[gdal-dev] Geotiff nodata handling

[Andrea Aime]

Hi,
I'm trying to wrap my head around how gdal handles nodata values.
In an image with this gdalinfo exceprt:

Band 1 Block=512x512 Type=Byte, ColorInterp=Red
  NoData Value=0
Band 2 Block=512x512 Type=Byte, ColorInterp=Green
  NoData Value=0
Band 3 Block=512x512 Type=Byte, ColorInterp=Blue
  NoData Value=0

and using software based on gdal (e.g. qgis), it seems that a pixel is
considered
nodata if any of the three pixel bands are zero (even just one).

I could swear I saw some gdal specific way to embed metadata
saying "if all three bands are zero, then the pixel is nodata"
(pull pixel as opposed band per band treatment), but I cannot
find it anymore.

Did I imagine it? If existing, do you have a feeling of how well
supported that is in common software?

Cheers
Andrea

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