feat(theming): Only convert a background image if it benefits from it

* WebP images are generally quite small, converting to pngs would increase
  the filesize a lot.
* Small JPEG and PNG images do not benefit from any conversion, so skip it.
* JPEG images will get quite bigger when converted to PNG so instead convert to progressive JPEG

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

apps/theming/lib/ImageManager.php

@@ -243,7 +243,7 @@ class ImageManager {
 			throw new \Exception('Unsupported image type');
 		}
 
-		if ($key === 'background' && strpos($detectedMimeType, 'image/svg') === false && strpos($detectedMimeType, 'image/gif') === false) {
+		if ($key === 'background' && $this->shouldOptimizeBackgroundImage($detectedMimeType, filesize($tmpFile))) {
 			// Optimize the image since some people may upload images that will be
 			// either to big or are not progressive rendering.
 			$newImage = @imagecreatefromstring(file_get_contents($tmpFile));
@@ -258,7 +258,11 @@ class ImageManager {
 			$outputImage = imagescale($newImage, $newWidth, $newHeight);
 
 			imageinterlace($outputImage, 1);
-			imagepng($outputImage, $tmpFile, 8);
+			if (strpos($detectedMimeType, 'image/jpeg') !== false) {
+				imagejpeg($outputImage, $tmpFile, 90);
+			} else {
+				imagepng($outputImage, $tmpFile, 8);
+			}
 			imagedestroy($outputImage);
 
 			$target->putContent(file_get_contents($tmpFile));
@@ -269,6 +273,31 @@ class ImageManager {
 		return $detectedMimeType;
 	}
 
+	/**
+	 * Decide whether an image benefits from shrinking and reconverting
+	 *
+	 * @param string $mimeType the mime type of the image
+	 * @param int $contentSize size of the image file
+	 * @return bool
+	 */
+	private function shouldOptimizeBackgroundImage(string $mimeType, int $contentSize): bool {
+		// Do not touch SVGs
+		if (strpos($mimeType, 'image/svg') !== false) {
+			return false;
+		}
+		// GIF does not benefit from converting
+		if (strpos($mimeType, 'image/gif') !== false) {
+			return false;
+		}
+		// WebP also does not benefit from converting
+		// We could possibly try to convert to progressive image, but normally webP images are quite small
+		if (strpos($mimeType, 'image/webp') !== false) {
+			return false;
+		}
+		// As a rule of thumb background images should be max. 150-300 KiB, small images do not benefit from converting
+		return $contentSize > 150000;
+	}
+
 	/**
 	 * Returns a list of supported mime types for image uploads.
 	 * "favicon" images are only allowed to be SVG when imagemagick with SVG support is available.

apps/theming/tests/ImageManagerTest.php

@@ -337,9 +337,12 @@ class ImageManagerTest extends TestCase {
 
 	public function dataUpdateImage() {
 		return [
-			['background', __DIR__ . '/../../../tests/data/testimage.png', true, true],
-			['background', __DIR__ . '/../../../tests/data/testimage.png', false, true],
-			['background', __DIR__ . '/../../../tests/data/testimage.jpg', true, true],
+			['background', __DIR__ . '/../../../tests/data/testimage.png', true, false],
+			['background', __DIR__ . '/../../../tests/data/testimage.png', false, false],
+			['background', __DIR__ . '/../../../tests/data/testimage.jpg', true, false],
+			['background', __DIR__ . '/../../../tests/data/testimage.webp', true, false],
+			['background', __DIR__ . '/../../../tests/data/testimage-large.jpg', true, true],
+			['background', __DIR__ . '/../../../tests/data/testimage-wide.png', true, true],
 			['logo', __DIR__ . '/../../../tests/data/testimagelarge.svg', true, false],
 		];
 	}