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.