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

[Stephen Woodbridge]

On 5/15/2014 1:56 PM, Even Rouault wrote:
> 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.

OK, I will look at gdaldem, but not wanting to give up on this yet, is 
there a way to change the NoData pixels from -32767 to say -20 then 
scale 1.0 and offset the value by 10.

This would make NoData=0

old   => new
  -5        5
   0       10
   1       11
100      110

then I would have all positive values and the range would be manageable?

The scale and offset are already supported in VRT, but I'm not sure if 
it is possible to remap -32767 values to a new value.

Thanks for your response.

-Steve

>>
>> 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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