[Qgis-developer] Ticket 778 and slow loading rasters

Peter Ersts ersts at amnh.org
Wed Apr 16 10:31:53 EDT 2008

Howdy all,

Carlos Dávila has been good enough to keep reminding me of Ticket 778 
(http://trac.osgeo.org/qgis/ticket/778) and supplied me with one of his 
ecw images. The image that Carlos sent me is 53k pixels by 14k pixels 
and it took about 106 seconds to load on my older desktop.

The slowness on initial load has to do with determining the min and max 
values for contrast enhancement, the default is a linear min-max 
stretch. On initial load of a raster, we have it set so that GDAL 
provides an approximation of the min and max for each band, later the 
user can elect to load the actual min max -- very slooooow right now. 
Even with only asking for an approximation, with an image that size 
there is going to be a bit of a delay. If you set the default contrast 
enhancement to "no stretch" these same data load in only 2 seconds.

Originally, we had the default contrast enhancement set to "no stretch" 
but that was met with some pretty strong opposition, so simply changing 
the default behavior is not an option. So I propose a slight compromise 
and would like to know how everyone feels about it.

--> 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
--> Add an "approximation" and "actual" option to the raster properties 
window for the "Load min max from band" button

Rational: very large images are probably a mosaic of some type and 
already have been preprocessed to some extent and therefore probably 
don't actually need to be stretched. This way, smaller images will still 
always be stretch and users will see what they expect and larger data 
will load considerably faster. The threshold will have to be though 
about, but I was thinking something around 75 million pixels. Something 
larger than a Landsat ETM.

Let me know your thoughts or if you have any reservations.


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
Tel: Home Office (518)-632-4745 or NYC Office (212)-496-3488
Web: http://biodiversityinformatics.amnh.org
Web: http://cbc.amnh.org

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