[GRASS-user] Re: Quantization of a true color GeoTIFF

[WolfgangZ]

WolfgangZ schrieb:
> Hamish schrieb:
>> Hi,
>>
>> I have a 16244x12706 GeoTIFF file (600dpi scan) which is made up of 7
>> colors but came as a 3-band 24bit True Color image.
>>
>> I can import with r.in.gdal as red,green,blue bands, but it's huge and
>> slow.
>>
>> I can't use pnmquant as it overflows the RAM. (I have 2GB physical + 2GB
>> swap)
>>
>> I tried Indexing it to a small number of colors with the Gimp, but after
>> the process the colors were all changed. I can index as 256 colors with
>> little color change in the Gimp, but that is still 249 more than I need.
>>
>> I want a CELL map with cat=[1,2,3,4,5,6,7] values with RGB set for each
>> category by r.colors.
>>
>> I guess I can use the Gimp to index to 7 (wrong) colors, import that into
>> GRASS, then use r.colors + the Gimp's eye-dropper tool on the original to
>> fix them.
>>
>> Staying with the original 3-band import, how to find the 7 peaks in the
>> r.composite histogram, filter out anything that isn't a peak by changing
>> its value to be that of the nearest peak, and reclass to 1-7? As
>> r.reclass can take ranges for input, I guess I can be a bit sloppy in
>> finding the peaks and use the result of r.stats as input to r.reclass,
>> then extract a "hardcopy" from the reclass map with r.mapcalc a=b.
>>
>>
>> ?
>> any one have a bright idea how to do this with minimal data loss
>> ?
>>
>>
>> thanks,
>> Hamish
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>>
> 
> 
> you could try the script rgb2pct which is included in the gdal 
> utilities. with "-n number" you can set the number of colors.
> 
> regards
> wolfgang
> 
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at least rgb2pct is included in the wftools http://fwtools.maptools.org/ 
  not sure if it officially belongs to gdal utilities.