[Qgis-developer] Ticket 778 and slow loading rasters
ersts at amnh.org
Wed Apr 16 13:33:34 EDT 2008
Frank Warmerdam wrote:
> I would be interested to investigate why getting approximate statistics
> for this file is so slow. I does not seem like it ought to be.
> In fact the ECW driver in GDAL is somewhat special in that it does not
> have fixed overview levels, which means that approximate statistics are
> computed by fetching a sampling of full resolution blocks. I think it
> would be better to fix this by modifying GDAL to select an appropriate
> resolution and read the whole image highly decimated to get statistics
> for formats like ECW with "arbitrary" overviews.
> I'll work on this now and see if it helps alot.
Thanks Frank, that sounds great but we may still want to consider a QGIS
work around for the time being as well. The threshold idea I am
proposing would only take about 3 or 4 lines of code.
FYI, I used the GDAL 1.5 stable branch and version 3.3 (2006-08-29) of
ECW JPEG 2000 SDK.
>> --> Keep the default contrast enhancement as linear min-max on initial
>> --> If the total number of pixels in the image is greater than some
>> threshold revert back to "no stretch" as the default on initial load
> I would like to think this isn't necessary. I am however confused
> about why 8bit images are stretched by default. The default rule in
> OpenEV for instance is to do no stretching for 8bit images, but to
> use linear stretch for non-8bit images.
There never really was a concerted design decision about stretching
8-bit data. I can see an argument for or not doing it, but with
everything equal and the speed at which GDAL can give an approximate min
max, I say why not.
Peter J. Ersts, Project Specialist
American Museum of Natural History
Center for Biodiversity and Conservation
Central Park West at 79th Street
New York, New York 10024
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