[mapserver-users] How MapServer uses overviews in GDAL?

[Frank Warmerdam]

Juozas Urbonas wrote:
> Hi All.
>  
> I'm building a GDAL plugin on top of some ActiveX map component, I want 
> to use it with MapServer and provide WMS service. What is the best way 
> to implement overviews? I was thinking about arbitrary overviews 
> approach, but does MapServer utilizes arbitrary overviews, because my 
> HasArbitraryOverviews is not getting called by MapServer. 
> Or maybe discrete overviews is the only way to go with MapServer?
> Please advise.

Juozas,

Use of GDAL managed overviews is handled entirely within GDAL.  MapServer
just calls RasterIO() asking for a region of the file on disk read at a
particular resolution and GDAL takes care of using the most appopropriate
overviews, and such.

"arbitrary overviews" is just a flag that a GDAL driver can return for
formats which effectively can read efficiently at any resolutions.  In
practice it is not widely the case.  But it isn't possible for you to
prepare arbitrary overviews for a dataset except by encoding it in one
of the particular formats.

Generally speaking you would use gdaladdo to add descrete overview levels
to an existing GDAL supported raster file, after which they would be used
automatically by GDAL and MapServer.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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