public SpriteSheet get(String fileName, int spritesPerX, int spritesPerY,
			int spriteBorderSize, int filter) throws IOException {
		SoftReference<SpriteSheet> ref = loadedSpriteSheets.get(fileName);
		SpriteSheet current = ref == null ? null : ref.get();
		if (current != null) {
			return current;
		} else {
			loadedSpriteSheets.remove(fileName);
			SpriteSheet result = new SpriteSheet(bitmaps.get(fileName, filter),
					spritesPerX, spritesPerY, spriteBorderSize);
			loadedSpriteSheets.put(fileName, new SoftReference<SpriteSheet>(
					result));
			return result;
		}
	}