[NeXus-committee] Next DECTRIS Summary

[Pete R Jemian]

Armando is correct.

On 01/22/2013 02:58 AM, "V. Armando Solé" wrote:

> ... It seems logical that if NXdata is a group
> defining a plot, the attributes to define the plot belong to the group
> and not to the datasets inside (kudos to the guys that came out with
> that idea). ...

"An important motivation for the design of NeXus was to simplify the 
creation of a default plot view." 
(http://download.nexusformat.org/doc/html/motivations.html#motivations-for-the-nexus-standard-in-the-scientific-community)

"NXdata is used to identify the default plottable data." 
(http://download.nexusformat.org/doc/html/design.html#nexus-base-classes)

Listen to these words; they are fundamental to this discussion.

The discussion has been to make the NeXus standard _do as we say_, so 
that it is the NXdata group attributes that specify the default plot.

In our current standard, we have it different where it is the dataset 
attributes that seem to specify the default plot.  That fails for some. 
  The data from DECTRIS point out one such case.  To fix what is now 
obviously a broken condition, we must change the NeXus standard and use 
the NXdata group to identify the plottable data.

The change discussed will simplify the procedure to find the plottable 
data.  Find the current procedure at this URL: 
http://download.nexusformat.org/doc/html/datarules.html#find-the-plottable-data. 
  Here's the revision to the simpler method

1. Open the first top level NeXus group with class ``NXentry``.

2. Open the first NeXus group with class ``NXdata``.

3. The default plottable dataset is named in the
    *plot* attribute (such as ``plot="data"`` to plot
    the dataset named *data*).

4. If there is no such attribute (indicating a data file written
    to an older version of the NeXus standard), ...

...

And while we are at it...

We should also move the specification of the dimension scales from the 
datasets to the NXdata group, as well.  Same justification as above.

We should also have an attribute on the NXentry group to indicate which 
NXdata should be selected for the default plot, if several NXdata groups 
are present.  Then selection of which NXdata in step 2 above becomes 
certain.  At present, it is not clear which NXdata will be presented as 
the "first" group.

Regards,
    Pete