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nxdata stricter suggestion on when axisname indices can be omitted + clarify 3D or higher #1550

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nxdata stricter suggestion on when axisname indices can be omitted + clarify 3D or higher #1550

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406b742
omitting AXISNAME_indices can only be unambiguous in the case of 1D axes
woutdenolf Mar 12, 2025
975c349
make clear that 3- or higher-dimensions are allowed for data and axes
woutdenolf Mar 12, 2025
a778dfe
use "equally" instead of "equivalently"
woutdenolf Mar 18, 2025
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15 changes: 8 additions & 7 deletions base_classes/NXdata.nxdl.xml
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Expand Up @@ -75,11 +75,12 @@

More complex cases are supported

* histogram data: ``x`` has one more value than ``data``.
* alternative axes: instead of a single ``x`` axis you can have several axes, one of which being the default.
* signals with more than one dimension: ``data`` could be 2D with axes ``x`` and ``y`` along each dimension.
* axes with more than one dimension: ``data`` could be 2D with axes ``x`` and ``y`` also being 2D, providing a
unique ``(x, y)`` coordinate for each ``data`` point.
* Histogram data. For example ``x`` has one more value than ``data``.
* Alternative axes. For example instead of a single ``x`` axis you can have several axes, one of which being the default.
* Signals with more than one dimension. For example ``data`` could be 2D with axes ``x`` and ``y`` along each dimension.
Equally 3- or higher-dimensional data is allowed.
* Axes with more than one dimension. For example ``data`` could be 2D with axes ``x`` and ``y`` also being 2D, providing a
unique ``(x, y)`` coordinate for each ``data`` point. Equally 3- or higher-dimensional axes are allowed.

**Signals:**

Expand Down Expand Up @@ -141,8 +142,8 @@
field name in the :ref:`axes </NXdata@axes-attribute>` attribute are used.

4. When :ref:`AXISNAME_indices </NXdata@AXISNAME_indices-attribute>` is the same as the indices of "AXISNAME" in the
:ref:`axes </NXdata@axes-attribute>` attribute, there is no need to provide
:ref:`AXISNAME_indices </NXdata@AXISNAME_indices-attribute>`.
:ref:`axes </NXdata@axes-attribute>` attribute and :ref:`AXISNAME </NXdata/AXISNAME-field>` is one-dimensional,
the :ref:`AXISNAME_indices </NXdata@AXISNAME_indices-attribute>` attribute can be omitted.
Comment on lines +145 to +146
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@woutdenolf woutdenolf Mar 12, 2025

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Addresses @PeterC-DLS comment #1396 (comment)

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@woutdenolf woutdenolf Mar 12, 2025

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For reference: for higher dimensional axes the AXISNAME_indices attribute can never be omitted unambiguously by using the indices in @axes instead. For example @axes=["x", "x", "z"] could be @x_indices=[0,1] or @x_indices=[1,0].

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@PeterC-DLS PeterC-DLS Mar 13, 2025

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That is a bit too strong as you could determine the indices ordering from the signal field shape if the pertinent dimensions are distinct: in the example, signal[23,47,17] with x[47,23] is unambigously inferred to have @x_indices=[1,0].

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@woutdenolf woutdenolf Mar 18, 2025

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Sure it can be omitted for nD if it can be derived "unambigously". But then to define what "unambigously" means in this context would be rather convoluted. For example it won't work when signal[23,23,17].

So your original suggestion to omit AXISNAME_indices only for 1D still makes sense imo.


5. The indices of "AXISNAME" in the :ref:`axes </NXdata@axes-attribute>` attribute must be a subset of
:ref:`AXISNAME_indices </NXdata@AXISNAME_indices-attribute>`.
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