add color extensions for WCAG-conform contrast ratio calculation

2021-02-16 12:31:57 +01:00
parent 9b40fed92e
commit f0981046b0
1 changed files with 61 additions and 0 deletions
@@ -0,0 +1,61 @@
+using System;
+using System.Drawing;
+using System.Linq;
+
+namespace zero.Core.Extensions
+{
+  public static class ColorExtensions
+  {
+    /// <summary>
+    /// Get color object from a hex string
+    /// </summary>
+    public static Color FromHex(this string color)
+    {
+      if (color.StartsWith("#"))
+      {
+        color = color[1..];
+      }
+
+      if (color.Length != 6)
+      {
+        throw new Exception("Hex color has to be in format #RRGGBB");
+      }
+
+      return Color.FromArgb(
+        int.Parse(color.Substring(0, 2), System.Globalization.NumberStyles.HexNumber),
+        int.Parse(color.Substring(2, 2), System.Globalization.NumberStyles.HexNumber),
+        int.Parse(color.Substring(4, 2), System.Globalization.NumberStyles.HexNumber)
+      );
+    }
+
+
+    /// <summary>
+    /// Get contrast ratio between two color values
+    /// <see cref="https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js#L95" />
+    /// </summary>
+    public static double GetContrastRatio(this Color color1, Color color2)
+    {
+      double lum1 = color1.GetLuminance();
+      double lum2 = color2.GetLuminance();
+      double brightest = Math.Max(lum1, lum2);
+      double darkest = Math.Min(lum1, lum2);
+      return (brightest + 0.05) / (darkest + 0.05);
+    }
+
+
+    /// <summary>
+    /// Get luminance value of a color based on WCAG-conform JS implementation
+    /// <see cref="https://github.com/LeaVerou/contrast-ratio/blob/gh-pages/color.js#L42" />
+    /// </summary>
+    public static double GetLuminance(this Color color)
+    {
+      double[] a = new double[3] { color.R, color.G, color.B }.Select(v =>
+      {
+        v /= 255;
+        return v <= 0.03928 ? v / 12.92 : Math.Pow((v + 0.055) / 1.055, 2.4);
+      }).ToArray();
+
+      return a[0] * 0.2126 + a[1] * 0.7152 + a[2] * 0.0722;
+    }
+  }
+}