[GRASSLIST:3645] Re: more r.mapcalc troubles

[Maciek Sieczka]

> > I made a following raster for my vignetting correction purposes:
> >
> >     r.mapcalc 'kor_lx = ( 1218.01 / ( sqrt(  (double(x()) - 941.0) ^2.0
> >     + (double(y()) - 928.0)^2.0) ^2.0 + 1218.01^2.0) ) ^4.0'
> >
> > the value range for 'kor_lx' file is (read from the color file in the
> > 'colr' folder):
> >
> >     0.00000000000002026206628416476 - 0.000000000000454355718931865126
> >
> > I checked if r.mapcalc hadles the 'kor_lx''s values properly by doing:
> >
> >     r.mapcalc 'copy=kor_lx'
> >
> > the data range in the 'colr' file for 'copy' was DIFFERENT (the min
> > value is higher than in 'kor_lx'):
> >
> >     0.000000000000028123670807620238 - 0.000000000000454355718931865126

> FWIW, I can't reproduce that behaviour.
>
> Having created an X/Y location with the following region:
>
> GRASS:~ > g.region -p
> projection: 0 (x,y)
> zone:       0
> north:      1824
> south:      0
> west:       0
> east:       1868
> nsres:      1
> ewres:      1
> rows:       1824
> cols:       1868
>
> I ran the above r.mapcalc command, followed by "r.colors kor_lx
> color=grey",
> and obtained the same range as yourself in the colour table:
>
> % 0.00000000000002026206628416476 0.000000000000454355718931865126
> 0.00000000000002026206628416476:0 0.000000000000454355718931865126:255
>
> However, if I run "r.mapcalc 'copy=kor_lx'", the copy has an identical
> colour table:
>
> % 0.00000000000002026206628416476 0.000000000000454355718931865126
> 0.00000000000002026206628416476:0 0.000000000000454355718931865126:255

Fixed that.
The reason was trivial - there was a mask made of the aerial photo borders
which I forgot
and I didn't know that r.mapcalc ignores the mask when no raster is given as
an argument i.e. "r.mapcalc 'raster=100'" (like in case of the formula on
the very top of the email).
So there was a difference beetween the 'kor_lx' and 'copy' because only
'copy' was masked.

Just to finish this toppic, could you only explain why it is so?

> Also, I get the same results with both 5.0.2 and a recent 5.3
> snapshot.
>
> BTW, if I do:
>
> r.mapcalc 'kor256=#kor_lx'
> r.info -r kor256
>
> GRASS:~ > r.mapcalc 'kor256=#kor_lx'
> 100%
> GRASS:~ > r.info -r kor256
> min=0
> max=255
>
> Presumably, you are using a different colour table; which one?

The reason was as above.

> > Is there a way to learn about the value range etc. in case of such a low
> > value raster in Grass 5.03?

> I don't know. The range is actually stored as binary data, so the
> information exists. However, r.info displays it using "%f", so small
> values will be rounded to zero. I don't know of any other program
> which will provide this information.

some solution: "r.colors raster_name color=rules" prints the value range
with a pretty good precission

for instance:

    r.colors kor_lx_bis color=rules
    Enter rules, "end" when done, "help" if you need it.
    fp: Data range is 0.0000000000000281236708076 to
0.0000000000004543557189319

Maciek