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FITS /BITS
[VECTOR\]FITS OutputFile.fits FROM InputFile.gdf /BITS Nbit
The number of bits offers two controls: the data size and the encoding
scheme. This value will be found in the resulting header under the card
BITPIX.
Encoding scheme:
It is controled by the Nbit sign.
Positive sign means indexed encoding: the values are scaled as inte-
gers distributed among the 2**abs(Nbit) values. The forward and
back-conversion from actual values to indexed values are based on
the data extrema. Beware they must be up-to-date in the GDF header
before converting a BITPIX>0 FITS (see command V\HEADER /EXTREMA)!
Blank values are identified as the last integer of the indexed
scale.
Negative sign means the actual values are saved "as is" (IEEE float-
ing point format). Blank values are saved as IEEE NaN.
Data size:
It is controled by the Nbit absolute value: 16, 32, or 64 will con-
sume respectively 2, 4 and 8 bytes per value, with a direct impact
of the file size.
Nbits=+16 is worth only if the data dynamic (range of values) is low,
otherwise value precision loss will happen (2**16 values are not enough
to cover properly a large range of values). In the other cases, Nbit<0
is more modern and should be prefered. The command supports Nbits +16,
+32, -32, -64.
Gildas manager
2024-03-28