[trans] - Model comments

[CresitelloDittmar, Mark]

Gerard/All,

Just attempting to separate Transform model discussion separate from the
vo-dml Composition rule
Gerard's last mail with suggested model changes:
   http://mail.ivoa.net/pipermail/dm/2020-March/005994.html

1) What AST implements as Frame is modeled in the coords model..
    The version coming out next (post RFC comments), uses CoordSys to
combine the "Frame" and "CoordSpace" information which == ast:Frame
2) The Transform model has also been updated to include a "SystemMap" which
references source and target coords:CoordSys with a trans:TMapping
    This has not been released on Volute, because David is working through
the implementation thereof with AST to look for issues in compatibility
with ast:FrameSet.

In the coords model, the visibility of individual axes is severely
limited.  For the most part, our data reside in standard coordinate spaces,
and the specific axes (other than Pixel), are rarely explicitly defined.
So the AxisMap design doesn't, I think, play well with the typical data
product.

I also don't see that supporting the compound mapping, though it may be
modifiable to do so.
The scenario for the compound mapping is something like
     Frame A  (x,y) ==> [ shift ] => [scale]  => [Permute] => | Poly2D | =>
(X',Y') Frame B
                                                              | Poly2D |

With a similar set of operations for the 'inverse' path.  There is no
"Frame" spec between each individual operation.

Mark
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.ivoa.net/pipermail/dm/attachments/20200310/ccee32aa/attachment.html>