[NeXus-tech] Proposal for dealing with start and end positions per scan point

[Andreas Förster]

Thank you, Herbert!  This addresses our (users') concerns.

Best regards.


Andreas


On Thu, May 28, 2015 at 9:25 PM, Herbert J. Bernstein <yayahjb at gmail.com>
wrote:

> This is a proposal in response to Dectris requests.  I would suggest
> we discuss it at the next Telco.  I have added preliminary support
> code to CBFlib_bleeding_edge in the cbflib soureforge project.
>
> This is a proposed revision to the base class NXtransformations adding
> certain optional items that would be useful in MX data processing.
> The currently posted NXtransformations has caused some confusion
> because of the variable axis names.  For clarity, let is use "(axis)"
> as a place holder for an axis name, e.g. for
> "GONIOMETER_PHI"
>
> Then the current form of NXtransformations is the class of a group
> containing some number of fields (HDF5 datasets), each of the form
>
> (axis)[np]:NX_NUMBER
>   @transformation_type: NX_CHAR
>   @vector[3]: NX_NUMBER
>   @offset[3]: NX_NUMBER
>   @offset_units: NX_CHAR
>   @depends_on: NX_CHAR
>
> we propose to permit the optional addition of three new fields:
>
> (axis)_end[np]:NX_NUMBER
> (axis)_range_average:NX_NUMBER
> (axis)_range_total[2]:NX_NUMBER
>
> and the optional addition of two new attributes for clarity
>
>   @end_point:NX_CHAR
>   @start_point:NX_CHAR
>
> (axis) is an array of the dimension of the number of frames containing
> the starting set points of this axis when collecting data for the
> corresponding frame.
>
> (axis)_end is an array of the dimension of the number of frames
> containing the ending set points of this axis when collecting data for
> the corresponding frame.
>
> (axis)_range_average is the average to the magnitudes of the ranges of
> set point values (i.e. the average of the magnitudes of the
> differences between corresponding values in (axis) and (axis)_end),
> appropriately corrected for the periodicity of rotational settings,
> e.g.  if (axis)=[359,0,1] and (axis)_end=[358,359,0] then
> (axis)_range_average = 1, exactly.
>
> (axis)_range_total is an array of two values, the minimum and the
> maximum of the values in (axis) and (axis)_end, with values
> appropriately corrected for the periodicity of rotation settings to
> minimize the excursion e.g. if (axis)=[359,0,1] and
> (axis)_end=[358,359,0] , then (axis)_range_total =[-2,1] consistent
> with the data values.
>
> @end_point is an optional attribute to (axis) giving the path to (axis)_end
> @start_point is an optional attribute to (axis)_end giving the path to
> (axis)
>
> Note 1:  When np>1 and the scan is known to be a continuous
> open-shutter scan, then none of the optional fields are necessary,
> inasmuch as they can be computed from the values in (axis).  However,
> when np=1 or if the scan is discontinuous, the values in (axis)_end
> will not be computable from the values in (axis).
>
> Note 2:  The values in (axis)_range_average and (axis)_range_total are
> normally computable from the values in (axis) and (axis)_end, but for
> many processing programs one, the other or both will need to be
> computes, and these field names provide a convenient, documented place
> in which to store the information in the file for processing, avoiding
> conflicts in naming from different experimenters making idiosyncratic
> name choices.
>
> Note 3:  The two optional attributes @end_point and @start_point are
> also redundant but again are scratch information people are likely to
> generate for processing support and providing these specific names
> helps to avoid conflicts in naming.
>
> --- HJB
>



-- 
**************************************************
Andreas Förster, PhD
Application Scientist MX

DECTRIS Ltd.
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Switzerland

+41 56 500 2100 general
+41 56 500 2101 fax
+41 56 500 2176 direct
andreas.foerster at dectris.com
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