[vo-dml] Clarification on composition rule..

[David Berry]

On Tue, 10 Mar 2020 at 10:54, Gerard Lemson <glemson1 at jhu.edu> wrote:

> As to the case that started this discussion, I am still wondering whether the issue could be resolved more elegantly once an explicit representation of the things that are being mapped/transferred is added to the model.

>From the point of view of working with a similar system (AST) for many
years, it has been very useful to separate the representation of the
variables being transformed  ("Frames") from the representation of the
transformation itself ("Mappings") . Doing so means that you can
combine Mapping objects together into compound Mappings in various
ways without needing to worry about  the associated Frames. Most real
world Mappings are formed by combining lots of simpler atomic mappings
together into a compound Mapping. If each atomic Mapping included a
description of the associated input and output variables, you would
end up with a large amount of redundant information in the compound
Mapping.

Of course, at the end of the day you need the variable descriptions to
be stored some where. We have found it useful to define a higher level
"FrameSet" class for this purpose, which contains a tree structure of
related Frames connected together by (potentially compound) Mappings.
So for instance a FITS WCS header could be described by a FrameSet
that contained separate Frames describing pixel coordinates, WCS
coordinates, focal plane coordinates, CCD coordinates, etc, together
with Mappings that transform positions between each of these
coordinate systems.

David