[Nexus] item discussed now

[Tobias Richter]

Hi Eugen,

The centre is the lowest common denominator. It mostly works for the NXsample and possibly for NXslit, I would say.
For all other cases ideally we need to define a prominent reference point. Like first pixel for NXdetetcor. That’s best done case by case.
In an abstract way I cannot guarantee that we’ll always find a suitable generic location independent of the specific model or construction. But we can try.

Regards,

Tobias


> On 18 Feb 2015, at 14:03, Eugen Wintersberger <eugen.wintersberger at desy.de> wrote:
> 
> Hi folks
>  though that I did not attend todays telecon - I would like to make a
> remark here as I am currently working with transformations quite a lot
> while designing the NeXus layout for several of our beamlines at PETRA
> III.
> 
> On Wed, 2015-02-18 at 07:36 -0500, Herbert J. Bernstein wrote: 
>> As per our discussion on the hangout today, please note the following
>> clarification:
>> 
>> When specifying the position of a component with a depends_on field,
>> the specific point in the component being positioned will either be
>> the center of the first pixel for a detector 
> 
> This is quite fine. One can mange this as every 2D detector has
> something like a first pixel. 
> 
>> or the center of the
>> component for components not having an equivalent to a "first pixel".
> 
> This is a bit more complex. What would be the "center" of a component. 
> The entire situation remains me a bit on 3D modelling software which
> follows a composite geometry approach. All of their 3D primitives have a
> well defined center. However, one has to consult the manual of the
> software package to learn where the center for a particular primitive is
> (different software packages can make different choices where to place
> the center for a particular primitive). At least all software packages I
> kow use this "center" as the origin for the primitive local coordinate
> frame. In my opinion this is what we are missing - a center for each
> "component" onto which we can bind the origin of the local coordinate
> frame.  
> 
> The question that arises for me is: is it possible to define such a
> center for our base classes (for those who can be used along with
> transformations) in a generic manner (in a way independent of the
> particular mechanical construction)?
> 
> BTW. This discussion does not render transformations useless. From my
> experience at PETRA III they have already proven quite useful when
> describing the dependencies of axes. However, I guess this discussion
> can greatly improve the usability of transformations.  
> 
> regards
>  Eugen
> 
>> 
>> On Wed, Feb 18, 2015 at 7:29 AM, Pete Jemian <prjemian at gmail.com> wrote:
>>> 
>>> use this URL for the transformations
>>> http://download.nexusformat.org/doc/html/design.html#coordinate-transformations
>>> 
>>> On 2/18/2015 6:10 AM, Pete Jemian wrote:
>>>> 
>>>> https://github.com/nexusformat/definitions/blob/depends_on/nxdl.xsd
>>>> 
>>>> 
>>>> On 2/18/2015 6:05 AM, Pete Jemian wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> https://github.com/nexusformat/definitions/issues/273#issuecomment-74158201
>>>>> 
>>>>> 
>>>> 
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