[COG] Reading from the overviews in cog

[Grégory Bataille]

makes sense, although I did *want* to use the buf_xsize, buf_ysize :)

Thanks

---
Gregory Bataille


On Tue, Jun 12, 2018 at 6:26 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> On mardi 12 juin 2018 16:47:08 CEST Grégory Bataille wrote:
> > 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!
>
> You probably misused the (xsize, ysize) parameters instead of the
> (buf_xsize, buf_ysize).
>
> (xoff, yoff, xsize, ysize) are in full resolution image coordinates
> buf_xisze, buf_ysize is the dimension of the output buffer
>
> > 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
>
> Little demo. Let's generate a 1024x1024 raster, with 512x512 overview
>
> $ gdal_translate byte.tif test.tif -outsize 1024 1024
> $ gdaladdo test.tif 2
>
> 1) Reading whole raster at full resolution:
>
> $ CPL_DEBUG=ON python -c "from osgeo import gdal; ds =
> gdal.Open('test.tif'); ds.ReadAsArray()"
> [...]
> GDAL: GDALOpen(test.tif, this=0x1685430) succeeds as GTiff.
> [...]
>
> 2) Reading whole raster at overview resolution:
>
> $ CPL_DEBUG=ON python -c "from osgeo import gdal; ds =
> gdal.Open('test.tif'); ds.ReadAsArray(buf_xsize=512, buf_ysize=512)"
> [...]
> GDAL: GDALOpen(test.tif, this=0xdae480) succeeds as GTiff.
> [...]
> GTiff: ScanDirectories()
> GTiff: Opened 512x512 overview.
> [...]
>
> >
> > 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
>
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/cog/attachments/20180612/26e09027/attachment-0001.html>