[gdal-dev] Masks vs nodata?

[Sean Gillies]

Hi Even,

On Mon, Apr 20, 2015 at 11:20 AM, Even Rouault <even.rouault at spatialys.com>
wrote:

> Sean,
>
> >
> > Not being able to unset nodata values for datasets is tripping up GDAL
> > users at work. I'm considering measures that require 1-bit internal masks
> > (we're primarily using GeoTIFF) instead of nodata values because, unlike
> > nodata, the masks can be "cleared" by setting them uniformly to True
> using
> > the GDAL API (GDALFillRaster() specifically).
>
> If there are external masks .msk, you'd better just deleting the file. That
> will save a bit some processing time for algorithms that deal with masks.
>
> >
> > Background: the GeoTIFF driver's SetNoDataValue() method doesn't let us
> > remove or set a null or empty nodata value or unset the bNoDataSet flag.
>
> A ClearNoDataValue() could be an interesting addition to the GDAL API
> indeed.
> I'm pretty sure there are tickets about that.
>
> > Passing a value outside the range of values in the dataset can be
> > problematic, especially for 8-bit data. Passing -inf, nan, or 256 to
> > SetNoDataValue() has the same effect on derived masks as passing 0.
>
> Looking at gcore/gdalnodatamaskband.cpp, it indeeds casts dfNoDataValue to
> GByte without checking. Should probably do better checking, although
> setting
> out-of-range nodata value is a bit in the unspecified behaviour area.
>
> >
> > I'm looking for expert advice on consequences of using masks instead of
> > nodata values. Are there features of GDAL that rely on nodata values and
> > don't respond to masks?
>
> I wouldn't be surprised that some algorithms there or there only work with
> the
> nodata concept and not with the more general concept of masks (GDALContour
> for
> example from a quick searching)
>
> > Are there adverse implications for
> > interoperability?
>
> In TIFF, I don't think one of nodata or nodata masks can be considered as
> more
> interoperable as the other. the GDAL_NODATA TIFF tag is a GDAL specific
> TIFF
> tag. If you use external masks (.msk), they are a GDAL specific thing. And
> if
> you use TIFF internal masks, they are quite an esoteric formulation of
> TIFF.
>
> > Would it be better to simply accept nodata values as
> > being part of the structure of datasets, like tiling and interleaving,
> and
> > immutable once set?
>
> Some drivers accept modifying geotransform or projection, so it is not
> really
> clear what users might want to change after a dataset has been created.
> But in
> your use case, it seems that the users do have use case for clearing an
> existing nodata value, so I'm not sure to follow your reasonning about what
> should be immutable.
>
> The easiest way to clear nodata currently is to use :
> gdal_translate -a_nodata none in.tif out.tif
>

Yes. Thanks for the advice, Even. I'm not ready to take on driver
enhancements, so I'll work on adapting workflows to the constraints of
nodata.
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