include some extra data to validate the state of the object.

they are compatible.  This was done in a way that is backwards compatible with
previous versions of dlib.  That is, we can still load data serialized by
previous dlib versions.  However, older versions of dlib can't load the new
serialization format.

matrices of float values.

out what the transformation matrix is.

for ranking functions which output constant values.

pixel types without any information loss.

images with unsigned char pixels.

Previously, we used an ASCII based format.  It now uses a much more efficient
binary format.  The deserialization routines have been made backwards
compatible with the previous format.  So dlib can still deserialize older data
but older software won't be able to read the new format.

recent changes make its feature representation incompatible with
previous versions.