[NeXus-tech] How to write the "axes" attribute in this case?

[Tobias.Richter at diamond.ac.uk]

There is no ratified solution that works for cases like that. The canSAS way was proposed in 2012 but the decision was postponed.
I've recently documented that for Herbert (attached). We're increasingly writing that out here at Diamond as well. It would be good to get some more support for that, especially since you co-authored that, Pete. ;-)

Tobias


________________________________________
From: NeXus-tech [nexus-tech-bounces at nexusformat.org] on behalf of Pete Jemian [prjemian at gmail.com]
Sent: 19 July 2014 15:45
To: NeXus Technical sub-committee list
Subject: [NeXus-tech] How to write the "axes" attribute in this case?

What is the recommended way to do this?

Consider a pinhole SAS experiment with 2-D arrays of intensity data "I"
and reciprocal space vector magnitude |Q| as "Q", one way to provide
this data using the basic NeXus structure is:

/: NeXus HDF5 file root
   entry:NXentry
     data:NXdata
       Q:NX_NUMBER[nx,ny]
       I:NX_NUMBER[nx,ny]
         @signal=1

How to write the/entry/data/I/@axes attribute?

The default visualization of I(Q) is parametric in this case, where the
Q value associated with a given I are found by corresponding row and
columns in the two arrays.

Since /entry/data/Q is a 2-D array, we cannot use it in the "axes"
attribute of /entry/data/I because, in the NXdata specification, axes
"defines the names of the dimension scales (independent axes) for this
data set as a colon-delimited array."  Dimension scales, each defining
an axis of the data, are rank=1 arrays.  Since the values in any column
of "Q" are different than any other column, we cannot simply create two
1-D arrays and use them as dimension scales in the "axes" attribute.

Is it permissable to leave out the "axes" attribute?

Otherwise, to provide a default plot, should the writer "dereference"
these two arrays into two 1-D arrays of I(|Q|), each of length nx*ny?

(Because the source of this data was a 2-D detector, it is less
desirable to proceed with this reduction, just to satisfy NeXus.  With
reduction to 1-D, one loses access to intuition about unresolved
artefacts in the visualized image, such as uneven detector efficiency
and line dropouts.)
Provide both 2-D and also 1-D reductions?
Same NXdata group?
If the 1-D reduction is written into a separate NXdata group, how is the
"preferred" NXdata group noted for default visualization?

Thanks,
    Pete

_______________________________________________
NeXus-tech mailing list
NeXus-tech at nexusformat.org
http://lists.nexusformat.org/mailman/listinfo/nexus-tech

-- 
This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail.
Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. 
Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message.
Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom