[gdal-dev] Support for the LUT transformation of the vrt bands

[Frank Warmerdam]

Tamas Szekeres wrote:
> Folks,
> 
> In my recent work I need a limited support for a LUT based
> transformation of the bands to apply some colorization  and threshold
> operations created with the gdal_grid application. I wonder if we
> could implement this enhancement in the gdal vrt driver, and the
> dataset could be created from the scripting interfaces (like C#)
> either. By adding this functionality the vrt driver the config could
> be stored in a separate configuration profile (XML) that could be
> preserved and used for subsequent transformations.
> 
> In my proposal this functionality would be implemented in a new
> VRTRasterBand subclass, like subClass="VRTLUTRasterBand", and we
> 
> could specify the lookup table in the following pseudo fromat:
> 
> <LUT>[source value1]:[dest value1],[source value2]:[dest value2] ...</LUT>
> 
> We could also specify the interpolation mode when gathering the
> intermediary values (linear or none at the moment)
> 
> There would be another transformation parameter assigned to a LUT
> denoting how the scalar value is computed in case when we have a
> complex source band (the possible values are
> magnitude/phase/real/imaginary)
> 
> For example:
> <VRTRasterBand dataType="Byte" band="1" subClass="VRTLUTRasterBand">
>   <LUT interpolation="linear"
> scalarvalue="magnitude">23400.34:0,75234.45:255</LUT>
>   <SimpleSource>
>       <SourceFilename relativeToVRT="1">source.tif</SourceFilename>
>       <SourceBand>4</SourceBand>
>   </SimpleSource>
> </VRTRasterBand>
> 
> 
> I wouldn't address creating complex bands with this method at the
> moment. However we could probably extend this by adding multiple LUT
> nodes and data sources to a single VRTLUTRasterBand.
> 
> In the future we could possibly add helper functions to create
> predefinied LUT XML nodes based on some common use cases (like
> equidinstant or histogram based partitions of the source values) but
> currently it's up to the user and script developer how to create the
> desired vrt xml. However a graphical application could easily
> visualize the LUT by applying the vrt on a sample image with linear
> range of the values.
> 
> I'd like to know whether the idea above would be reasonable to
> implement. Should I create an RFC on this?

Tamas,

I would like to suggest that you handle this as additional parameters
on the VRTComplexSource rather than as a distinct band object type.

The VRTComplexSource can already apply rescaling on the fly, and it
seems like a reasonable extension to add interval based LUTs.  In
some cases it may also be helpful to be able to apply different LUTs
to different sources in a virtual mosaic.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org