Since I was apparently using an outdated API and STW is moving to more flashy features, I figured it's time to move on. And it turned out to be very little effort, and actually much snappier on my shared host. Just using thumbnails for a few personal projects, so didn't really need a distributed service, or spooler, or anything fancy in the first place.

I'm sharing the code here for the interested. It comes with no support whatsoever. Using it is fairly simple, but requires above average experience with configuration/code. Further security precautions might be necessary for other setups.

Anyhow, I've deployed a transparent thumbnail generation feature. It sits on a separate virtual host, and expects URLs to be thumbnailed in double-urlencoded form, plus .jpeg suffix:

http://preview.example.com/http%253A%252F%252Fwww.google.com%252F.jpeg

If the image exists, Apache will deliver it straight away. Otherwise a generation script will engage. Here's the magic .htaccess to make that work:

#-- transparent preview.php image generation
#
RewriteEngine On
RewriteBase  /

# Probes if requested filename matches "http.....jpeg"
# If such a files does not exist, then redirect through img generator.
#
RewriteCond  %{REQUEST_URI}    /((http.+)\.(png|jpeg))$
RewriteCond  %{DOCUMENT_ROOT}/%1     !-f
RewriteRule  ^.+$           ./.preview.php?url=%2&type=%3  [L]

# Fallback for empty/failed thumbnails
RewriteCond  %{REQUEST_URI}    /((http.+)\.(png|jpeg))$
RewriteCond  %{DOCUMENT_ROOT}/%1     -f
RewriteCond  %{DOCUMENT_ROOT}/%1     !-s
RewriteRule  ^.+$                    .placeholder.jpeg

#-- security: move preview.php out of image directory and disable exec
#
#Options -Exec -MultiViews
#php_value engine Off

And this is the very simplistic thumbnail generation script (requires wkhtmltoimage):

<?php
/**
 * Generates website preview images.
 *
 * Needs transparent .htaccess RewriteRule and .wkhtmltoimage
 * in the current dir (path should either be writable or suphp
 * enabled).
 *
 * Images must be requested using double-urlencoded notation:
 * http://img.example.com/http%253A%252F%252Fwww.google.com%252F.jpeg
 * (They are stored single-urlencoded on disk. So that that RewriteCond
 * checks can associate them.)
 *
 * A cron job should regularily clean out older thumbnails.
 *
 */


#-- parameters
$restrict_referer = "#^$|^\s*http://([\w.]+\bEXAMPLE.COM|www)/#e";
$type = "jpeg";
# The incoming URL was double-urlencoded, so needs to be cleaned up once,
# filter_var for minimum validation (does not guarantee http URLs)
$url = filter_var(urldecode($_GET["url"]), FILTER_VALIDATE_URL);


#-- restrain referer
if (!preg_match($restrict_referer, $_SERVER["HTTP_REFERER"])) {
   die("Referer mismatch ".header("Status: 403"));
}


#-- execute wkhtmltoimage
if (preg_match("{^https?://[\w\-.?&#=+%/~:,;()\[\]\pL]+$}u", $url)) {

    # escape and wrap cmdline arguments
    # the image filename will be just "URL"+".JPEG"
    $img_fn = urlencode("$url.$type");
    touch($img_fn);
    $img = escapeshellarg($img_fn);
    $url = escapeshellarg($url);

    # run WebKit binary to create image file in current directory
    exec ("nice ../../files/wkhtmltoimage --format '{$type}' --quality 80 ".
          " --zoom 0.32 --width 320 --height 240 ".
          " --disable-local-file-access --disable-javascript ".
          " {$url} {$img} ".            #--disable-plugins
          " 2>/dev/stdout");
    # --crop-h 250 --crop-w 400
}


#-- redirect to image,
#   when generation was successful
if (isset($img_fn) && file_exists($img_fn)) {
    $img = urlencode($img_fn);
    header("Location: ./$img");
    print "<a href=$img>$img</a>";
}

?>

Notice that at least the referer needs updating. Current dir must be writable (suexec/suphp setup or chmod permissions). And again, only reall suitable for minor usage. (I might do a spooler-version, doesn't seem much effort either.)

Oh, and as cron-job something like:

# Refresh every 21 days
#
find /usr/web123/html/preview/ -name "*.jpeg" -mtime +21 -delete

# Remove empty files after 3 days
#
find /usr/web123/html/preview/ -name "*.jpeg" -empty -mtime +2 -delete