[FWTools] Speed of rendering large (2GB) ECW image at normal screen size

[Frank Warmerdam]

On 4/28/05, Luca Manganelli <luca76 at gmail.com> wrote:
> Hi all,
> 
> today I downloaded the latest release of FWTools for windows (0.9.8).
> I tried the demo in python, OpenEV_FW, to open a locally stored large
> ECW raster file (about 2GB). The demo was incredibly slow in
> visualizing in its small window (about 800 x 600).
> Do you know if it is a fault of the demo or of the library?

Luca, 

I believe this is mostly a defect of OpenEV's raster handling strategy
and the way that GDAL represents ECW files to it.  In particular, 
OpenEV creates a texture for each 256x256 chunk of the full res file.
In overview on very large files this means that virtually every pixel
on screen is a separate texture, and results in a seperate io request
to the file - a most pathological case. 

I would add this isn't a problem for MapServer.  In the raster code
we take care to only make one request to the file per band, so at
worst there are 3 reads from the ECW file for any given render, and
they are done in such a way that full advantage is taken of the ECW
overview capability.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent