Spectral DM document update

[Paul Harrison]

On 09.10.2006, at 16:53, Doug Tody wrote:

> On Mon, 9 Oct 2006, Paul Harrison wrote:
>> On 09.10.2006, at 15:51, Doug Tody wrote:
>>> This approach would make sense, as not only is it more efficient
>>> for large spectral arrays of several thousand points, it is more
>>> consistent with the other serializations, which are also array- 
>>> based.
>>> Data handling and transformations between formats would thus be
>>> more straightforward.
>> I would argue that the whole point of the "pure XML" serialization  
>> is that it is verbose - XML is intended to be self-describing and  
>> naturally hierarchical, so expressing this serialization in a form  
>> that is as close to the DM as possible would seem sensible - i.e.  
>> <Points> with all their associated children rather than <Flux>  
>> with an array of points <Error> with an array etc. If you want to  
>> be storage efficient then just use the FITS serialization....
>
> This is the usual approach with XML, and in general I agree, but in
> this case we are talking about bulk data arrays which can be thousands
> of points in length.  Ignoring for the moment efficiency concerns, it
> is just a whole lot easier to deal with them as a single vector-valued
> data element.  Plus as I say, this is more consistent with the other
> serializations, facilitiating conversions, and with the likely use
> of this data in applications code - which will probably also be
> array based (all current spectral applications I am familiar with
> are array based).
>
> 	- Doug

Still not convinced this is an argument for making the XML  
serialization more vector-like, its all depends on what your idea of  
an array is - you could still have all the abcissae,ordinate,errors  
etc.  for a single spectral point localized in memory and then a  
whole spectrum

* in a modern OO language would be an array of "point objects"
* in a C like language it could be an array of structs
* even in FORTRAN it could be one a big array, just with a index  
increment of > 1 to obtain the next element of the same physical  
meaning e.g. Flux (assuming that all the values were stored as the  
same basic data type)


Paul Harrison
ESO Garching
www.eso.org