[gdal-dev] Can't edit alpha in palette through VRT

[Even Rouault]

On vendredi 24 mars 2017 13:36:35 CET Rahkonen Jukka (MML) wrote:
> Hi,
> 
> I am following the FAQ "How to create or modify an image color table?" from
> https://trac.osgeo.org/gdal/wiki/FAQRaster
> 
> I took out the palette from existing tiff file into VRT and I would like to
> change alpha value of the first input from 255 into 0.
> 
> I had originally this:
> 
> <VRTDataset rasterXSize="734" rasterYSize="565">
>   <VRTRasterBand dataType="Byte" band="1">
>     <ColorInterp>Palette</ColorInterp>
>     <ColorTable>
>       <Entry c1="255" c2="255" c3="255" c4="255" />
>       <Entry c1="0" c2="0" c3="0" c4="255" />
>   ...
> Then I edited the VRT into
> 
>     <ColorTable>
>       <Entry c1="60" c2="255" c3="255" c4="0" />
>       <Entry c1="0" c2="0" c3="0" c4="255" />
> ...
> 
> 
> Then I executed
> gdal_translate fixed_palette.vrt fixed palette.tif
> 
> However, the result is not exactly what I hoped for:
> 
> Band 1 Block=734x11 Type=Byte, ColorInterp=Palette
>   Color Table (RGB with 256 entries)
>     0: 60,255,255,255
>     1: 0,0,0,255
> 
> The value "60" for the red band is taken from my edited palette but my
> edited alpha value is  not honored. Am I doing something wrong?

No, this is a TIFF limitation. The format doesn't support the alpha component in color tables.
You might workaround this by setting the NODATA value to 0 instead.

> 
> -Jukka Rahkonen-


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