We revisit most of the concerns about metadata from Lecture 7 as they
apply to non-text and multimedia objects and resources, but some new
challenges arise because of the temporal character of audio and video
and the semantic opacity of the content. Because multimedia content
can't be (easily) processed to understand what the object means, there
is a "semantic gap" between the descriptions that people assign to
multimedia content and those that can be assigned by computers or
automated processes. On the other hand, technology for creating
multimedia can easily record contextual metadata at the same time.
Thesauri and other aids for professional "metadata makers" are
invaluable but rarely used by ordinary people when they tag photos or
videos.