[Qgis-developer] WMS Design Problem and Mac Crash

[Tim Sutton]

Hi

2007/2/12, Magnus Homann <magnus at homann.se>:

>
> I'm not sure this is the right place to discuss thsi, but it would be
> good to have a way to dra partial rasters, that is no reading the entire
> file into memory. Good for *huge* rasters and *very zoomed in* rasters.
>
> See ticket #574.
>
> Is this related in any way?
>
> Magnus

My understanding of gdal is that it does not read the entire file into
memory in these situations:

I asked Frank Warmerdam about this over on GDAL and his answer was
basically that performance is basically a factor of:

- presence of overviews ('pyramids' in QGIS)
- the file format characteristics

So in these situations where an overview exists the IO required for
e.g. a very zoomed out image is substantially less. I'm guessing that
simple formats like ArcInfo ASCII grid are not very effiecient in this
respect since they dont have pyramids or scanlines.

One thing I noticed while looking at the docs is that performance can
be dramatically improved for some drivers by using AdviseRead prior to
performing read operations:

http://www.gdal.org/classGDALDataset.html#03a02a76682d883b378a2241528775c7


Peter perhaps another goodie to add to our todo list for raster
revamping if you are interested...?

Regards

-- 
Tim Sutton

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