MDL-32471 introducing generate_image_thumbnail() function

This helper function uses GD library to generate a thumbnail of the
given image file.

lib/gdlib.php

@@ -222,3 +222,88 @@ function process_new_icon($context, $component, $filearea, $itemid, $originalfil
     return true;
 }
 
+/**
+ * Generates a thumbnail for the given image
+ *
+ * If the GD library has at least version 2 and PNG support is available, the returned data
+ * is the content of a transparent PNG file containing the thumbnail. Otherwise, the function
+ * returns contents of a JPEG file with black background containing the thumbnail.
+ *
+ * @param string $filepath the full path to the original image file
+ * @param int $width the width of the requested thumbnail
+ * @param int $height the height of the requested thumbnail
+ * @return string|bool false if a problem occurs, the thumbnail image data otherwise
+ */
+function generate_image_thumbnail($filepath, $width, $height) {
+    global $CFG;
+
+    if (empty($CFG->gdversion) or empty($filepath) or empty($width) or empty($height)) {
+        return false;
+    }
+
+    $imageinfo = getimagesize($filepath);
+
+    if (empty($imageinfo)) {
+        return false;
+    }
+
+    $originalwidth = $imageinfo[0];
+    $originalheight = $imageinfo[1];
+
+    if (empty($originalwidth) or empty($originalheight)) {
+        return false;
+    }
+
+    $original = imagecreatefromstring(file_get_contents($filepath));
+
+    if (function_exists('imagepng')) {
+        $imagefnc = 'imagepng';
+        $filters = PNG_NO_FILTER;
+        $quality = 1;
+    } else if (function_exists('imagejpeg')) {
+        $imagefnc = 'imagejpeg';
+        $filters = null;
+        $quality = 90;
+    } else {
+        debugging('Neither JPEG nor PNG are supported at this server, please fix the system configuration.');
+        return false;
+    }
+
+    if (function_exists('imagecreatetruecolor') and $CFG->gdversion >= 2) {
+        $thumbnail = imagecreatetruecolor($width, $height);
+        if ($imagefnc === 'imagepng') {
+            imagealphablending($thumbnail, false);
+            imagefill($thumbnail, 0, 0, imagecolorallocatealpha($thumbnail, 0, 0, 0, 127));
+            imagesavealpha($thumbnail, true);
+        }
+    } else {
+        $thumbnail = imagecreate($width, $height);
+    }
+
+    $ratio = min($width / $originalwidth, $height / $originalheight);
+
+    if ($ratio < 1) {
+        $targetwidth = floor($originalwidth * $ratio);
+        $targetheight = floor($originalheight * $ratio);
+    } else {
+        // do not enlarge the original file if it is smaller than the requested thumbnail size
+        $targetwidth = $originalwidth;
+        $targetheight = $originalheight;
+    }
+
+    $dstx = floor(($width - $targetwidth) / 2);
+    $dsty = floor(($height - $targetheight) / 2);
+
+    imagecopybicubic($thumbnail, $original, $dstx, $dsty, 0, 0, $targetwidth, $targetheight, $originalwidth, $originalheight);
+
+    ob_start();
+    if (!$imagefnc($thumbnail, null, $quality, $filters)) {
+        ob_end_clean();
+        return false;
+    }
+    $data = ob_get_clean();
+    imagedestroy($original);
+    imagedestroy($thumbnail);
+
+    return $data;
+}