protected function computeMarkerImages(Map $map)
    {
        $markerImages = array();

        foreach ($map->getMarkers() as $marker) {
            if ($marker->hasIcon() && !in_array($marker->getIcon(), $markerImages)) {
                $markerImages[] = $marker->getIcon();
            }

            if ($marker->hasShadow() && !in_array($marker->getShadow(), $markerImages)) {
                $markerImages[] = $marker->getShadow();
            }
        }

        return $markerImages;
    }