[gdal-dev] Mask value data type in Python wrappings

[Even Rouault]

Dave,

> >>> band.SetNoDataValue(0)                     # Set the NODATA value to
> >>> zero print "Nodata:", band.GetNoDataValue(),
> >>> type(band.GetNoDataValue())
> 
> Nodata: 0.0 <type 'float’>
> 
> >>> mb = band.GetMaskBand()
> >>> ma = mb.ReadAsArray()
> >>> 
> >>> 
> >>> print "Origin:", ma[0, 0], type(ma[0, 0])  # *** SHOULD BE ZERO!!! ***

--> (Implicit) mask bands (such as the one generated for mask flag = 
GMF_ALL_VALID or GMF_NODATA), once fetched a first time, are no longer 
influenced by later actions. Re-opening the file should expose an updated mask 
band.

> 
> Origin: 255 <type 'numpy.uint8'>
> 
> >>> print "(5, 5):", ma[5, 5], type(ma[5, 5])  # Correct
> 
> (5, 5): 255 <type 'numpy.uint8'>
> 
> >>> print np.any(ma==0)                        # Mask band is entirely 255
> 
> False
> 
> As you can see, the mask covers the entire image and not the zeros on the
> edge.
> 
> The band.GetNoDataValue() is returning a Python float()

The C++ method always return a double, whatever the raster data type, hence 
the same for the Python bindings.

> which leads me to
> suspect that there is incorrect type handling within GDAL’s Python
> wrapping such that the mask generation is trying to compare the band’s
> raster values (GDAL_UInt16) to whatever the NoDataValue type is stored
> internally.  

See my first explanation

Even

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