Re: HDF5 bitfields...

Hi John,

> >>>the scale/offset can be calculated easily from the data itself. often, 
> >>>people want to apply different scale/offset to different parts of the 
> >>>same array, eg vertical levels.
> >>>      
> >>>
> >>and you replied:
> >>    
> >>
> >>>    Hmm, how would you parameterize this?  Would a user select various 
> >>> parts
> >>>of the dataset's dataspace and specify scale/offset information for them?
> >>>      
> >>>
> >>When Harvey Davies was here from Australia for a visit about 8 years
> >>ago, we worked out two kinds of scaling for varying packing parameters
> >>along one or more dimensions of a variable: predefined scaling and
> >>adaptive scaling.  
> >>
> >>With predefined scaling, the scale and offset values associated with a
> >>packed variable were stored in auxiliary arrays, varying along just
> >>the subset of dimensions used by these arrays.  For example, to store
> >>a packed array of temperatures, one might use
> >>
> >>  dimensions:
> >>    time = ...
> >>    lat = ...
> >>    lon = ...
> >>    level = ...
> >>  variables:
> >>    byte temperature(time, level, lon, lat);
> >>    double temperature_scale_factor(level);
> >>    double temperature_add_offset(level);
> >>
> >>which would use a possibly different (scale_factor, add_offset) pair
> >>for packing temperatures on each atmospheric level.  This would allow
> >>for greater precision using the same number of bits (or fewer bits for
> >>the same precision) than using one packing parameter pair for all the
> >>data, because this variable tends to have values that depend on level.
> >>It wouldn't work so well with other variables that don't have a
> >>level-dependence.
> >>
> >>With adaptive scaling, the optimum scale and offset values were to be
> >>computed by the library for each slab of the variable as it was
> >>written, and stored in automatically-generated associated variables
> >>(or multidimensional attributes).
> >>
> >>Although we defined interfaces for these types of scaling, they were
> >>never implemented.  Implementing adaptive scaling seemed pretty
> >>ambitious, and even the predefined scaling would have required
> >>adoption of new conventions for naming associated variables, etc.  And
> >>the proposals actually foundered on inability to agree on all the gory
> >>details, such as determining whether to permit the types of the
> >>scaling parameters to be user-specifiable in adaptive scaling, etc.
> >>    
> >>
> >    Ok, I see.  Hmm...  I think that the adaptive scaling would actually be
> >somewhat easier that the predefined scaling you describe in HDF5.  With the
> >adaptive scaling, each chunk in the dataset could be scanned to compute the
> >optimum scale and offset values which would be stored with the chunk.  
> >Handling
> >predefined scaling that varied according to a position within the dataspace
> >seems like it would require accessing some information that was stored 
> >outside
> >each chunk and that might be a little unusual in the current implementation.
> >Predefined scaling that didn't vary across the dataspace would be easier than
> >either of those methods, of course.  Although it gets a little weird to 
> >define
> >any sort of scaling on non-numeric datatypes, we've got a mechanism for
> >disallowing that now.
> >
> >    Quincey
> >  
> >
> If we have the ability to store variable length "compressed formats", 
> then it seems like we can define some simple format for this, say a sequence
>   n  (int)
>   nbits (byte)
>   scale (float/double)
>   offset (float/double)
>   n nbit integers
> so that the compressed data is self contained, with no need for auxilary 
> variables or info stored outside the chunk.
    I thought you wanted the predefined scale/offset values to vary according
to the chunk's location in the dataspace?

> couldnt both the "predefined" and "adaptive" scaling be stored in this 
> way? if so, then the difference between the two would be in how the user 
> specifies, ie the API.
    If the predefined scaling didn't vary, this would be fine.

> Seems like we could start simple, just allow adaptive scaling on either 
> the whole array, or varying along a single dimension. ( i think we get 
> enough functionality to do that out of the ability to compress each 
> chunk independently. ) output type is restricted to float or double. so 
> all user has to specify is nbits and optionally a dimension.
    Yes, this would be possible at the API level, but currently the filter
parameters are "global" to the entire dataset, so there's no provision in the
internal code for "knowing" which chunk is being operated on or varying the
filter parameters according the chunk coordinates.  Of course, that can always
be changed, but this is a public sort of API and we have to make changes to it