[Gdal-dev] optimum overview settings?

[Frank Warmerdam]

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,
-- 
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