see:
http://www.unidata.ucar.edu/software/netcdf/docs/BestPractices.html#Packed
Data Values
regards,
john
On Thu, May 14, 2015 at 9:27 AM, Daniele Romagnoli <
daniele.romagnoli@xxxxxxxxxxxxxxxx> wrote:
> Hi.
> First of all, sorry if that's not a java specific topic.
>
> I'm working on supporting dataPacking when storing a dataset to a NetCDF
> output file. Suppose I have Double data in the range [-10 : 35] (as an
> instance, a temperature in celsius degrees) where the input _fillValue is
> -65536.
>
> I want to write it as packed data (as an instance, Byte... it's just an
> example).
> I'm using the formula provided here
> <https://www.unidata.ucar.edu/software/netcdf/workshops/2010/bestpractices/Packing.html>
> .
> Therefore, I'm computing dataMin, dataMax: -10, 35 in my case (-65536 is
> not part of the statistics computations).
> Then, I'm using them to compute add_offset and scale_factor:
>
> add_offset = dataMin
> scale_factor = (dataMax - dataMin) / (2^n - 1)
>
> I'm setting out a NetCDF Variable with short dataType and I'm rescaling
> the values on the fly using that formula:
>
> packed_value = nint((unpacked_value - add_offset) / scale_factor)
>
> Note that I'm avoiding to apply this transformation to samples with value
> = -65536 (being my NoData, _fillValue).
>
> The question is:
> I want to setup a _fillValue in the packed dataset.
> It need to respect the output dataType (so, it's a Byte).
> _fillValue = -65536 won't fit in the Byte range. What is the recommended
> approach to compute the "output" _fillValue?
>
> Moreover, what can I do to make sure that I still have a "reserved" value
> in the output range, to be used to represent _fillValue without risking
> overlaps with the converted range?
> Should I use some additional "offsets" when computing the
> add_offset/scale_factor to make sure as an instance, that the first value
> of dataType range is reserved for _fillValue? (as an instance, to make sure
> that _fillValue = 0 and valid range will be between 1 and 255)
>
> Please, let me know.
> Best Regards,
> Daniele
>
>
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