[GRASS-dev] Re: document what number is NULL

Markus Neteler neteler at osgeo.org
Tue Mar 4 04:53:49 EST 2008

On Tue, Feb 26, 2008 at 5:25 PM, Maciej Sieczka <tutey at o2.pl> wrote:
> Glynn Clements pisze:
>  > -2^31 (= 0x80000000 = -2147483648) is the null value
>  > for the CELL type, so you'll never see that value in a map.
>  > The FP nulls are the all-ones bit patterns. These corresponds to NaN
>  > according to the IEEE-754 formats, although it isn't the "default" NaN
>  > pattern generated by most architectures (which is usually 7fc00000 or
>  > ffc00000 for float and 7ff8000000000000 or fff8000000000000 for
>  > double, i.e. an all-ones exponent, the top-bit of the mantissa set,
>  > and either sign).
>  >
>  > So far as arithmetic is concerned, any value with an all-ones exponent
>  > and a non-zero mantissa is treated as NaN. But the GRASS
>  > G_is_[fd]_null_value() functions only consider the all-ones bit
>  > pattern to be null. I intend to change this in 7.x so that all FP NaN
>  > values are treated as null. This will mean that code which can
>  > generate NaNs doesn't have to explicitly convert them to the GRASS
>  > null value.
>  These should go to "Raster data processing in GRASS GIS". As I don't
>  really understand the part about floating point, I won't do it myself.
>  Somebody more savvy please do, putting it some simpler words if possible.

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