[Gdal-dev] Re: Rendering optimization [Re: Looking for huge vector dataset]

[Frank Warmerdam]

Schuyler Erle wrote:
> * On  6-Mar-2007 at 11:55PM PST, Mateusz Loskot said:
>>>> 1. load the whole geometry data into memory, into simple
>>>> structures  (arrays) - this is similar to how Manifold works
>>> This is doomed to lead into problems once you try to open a theme
>>> with 5 million polygons...
>> Yup, in many cases it is.  However, both approaches introduce some
>> problems.  In all-in-memory approach, the obvious assumption is a
>> user has a *lot* of RAM space, 4-8 GB.  But in turn, it provides the
>> fastest access to features (accessing RAM is faster than disk or
>> network).
> 
> This is sort of a blue sky idea, but we build overviews of rasters for
> exactly this sort of purpose. I can easily imagine designing some kind
> of OGR-based tool that constructs a directory of progressively
> generalized versions of a master vector dataset according to a
> hierarchy of scales, and then offers an access API that selects the
> generalization for a scale appropriate to the task at hand. I can't
> speculate whether this sort of tool would belong in GDAL/OGR the same
> way that gdaladdo etc. does.

Schuyler,

Traditionally I have left this to higher level configuration.  For
instance distinct layers in mapserver for different map scales.  With
raster data it is obvious from a given RasterIO request what overview
level to read from. For vector data it requires more information than is
currently available to OGR to know what level of generalization to read at.

But I could imagine us having something like a SetScale() method on the layer
that could be used internally to pick the appropriate generalization.
Nevertheless, I'm still most comfortable leaving this to higher level logic
on top of OGR.

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