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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