Only generate previews in powers of 4 and set min

Before we'd round up all preview request to their nearest power of two.
This resulted still in a lot of possible images. Generating a lot of
server load and taking up a lot of space.

This moves it to previews to be powers of 4: 64, 256, 1024 and 4096
Also the first two powers are always skipped (4, 16) as it doesn't make
sense to generate previews for that.

We cache preview pretty agressively and I feel this is a better
tradeoff.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

1 files changed, 11 insertions, 7 deletions

diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index 86579e3480b..1f7decf2b79 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -298,19 +298,23 @@ class Generator {
 
 		if ($height !== $maxHeight && $width !== $maxWidth) {
 			/*
-			 * Scale to the nearest power of two
+			 * Scale to the nearest power of four
 			 */
-			$pow2height = 2 ** ceil(log($height) / log(2));
-			$pow2width = 2 ** ceil(log($width) / log(2));
+			$pow4height = 4 ** ceil(log($height) / log(4));
+			$pow4width = 4 ** ceil(log($width) / log(4));
 
-			$ratioH = $height / $pow2height;
-			$ratioW = $width / $pow2width;
+			// Minimum size is 64
+			$pow4height = max($pow4height, 64);
+			$pow4width = max($pow4width, 64);
+
+			$ratioH = $height / $pow4height;
+			$ratioW = $width / $pow4width;
 
 			if ($ratioH < $ratioW) {
-				$width = $pow2width;
+				$width = $pow4width;
 				$height /= $ratioW;
 			} else {
-				$height = $pow2height;
+				$height = $pow4height;
 				$width /= $ratioH;
 			}
 		}