contrast function first PR

Author: FerrinThreatt

src/color/p5.Color.js

@@ -28,6 +28,7 @@ import {
 
   P3
 } from 'colorjs.io/fn';
+
 import HSBSpace from './color_spaces/hsb.js';
 
 const map = (n, start1, stop1, start2, stop2, clamp) => {
@@ -41,6 +42,17 @@ const map = (n, start1, stop1, start2, stop2, clamp) => {
 
 const serializationMap = {};
 
+const SRGB_THRESHOLD = 0.03928;
+const SRGB_DIVISOR = 12.92;
+const GAMMA_OFFSET = 0.055;
+const GAMMA_DIVISOR = 1.055;
+const GAMMA_EXPONENT = 2.4;
+const LUMINANCE_RED = 0.2126;
+const LUMINANCE_GREEN = 0.7152;
+const LUMINANCE_BLUE = 0.0722;
+const EPSILON = 0.05;
+const THRESHOLD = 4.5;
+
 class Color {
   // Reference to underlying color object depending on implementation
   // Not meant to be used publicly unless the implementation is known for sure
@@ -291,6 +303,58 @@ class Color {
     return colorString;
   }
 
+/**
+   * Checks if two colors contrast ratio is WCAG 2.1 compliant and returns the ratio
+   *
+   * @param {Color} other
+   * @returns {{ ratio: Number, passes: boolean }}
+   * @example
+   * <div>
+   * <code>
+   * 
+   * function setup() {
+   *   // Define colors
+   *   let color1 = color(255, 255, 255);
+   *   let color2 = color(0);
+   *  
+   *   // Test for contrast
+   *   let result = color1.contrast(color2)
+   * 
+   *   console.log(result)
+   * 
+   * }
+   * </code>
+   * </div>
+   */
+
+    contrast(new_other) { 
+      //Constants for contrast ratio cutoffs 
+      const CONTRAST_AA_NORMAL = 4.5;
+      const CONTRAST_AA_LARGE = 3.0;
+      const CONTRAST_AAA_NORMAL = 7.0;
+      const CONTRAST_AAA_LARGE = 4.5;    
+
+      //helper function for luminance aka brightness
+      let luminance = (c) => {
+        //putting RGB values into array and convert colors to value between 0-1
+        let rgb = [red(c), green(c), blue(c)].map(v => {
+          v /= 255;
+          return v <= SRGB_THRESHOLD
+            ? v / SRGB_DIVISOR
+            : Math.pow((v + GAMMA_OFFSET) / GAMMA_DIVISOR, GAMMA_EXPONENT);
+        });
+        return LUMINANCE_RED * rgb[0] + LUMINANCE_GREEN * rgb[1] + LUMINANCE_BLUE * rgb[2];
+      };
+
+      let l1 = luminance(this);
+      let l2 = luminance(new_other);
+      //Epsilon to avoid dividing by zero
+      let ratio = (Math.max(l1, l2) + EPSILON) / (Math.min(l1, l2) + EPSILON);
+
+      return { ratio, pass: ratio >= THRESHOLD };
+    
+    };
+
   /**
    * Sets the red component of a color.
    *