[COG] Reading from the overviews in cog

[Grégory Bataille]

Hey Even,

Yes I guess ;)

However, what you say is exactly what I did and what I had hoped but I
found that it was reading only the first 512x512 pixels of my big raster
and dumping at full res rather than downsampling it as I expected!
I expected so much the behavior you mention so much that it took me time to
understand what was actually happening!

Maybe I did something wrong then. I’ll retry again

Thanks for the quick answer as always
Greg

On Tue, 12 Jun 2018 at 16:42, Even Rouault <even.rouault at spatialys.com>
wrote:

> On mardi 12 juin 2018 16:20:47 CEST Grégory Bataille wrote:
> > Hello everyone,
> >
> > I’m trying to migrate my code to COG more and more rather than
> > pre-computing some views/data.
> >
> > I have had some great success with extracting partial full res raster
> > values and it’s great.
> >
> > One thing I have not been able to do so far though is « reading from the
> > overview »
> > Say for example I want to render a 512x512 tiles of data but really
> zoomed
> > out. Let’s say for example that this tile contains my entire
> 10’000x10’000
> > pixels geotiff. It’s obviously super inefficient to read from the raw
> > raster rather than reading from one of the pre-computed overviews.
> > Is there an API (python) to ask for raster data (between 2 points) (like
> > ReadAsArray which is the one I currently use) but passing it the output
> > size so that gdal can figure out which overview it’s better to read from?
> >
>
> Grégory,
>
> This is a GDAL question in fact :-)
>
> Yes, the full signature of Dataset.ReadAsArray() is
>
>     def ReadAsArray(self, xoff=0, yoff=0, xsize=None, ysize=None,
>                     buf_obj=None,
>                     buf_xsize=None, buf_ysize=None, buf_type=None,
>                     resample_alg=gdalconst.GRIORA_NearestNeighbour,
>                     callback=None,
>                     callback_data=None,
>                     interleave='band'):
>
>
> So specify buf_xsize and buf_ysize (or pass a buf_obj of the appropriate
> output size)
>
> Even
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>
> --
Greg
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