Python bindings: Add Dataset.ReadAsMaskedArray

[dbaston]

Resolves #12005

[rouault]

Perhaps indicate in the doc that when downsampling the mask resampling algorithm is Mode (ie majority) ?

[dbaston]

Documented this and also changed behavior of Band.ReadAsMaskedArray to match.

[dbaston]

Hmm, I'm not sure how my tests were passing locally before. Right now I'm puzzled by

(Pdb) band.GetMaskBand().ReadAsArray()
array([[  0, 255],
       [  0,   0]], dtype=uint8)
(Pdb) band.GetMaskBand().ReadAsArray(buf_xsize=1, buf_ysize=1, resample_alg=gdal.GRIORA_Mode)
array([[255]], dtype=uint8)

[rouault]

Right now I'm puzzled by

I believe this is due to: https://github.com/OSGeo/gdal/blob/master/gcore/overview.cpp#L4453 / https://github.com/OSGeo/gdal/blob/master/gcore/overview.cpp#L5798

[dbaston]

I don't think GRA_Mode downscaling works for NoData mask bands. If I'm following the code right, it performs a downscaled read from the parent band (using GRA_Mode, and excluding NoData pixels) and then sets all valid pixels to 255. Which is different from performing a full resolution read on the parent, setting valid pixels to 255, and then downscaling.

[rouault]

I don't think GRA_Mode downscaling works for NoData mask bands.

yes the logic I pointed out where the mask band of the mask band is the mask band itself should probably be modified for Mode, and just get the GetMaskBand() of the mask band, which would be a AllValid mask band

[dbaston]

I believe this is due to: https://github.com/OSGeo/gdal/blob/master/gcore/overview.cpp#L4453 / https://github.com/OSGeo/gdal/blob/master/gcore/overview.cpp#L5798

I don't see either of these lines hit in this case. The issue I see is

gdal/gcore/gdalnodatamaskband.cpp

Lines 404 to 407 in bfb3825

	eErr = m_poParent->RasterIO(
	GF_Read, nXOff, nYOff, nXSize, nYSize, pTemp, nBufXSize, nBufYSize,
	eWrkDT, nWrkDTSize, static_cast<GSpacing>(nBufXSize) * nWrkDTSize,
	psExtraArg);

where a downscaled read is performed from the parent band, and the result reclassified to 0/255. To get a correct result for GRIORA_Mode, I think we need to read from the parent band at full resolution, reclassify, and then downscale.

All this said, the current behavior of GRIORA_Mode is essentially GRIORA_Max and not necessarily undesirable as a default.