Feedbacks about AVM tags in Aladin

[Pierre Fernique]

Dear IVOA members,

We are pleased to announce that Aladin (beta release) is now supporting 
the AVM tags in JPEG images (IVOA DRAFT Note 2008 May 14 - 
http://www.virtualastronomy.org/AVM_Version1.1May312008.pdf). 
Concretely, it means that the Aladin users can click-and-drag a Spitzer 
or a Chandra Press Release image from their browser into Aladin and get 
a colored astrometrical calibrated images (from the "WCS" information 
provided in the AVM tags).

However, we were a little bit disappointed by our experience. We have 
several points that we would like to raise to the IVOA community:

1) Presently, the main (unique ?) method for incorporating AVM tags in 
the image is Fits-liberator. It is a Adobe Photoshop plugin. This plugin 
is free but not Photoshop. This plugin has a surprising behavior (bug 
?): it does not adjust the WCS parameters if the user crops, rotates or 
resamples the original image. And concretely, the press release images 
are often badly registered.
It existed a second method presented in the AVM document as an 
Adobe-independent method but unfortunatelly this tool has been 
deprecated when Adobe modified its XMP standard (see below)

2) Technically, the AVM tags have to be embedded in a XMP section inside 
a JPEG comment segment (or equivalent for TIFF and PNG). XMP (eXtensible 
Metadata Platform) is a XML format defined and used by Adobe for 
describing image additional information such as EXIF data related to 
camera picture metadata. Unfortunately, IVOA has no power for 
controlling XMP format. And concretly, XMP is evolving according to the 
Photoshop versioning and at the end some press release images used the 
old XMP format, and other the new one, or also a mixture of both.
- One syntax : <avm:Spatial.ReferencePixel> <rdf:Seq> 
<rdf:li>1044</rdf:li> <rdf:li>983.5</rdf:li> </rdf:Seq> 
</avm:Spatial.ReferencePixel>
- other syntax :  <rdf:Description  
avm:Spatial.Rotation="0.86206593355939" ...>

3) About WCS. Just the simplest WCS method is supported in the AVM tags. 
And for instance the CD matrix which was supported in AVM 1.0 has been 
surprisingly deprecated in AVM 1.1. We have presently only a pixel 
reference, a sky coordinate reference, a scale factor and a rotation. 
The original image size can be also provided in order to compute a bad 
ratio factor for bypass an user resampling (but working only if the 
reference pixel is the image center - see point 1).
To be honnest, the AVM 1.1 draft is also presenting the possibility for 
writing a full FITS header in a dedicated AVM tag but the syntax is not 
described (new line ? or 80 columns ?) and concretely, I was not able to 
find an example with a such tag. Perhaps because it could be difficult 
to put a full FITS header in a simple XML attribut (XMP new syntax).


So, concretly we are a little bit reluctant to extend Aladin for 
writting AVM tags in JPEG output images as XMP format can change again 
without any IVOA agreement (Adobe has a trademark on XMP, and retains 
control over the specification - see the recent example point 1 above). 
For my part, I would prefer to save the original full FITS header into a 
simple comment segment of the image. It is easy to read (string foo.jpg 
on Unix,  notepad2 foo.jpg on windows), easy to generate (jhead or 
wrjpgcom tool or any other jpeg tools) and it avoids to re-invent all 
FITS keywords (and notably the WCS tags) (also implemented in the cited 
Aladin beta release).

Could we compare our experience with some other IVOA members ? Other AVM 
reader or writer experiences ?

Best regards
Pierre Fernique

PS. Aladin beta release 5.901 is available via the official Aladin 
download page.