The main issue in supporting embedding is the protocol
used for commnication between the parent and child applications --
among other things, this needs to support negotiation about
window placement, color, resources, transfers of focus,
propogation of inputs, support for dialogue windows,
and menu integration.
In the Windows world, OLE 2 supports this ability;
on the Mac, Apple will provide OpenDoc;
on X platforms, the winner is still unclear.
If you want to take a look at the way embedding needs
to be dealt with in the X environment, you might be
interested in the work done by Jan Newmarch, from the
University of Canberra, when he took his sabbatical at OSF.
Jan built an embedding protocol on top of ICE, a virtual
message protocol layered over TCP, and released as part
of X11R6. The code plus demos is available from Jan
(jan@ise.canberra.edu.au), and possibly may be available
from the contrib area of the X Consortium.
Jan will be giving a talk at the upcoming X Conference
(Jan 30-Feb 1 in Boston) about this work.
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On the issue of filters -- as you note, one of the main
problems is characterizing the degradation factor
in translating from one format to another.
You might want to take a look at my article in Issue 5
of the X Resource, based on the talk I gave at the 1993
X Conference, on Describing Formats for X-based Data
Interchange.
It deals (among other things) with the more general problem of
characterizing the information carried in a selected part of a
document, and is a good starting place for dealing with
degradation issues as well.
-- Ellis