rPrime, gPrime, bPrime = X, C, 0
+	case c.Hue < 180.0:
+		rPrime, gPrime, bPrime = 0, C, X
+	case c.Hue < 240.0:
+		rPrime, gPrime, bPrime = 0, X, C
+	case c.Hue < 300.0:
+		rPrime, gPrime, bPrime = X, 0, C
+	case c.Hue < 360.0:
+		rPrime, gPrime, bPrime = C, 0, X
+	}
+	R := int64((rPrime + m) * 255)
+	G := int64((gPrime + m) * 255)
+	B := int64((bPrime + m) * 255)
+
+	return &Color{R, G, B}
+}
+
+func (c *HslColor) ScaleLightness(x float64) *HslColor {
+	newLightness := math.Round(math.Min(math.Max(0, c.Lightness*(1.0+x)), 1.0))
+	return &HslColor{c.Hue, c.Saturation, newLightness}
+}
+
+func (c *HslColor) ScaleSaturation(x float64) *HslColor {
+	newSaturation := math.Round(math.Min(math.Max(0, c.Lightness*(1.0+x)), 1.0))
+	return &HslColor{c.Hue, newSaturation, c.Lightness}
+}
+
 var x6colorIndexes = [6]int64{0, 95, 135, 175, 215, 255}
 var x6colorCube [216]Color