[gdal-dev] gdalinfo -mm also report n (number of grid cells that are not nodata)

[Even Rouault]

> 
> I checked, results with gdalinfo -stats are wrong because existing
> STATISTICS_* metadata are reported even if approximate statistics are not
> allowed.

No, if STATISTICS_APPROXIMATE=YES and is set in .aux.xml (because initial 
computation was done with -approx_stats) and you do gdalinfo -stats after, 
then statistics will be recomputed on all samples and 
STATISTICS_APPROXIMATE=YES  will be cleared

Demo:
$ gdalinfo -approx_stats test.tif
[...]
  Metadata:
    STATISTICS_APPROXIMATE=YES
    STATISTICS_MAXIMUM=206
    STATISTICS_MEAN=126.17083333333
    STATISTICS_MINIMUM=74
    STATISTICS_STDDEV=21.548781465291

$ gdalinfo -stats  test.tif
[...]
  Metadata:
    STATISTICS_MAXIMUM=255
    STATISTICS_MEAN=126.765
    STATISTICS_MINIMUM=74
    STATISTICS_STDDEV=22.928470838676



> The problem is, STATISTICS_APPROXIMATE is not set. Other software
> using GDAL to create raster datasets may use
> GDALRasterBand::SetStatistics() which does not indicate if stats are
> approximations., i.e. stats are approximations but there is no
> STATISTICS_APPROXIMATE=YES.

The idea is that if you use GDALRasterBand::SetStatistics()  then you are 
assumed to provide exact statistics. If they are only approximate, then you 
should also set STATISTICS_APPROXIMATE=YES with GDALSetMetadataItem()

> 
> GDAL assumes that STATISTICS_* metadata represent stats on all pixels, this
> is IMHO wrong. You can only hope that STATISTICS_* metadata represent stats
> on all pixels if a respective metadata item has been set to boolean true,
> something like STATISTICS_ALL_PIXELS=YES.

I'm really confused. Why introducing yet another item whereas 
STATISTICS_APPROXIMATE=YES is there for that purpose ?

> Even in this case, an option to
> force recomputing raster band stats would be very nice to have (verifying
> metadata).

ComputeStatistics() will recompute statistics. It is true that with gdalinfo -
stats, they are not recomputed if they already exist and were not approximate 
since it calls GetStatistics() and not ComputeStatistics(). An easy workaround 
is to delete the .aux.xml to force recomputation.

Even

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