document what number is NULL [Re: [GRASS-dev] r.mapcal rand() strangeness]

[Maciej Sieczka]

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.

Cheers,
Maciek