[Gdal-dev] optimum overview settings?

[Chapman, Martin]

Frank,

Hummm, I thought I didn't get any output when I tried ovr's other than
power of 2.  Maybe I had something else wrong in my code at the time.  I
will try it again.  That would be great if that's the case.

Thanks Frank,

Martin

-----Original Message-----
From: Frank Warmerdam [mailto:warmerdam at pobox.com] 
Sent: Friday, March 12, 2004 2:09 PM
To: gdal-dev at remotesensing.org
Subject: Re: [Gdal-dev] optimum overview settings?


Matt Wilkie wrote:
> Hi Listers,
> 
> What is the optimum number of overviews versus file size when stored 
> in
> geotiffs? And if the end goal is viewing perfomance, as measured by an

> application like ArcMap? Mapserver?
> 
> The gdal_utilities web page[0] gives "2 4 8 16" as an example, however
> significant file size savings can be realized if we use just 4-16.
What 
> is sacrificed by dropping 2?
> 
> Are powers of two just convenience? Would 3 9 18 36 work just as well?
> 
> And does the size of the image determine how many overview levels are
> useful? For example a 100mb image might not benefit much from an upper

> overview of 16, while a 3gb image might need to go all the way up 128
or 
> more.
> 
> Are there any guidelines for choosing overview levels?

Matt,

"Optimum number of overviews versus file size" does not compute.  If you
are just opitmizing for file size then the optimum number of overviews
is none.  Powers of 3 should be fine, and at PCI I used 3 9 27 81 243 as
the default set of overviews to build.  Powers of 2 are more
traditional.

Skipping the x2, x4 and x8 overviews will results in substantially
slower loads of regions at between x2 and x16 resolution.  In some
applications this may be significant (ie. MapServer) while in others it
won't matter much.

Chapman, Martin wrote:
 > Matt,
 >
 > You can create whatever overviews you want in an ovr file, but they
> definitely must be a power of two, so you can have 1, 2, 4, 8, 16, 32
> ,64 ,128 , 256, 512, 1024, 2048, 4096...ect...or 2, 16, 4096, or just
8.

I am not aware of a requirement that the overviews be powers of 2.  If
you can document a bug with non-powers-of-2 overview levels I would like
to know (via bugzilla).  There may be a requirement for integer overview
levels. I'm not too sure about that.

Best regards,
-- 
---------------------------------------+--------------------------------
---------------------------------------+------
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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