enh(theming): Adjust color utils to work as specified by WCAG (color contrast and luma calculation)

Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>

apps/theming/lib/Themes/CommonThemeTrait.php

@@ -40,7 +40,7 @@ trait CommonThemeTrait {
 	 */
 	protected function generatePrimaryVariables(string $colorMainBackground, string $colorMainText): array {
 		$isBrightColor = $this->util->isBrightColor($colorMainBackground);
-		$colorPrimaryElement = $this->util->elementColor($this->primaryColor, $isBrightColor);
+		$colorPrimaryElement = $this->util->elementColor($this->primaryColor, $isBrightColor, $colorMainBackground);
 		$colorPrimaryLight = $this->util->mix($colorPrimaryElement, $colorMainBackground, -80);
 		$colorPrimaryElementLight = $this->util->mix($colorPrimaryElement, $colorMainBackground, -80);
 

apps/theming/lib/Util.php

@@ -57,7 +57,7 @@ class Util {
 	 * @return bool
 	 */
 	public function invertTextColor(string $color): bool {
-		return $this->isBrightColor($color);
+		return $this->colorContrast($color, '#ffffff') < 4.5;
 	}
 
 	/**
@@ -81,7 +81,28 @@ class Util {
 	 * @param ?bool $brightBackground
 	 * @return string
 	 */
-	public function elementColor($color, ?bool $brightBackground = null) {
+	public function elementColor($color, ?bool $brightBackground = null, ?string $backgroundColor = null) {
+		if ($backgroundColor !== null) {
+			$brightBackground = $brightBackground ?? $this->isBrightColor($backgroundColor);
+			// Minimal amount that is possible to change the luminance
+			$epsilon = 1.0 / 255.0;
+			// Current iteration to prevent infinite loops
+			$iteration = 0;
+			// We need to keep blurred backgrounds in mind which might be mixed with the background
+			$blurredBackground = $this->mix($backgroundColor, $brightBackground ? $color : '#ffffff', 66);
+			$contrast = $this->colorContrast($color, $blurredBackground);
+
+			// Min. element contrast is 3:1 but we need to keep hover states in mind -> min 3.2:1
+			while ($contrast < 3.2 && $iteration++ < 100) {
+				$hsl = Color::hexToHsl($color);
+				$hsl['L'] = max(0, min(1, $hsl['L'] + ($brightBackground ? -$epsilon : $epsilon)));
+				$color = '#' . Color::hslToHex($hsl);
+				$contrast = $this->colorContrast($color, $blurredBackground);
+			}
+			return $color;
+		}
+
+		// Fallback for legacy calling
 		$luminance = $this->calculateLuminance($color);
 
 		if ($brightBackground !== false && $luminance > 0.8) {
@@ -139,12 +160,38 @@ class Util {
 	}
 
 	/**
+	 * Calculate the Luma according to WCAG 2
+	 * http://www.w3.org/TR/WCAG20/#relativeluminancedef
 	 * @param string $color rgb color value
 	 * @return float
 	 */
 	public function calculateLuma(string $color): float {
-		[$red, $green, $blue] = $this->hexToRGB($color);
-		return (0.2126 * $red + 0.7152 * $green + 0.0722 * $blue) / 255;
+		$rgb = $this->hexToRGB($color);
+
+		// Normalize the values by converting to float and applying the rules from WCAG2.0
+		$rgb = array_map(function (int $color) {
+			$color = $color / 255.0;
+			if ($color <= 0.03928) {
+				return $color / 12.92;
+			} else {
+				return pow((($color + 0.055) / 1.055), 2.4);
+			}
+		}, $rgb);
+
+		[$red, $green, $blue] = $rgb;
+		return (0.2126 * $red + 0.7152 * $green + 0.0722 * $blue);
+	}
+
+	/**
+	 * Calculat the color contrast according to WCAG 2
+	 * http://www.w3.org/TR/WCAG20/#contrast-ratiodef
+	 * @param string $color1 The first color
+	 * @param string $color2 The second color
+	 */
+	public function colorContrast(string $color1, string $color2): float {
+		$luminance1 = $this->calculateLuma($color1) + 0.05;
+		$luminance2 = $this->calculateLuma($color2) + 0.05;
+		return max($luminance1, $luminance2) / min($luminance1, $luminance2);
 	}
 
 	/**

apps/theming/tests/UtilTest.php

@@ -35,19 +35,20 @@ use OCP\Files\NotFoundException;
 use OCP\Files\SimpleFS\ISimpleFile;
 use OCP\Files\SimpleFS\ISimpleFolder;
 use OCP\IConfig;
+use PHPUnit\Framework\MockObject\MockObject;
 use Test\TestCase;
 
 class UtilTest extends TestCase {
 
 	/** @var Util */
 	protected $util;
-	/** @var IConfig */
+	/** @var IConfig|MockObject */
 	protected $config;
-	/** @var IAppData */
+	/** @var IAppData|MockObject */
 	protected $appData;
-	/** @var IAppManager */
+	/** @var IAppManager|MockObject */
 	protected $appManager;
-	/** @var ImageManager */
+	/** @var ImageManager|MockObject */
 	protected $imageManager;
 
 	protected function setUp(): void {
@@ -59,11 +60,29 @@ class UtilTest extends TestCase {
 		$this->util = new Util($this->config, $this->appManager, $this->appData, $this->imageManager);
 	}
 
+	public function dataColorContrast() {
+		return [
+			['#ffffff', '#FFFFFF', 1],
+			['#000000', '#000000', 1],
+			['#ffffff', '#000000', 21],
+			['#000000', '#FFFFFF', 21],
+			['#9E9E9E', '#353535', 4.578],
+			['#353535', '#9E9E9E', 4.578],
+		];
+	}
+
+	/**
+	 * @dataProvider dataColorContrast
+	 */
+	public function testColorContrast(string $color1, string $color2, $contrast) {
+		$this->assertEqualsWithDelta($contrast, $this->util->colorContrast($color1, $color2), .001);
+	}
+
 	public function dataInvertTextColor() {
 		return [
 			['#ffffff', true],
 			['#000000', false],
-			['#0082C9', false],
+			['#00679e', false],
 			['#ffff00', true],
 		];
 	}