[gdal-dev] How to map Int32 or Float32 GTiff to color map?

[Even Rouault]

Le jeudi 15 mai 2014 16:25:12, Stephen Woodbridge a écrit :
> Hi all,
> 
> I'm trying to create a VRT file for a GTiff that is Int32 or Float32 to
> add a ColorTable but I seem to be missing a key piece of information.
> How are the pixel values mapped to the color table entries? via the
> histogram?
> 
> My GTiff has noData=-32767 and good values the range from say -5 to 100.
> So do I create a color table that is based on the 256 buckets in the
> histogram?

No way you can use a GDAL color table to map negative values...
Perhaps you should use gdaldem color-relief to generate a RGB output. Note 
that gdaldem color-relief is compatible with VRT output : it uses the LUT 
mechanism to do so.

> 
> So for example I get:
> 
> Band 1 Block=256x256 Type=Float32, ColorInterp=Gray
>    Min=-32767.000 Max=33.980   Computed Min/Max=-54.985,34.320
>    Minimum=-32767.000, Maximum=33.980, Mean=-15887.345, StdDev=16386.592
>    256 buckets from -32831.1 to 98.0444:
>    1545169 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
> 0 0 0 0 0 0 27375 1924356
>    NoData Value=-32767
>    Metadata:
>      STATISTICS_MAXIMUM=33.979999542236
>      STATISTICS_MEAN=-15887.344614501
>      STATISTICS_MINIMUM=-32767
>      STATISTICS_STDDEV=16386.59208246
> 
> 
> It seems that the NoData value is included in the stats which really
> skews the numbers compressing all my values into two buckets! How can I
> avoid that? I want to ignore the NoData values and spread my good values
> into the remaining buckets. How can I do this?

You can't. There's a ticket I think about the fact we should ignore nodata 
value in stats.

> 
> Thanks,
>    -Steve
> 
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