2014 How to find default data

(written for NIAC2014_Meeting)

One of the motivations for NeXus is simple plotting.

The procedure to find the default data to be plotted is convoluted. In some cases (files with multiple NXentry and/or NXdata groups), it is not certain which data will be found.

This proposal is to add a new and simpler mechanism to manage (both set and determine) the path to the default data. The intent is that this addition to NeXus preserves backwards compatibility and becomes the standard way for new data files to identify the default data to be plotted.

Proposal

It is proposed to add a deterministic method to identify the default data for visualization in a data file. It is expected that this will become the preferred method from now on.

• add default_NXentry attribute to the root of the file that states which NXentry is the default. The value is the name of the NXentry group.
• add default_NXdata attribute to each NXentry that states which NXdata is the default. The value is the name of the NXdata group.
• add signal attribute to each NXdata that states which dataset is the default. The value is the name of the dataset to be plotted.

These default attributes only describe child elements, not child/object or ../object or other hierarchy.

The procedure to identify the default data to be plotted is quite simple, given any NeXus file, any NXentry, or any NXdata. Follow the chain as it is described from that point.

Conclusion

Ratified in a slightly modified form at NIAC 2014. See Niac 2014 minutes

Summary of that modification:

• add default attribute to the root of the file that states which NXentry is the default and only to resolve ambiguity when more than one NXentry exists. The value is the name of the NXentry group.
• add default attribute to each NXentry that states which NXdata is the default and only to resolve ambiguity when more than one NXdata exists. The value is the name of the NXdata group.
• add signal attribute to each NXdata that states which dataset is the default. The value is the name of the dataset to be plotted.