Fix contrast color calculation

Author: nickygerritsen

webapp/src/Twig/TwigExtension.php

@@ -1145,12 +1145,38 @@ public function fileTypeIcon(string $type): string
         return 'fas fa-file-' . $iconName;
     }
 
-    public function problemBadge(ContestProblem $problem, bool $grayedOut = false): string
+    private function relativeLuminance(string $rgb): float
+    {
+        // See https://en.wikipedia.org/wiki/Relative_luminance
+        [$r, $g, $b] = Utils::parseHexColor($rgb);
+
+        [$lr, $lg, $lb] = [
+            pow($r / 255, 2.4),
+            pow($g / 255, 2.4),
+            pow($b / 255, 2.4),
+        ];
+
+        return 0.2126 * $lr + 0.7152 * $lg + 0.0722 * $lb;
+    }
+
+    private function apcaContrast(string $fgColor, string $bgColor): float
+    {
+        // Based on WCAG 3.x (https://www.w3.org/TR/wcag-3.0/)
+        $luminanceForeground = $this->relativeLuminance($fgColor);
+        $luminanceBackground = $this->relativeLuminance($bgColor);
+
+        $contrast = ($luminanceBackground > $luminanceForeground)
+            ? (pow($luminanceBackground, 0.56) - pow($luminanceForeground, 0.57)) * 1.14
+            : (pow($luminanceBackground, 0.65) - pow($luminanceForeground, 0.62)) * 1.14;
+
+        return round($contrast * 100, 2);
+    }
+
+    /**
+     * @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.
@@ -1160,8 +1186,24 @@ 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';
+        // Pick the text color with the biggest absolute contrast.
+        $contrastWithWhite = $this->apcaContrast('#ffffff', $rgb);
+        $contrastWithBlack = $this->apcaContrast('#000000', $rgb);
+
+        $foreground = (abs($contrastWithBlack) > abs($contrastWithWhite)) ? '#000000' : '#ffffff';
+
+        return [$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';
@@ -1181,17 +1223,9 @@ 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';