Allow for RGBA

Author: vmcj

webapp/src/Utils/Utils.php

@@ -343,19 +343,25 @@ public static function convertToColor(string $hex): ?string
     }
 
     /**
-     * Parse a hex color into it's three RGB values.
+     * Parse a hex color into it's four RGBA values.
      *
-     * @return array{int, int, int}
+     * @return array{int, int, int, int}
      */
     public static function parseHexColor(string $hex): array
     {
+        $tmp = substr($hex, 1);
+        if (strlen($tmp) % 3 == 0) {
+            $mult = strlen($tmp) / 3;
+            $hex += 'f' * $mult;
+        }
         // Source: https://stackoverflow.com/a/21966100
         $length = (strlen($hex) - 1) / 3;
         $fact = [17, 1, 0.062272][$length - 1];
         return [
             (int)round(hexdec(substr($hex, 1, $length)) * $fact),
             (int)round(hexdec(substr($hex, 1 + $length, $length)) * $fact),
             (int)round(hexdec(substr($hex, 1 + 2 * $length, $length)) * $fact)
+            (int)round(hexdec(substr($hex, 1 + 3 * $length, $length)) * $fact)
         ];
     }
 
@@ -371,21 +377,47 @@ public static function componentToHex(int $component): string
     /**
      * Convert an RGB triple into a CSS hex color.
      *
-     * @param array{int, int, int} $color
+     * @param array{int, int, int}|array{int, int, int, int} $color
      */
     public static function rgbToHex(array $color): string
     {
         $result = "#";
+        if (count($color) === 3) {
+            $color[] = 255;
+	} elseif (count($color) !== 4) {
+            throw Exception("Invalid RGB number.");
+	}
         for (int $i=0; $i<count($color); $i++) {
             $result += static::componentToHex($color[$i]);
         }
         return $result;
     }
 
-    public static function relativeLuminance(string $rgb): float
+    /**
+     * @param array{int, int, int}|array{int, int, int, int} $rgba
+     * @param array{int, int, int}                           $bg
+     *
+     * @return array{int, int, int}
+     */
+    public static function blendAlphaBackground(array $rgba, array $bg): array
+    {
+        if (count($rgba) === 3) {
+            return $rgba;
+	} elseif (count($rgba) === 4) {
+            $result = [];
+            for (int $i=0; $i<3; $i++) {
+                $result[] = $rgba[$i] * $rgba[3] + $bg[$i] * (1 - $rgba[3]);
+            }
+	    return $result;
+	}
+	throw Exception("Invalid RGB number.");
+    }
+
+    public static function relativeLuminance(string $hexRGB, string $rgb_background = "#fff"): float
     {
         // See https://en.wikipedia.org/wiki/Relative_luminance
-        [$r, $g, $b] = static::parseHexColor($rgb);
+        $rgba = static::parseHexColor($hexRGB);
+        [$r, $g, $b] = static::blendAlphaBackground($rgba, $rgb_background);
 
         [$lr, $lg, $lb] = [
             pow($r / 255, 2.4),