Fix contrast color calculation

Author: nickygerritsen

webapp/src/Twig/TwigExtension.php

@@ -1148,12 +1148,11 @@ public function fileTypeIcon(string $type): string
         return 'fas fa-file-' . $iconName;
     }
 
-    public function problemBadge(ContestProblem $problem, bool $grayedOut = false): string
+    /**
+     * @return array{string, string}
+     */
+    private function hexToForegroundAndBorder(string $rgb): array
     {
-        $rgb = Utils::convertToHex($problem->getColor() ?? '#ffffff');
-        if ($grayedOut || empty($rgb)) {
-            $rgb = Utils::convertToHex('whitesmoke');
-        }
         $background = Utils::parseHexColor($rgb);
 
         // Pick a border that's a bit darker.
@@ -1163,8 +1162,29 @@ public function problemBadge(ContestProblem $problem, bool $grayedOut = false):
         $darker[2] = max($darker[2] - 64, 0);
         $border    = Utils::rgbToHex($darker);
 
-        // Pick the foreground text color based on the background color.
-        $foreground = ($background[0] + $background[1] + $background[2] > 450) ? '#000000' : '#ffffff';
+        [$r, $g, $b] = $background;
+
+        // Calculate relative luminance
+        $r = ($r / 255 <= 0.03928) ? ($r / 255) / 12.92 : pow(($r / 255 + 0.055) / 1.055, 2.4);
+        $g = ($g / 255 <= 0.03928) ? ($g / 255) / 12.92 : pow(($g / 255 + 0.055) / 1.055, 2.4);
+        $b = ($b / 255 <= 0.03928) ? ($b / 255) / 12.92 : pow(($b / 255 + 0.055) / 1.055, 2.4);
+
+        $luminance = 0.2126 * $r + 0.7152 * $g + 0.0722 * $b;
+
+        $foreground = ($luminance > 0.179) ? '#000000' : '#FFFFFF';
+
+        return [$foreground, $border];
+    }
+
+    public function problemBadge(ContestProblem $problem, bool $grayedOut = false): string
+    {
+        $rgb = Utils::convertToHex($problem->getColor() ?? '#ffffff');
+        if ($grayedOut || empty($rgb)) {
+            $rgb = Utils::convertToHex('whitesmoke');
+        }
+
+        [$foreground, $border] = $this->hexToForegroundAndBorder($rgb);
+
         if ($grayedOut) {
             $foreground = 'silver';
             $border = 'linen';
@@ -1184,17 +1204,9 @@ public function problemBadgeMaybe(ContestProblem $problem, ScoreboardMatrixItem
         if (!$matrixItem->isCorrect || empty($rgb)) {
             $rgb = Utils::convertToHex('whitesmoke');
         }
-        $background = Utils::parseHexColor($rgb);
 
-        // Pick a border that's a bit darker.
-        $darker = $background;
-        $darker[0] = max($darker[0] - 64, 0);
-        $darker[1] = max($darker[1] - 64, 0);
-        $darker[2] = max($darker[2] - 64, 0);
-        $border    = Utils::rgbToHex($darker);
+        [$foreground, $border] = $this->hexToForegroundAndBorder($rgb);
 
-        // Pick the foreground text color based on the background color.
-        $foreground = ($background[0] + $background[1] + $background[2] > 450) ? '#000000' : '#ffffff';
         if (!$matrixItem->isCorrect) {
             $foreground = 'silver';
             $border = 'linen';