private byte[] readSubBlock(final InputStream is) throws IOException {
        final int blockSize = 0xff & BinaryFunctions.readByte("blockSize", is, "GIF: corrupt block");

        return BinaryFunctions.readBytes("block", is, blockSize, "GIF: corrupt block");
    }
Extracts embedded XML metadata as XML string.
<p>

@param byteSource File containing image data.
@param params     Map of optional parameters, defined in ImagingConstants.
@return Xmp Xml as String, if present. Otherwise, returns null.
src/main/java/org/apache/commons/imaging/formats/gif/GifImageParser.java