[gdal-dev] Convert alpha to nodata?

[Robert Coup]

Hi Even,

Thanks for your reply!

On Fri, 19 Jan 2024 at 17:32, Even Rouault <even.rouault at spatialys.com>
wrote:

> I can't think an easy way of doing what you want.
>

Is there a functional gap? *Conceptually* it feels to me like
VRTRasterBand.ComplexSource.UseMaskBand should "mark" a pixel to be
nodata-equivalent (NaN?), and then VRTRasterBand.NoDataValue should "write"
the nodata-marked pixels with the value.

<VRTDataset ...>
  ...
  <VRTRasterBand dataType="Int16" band="1">
    <NoDataValue>-1</NoDataValue>
    <ComplexSource>
      <SourceFilename>in.tif</SourceFilename>
      <SourceBand>1</SourceBand>
      <UseMaskBand>true</UseMaskBand>
      ...
    </ComplexSource>
    ...
  </VRTRasterBand>


But maybe that's not how it's modelled inside?


> Well, you can definitely do that using a VRT Python pixel function, that
> would essentially do your --calc expression
>

Thanks for confirming it's possible, I'll have a play.

>From what I can see the python pixel functions are only called once,
use numpy and (IIUC) vectorised CPU operations... would that make it
faster/comparable to one of the C pixel functions that's doing a naive
array loop over rows & columns?


> A nice exercice for (Pythonist) contributors would be to add a -f VRT
> capability to gdal_calc.py that would essentially automate the writing
> of such VRT using Python pixel functions. Just saying...


*adds to a long list of nice exercises*

Thanks

Rob :)
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