snapshot of https://github.com/go-gitea/gitea.git (main, 2026-09-15; blobs > 131072B omitted)

modules/util/color.go

@@ -0,0 +1,74 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"fmt"
+	"math"
+	"strconv"
+	"strings"
+)
+
+// HexToRBGColor parses 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
+	if strings.HasPrefix(colorString, "#") {
+		hexString = colorString[1:]
+	}
+	// only support transfer of rgb, rgba, rrggbb and rrggbbaa
+	// if not in these formats, use default values 0, 0, 0
+	if len(hexString) != 3 && len(hexString) != 4 && len(hexString) != 6 && len(hexString) != 8 {
+		return 0, 0, 0
+	}
+	if len(hexString) == 3 || len(hexString) == 4 {
+		hexString = fmt.Sprintf("%c%c%c%c%c%c", hexString[0], hexString[0], hexString[1], hexString[1], hexString[2], hexString[2])
+	}
+	if len(hexString) == 8 {
+		hexString = hexString[0:6]
+	}
+	color, err := strconv.ParseUint(hexString, 16, 32)
+	color32 := uint32(color)
+	if err != nil {
+		return 0, 0, 0
+	}
+	r := float64(uint8(0xFF & (color32 >> 16)))
+	g := float64(uint8(0xFF & (color32 >> 8)))
+	b := float64(uint8(0xFF & color32))
+	return r, g, b
+}
+
+// linearizeChannel undoes the sRGB transfer function, channel is in 0..255 range
+func linearizeChannel(channel float64) float64 {
+	srgb := channel / 255
+	if srgb <= 0.04045 {
+		return srgb / 12.92
+	}
+	return math.Pow((srgb+0.055)/1.055, 2.4)
+}
+
+// getRelativeLuminance returns relative luminance for a SRGB color - https://www.w3.org/TR/WCAG20/#relativeluminancedef
+// Keep this in sync with web_src/js/utils/color.ts
+func getRelativeLuminance(color string) float64 {
+	r, g, b := HexToRBGColor(color)
+	return 0.2126*linearizeChannel(r) + 0.7152*linearizeChannel(g) + 0.0722*linearizeChannel(b)
+}
+
+// GetPerceivedBrightness weights the gamma-encoded channels, so it stays in the same space as the
+// raw channels and can scale them proportionally. Not a luminance, don't use it for contrast.
+func GetPerceivedBrightness(color string) float64 {
+	r, g, b := HexToRBGColor(color)
+	return (0.2126729*r + 0.7151522*g + 0.0721750*b) / 255
+}
+
+func UseLightText(backgroundColor string) bool {
+	return getRelativeLuminance(backgroundColor) < 0.36 // matches APCA better than WCAG's own 0.179
+}
+
+// ContrastColor returns a black or white foreground color that the highest contrast ratio.
+func ContrastColor(backgroundColor string) string {
+	if UseLightText(backgroundColor) {
+		return "#fff"
+	}
+	return "#000"
+}