[Qgis-developer] speed issues - using compressed imagery as background for digitizing

[karsten vennemann]

Hi All,

I have  been working with a client that previously has been using ArcGIS for
digitizing areas in the field on tablet laptops. Due to some frustrations
with ArcGIS and ArcPad we went ahead and implemented a simple python plug-in
that is based on the stream digitizing python plug-in, but adds a (csv file)
configure simple custom form with prefilled drop down boxes. The whole
things works great. Except one issue is that using compressed imagery such
as ecw or *.sid images the speed of the application is pretty slow when
digitizing (compared to ArcGIS).

Thus I have some questions for ideas or input on this.

I am assuming that the speed in QGIS totally depends on the driver used for
the ecw and .sid images - which as I understand is the ERMapper /Erdas
driver. 
Is there anything that could be done that the stream digitizing of polygons
(which itself without compressed imagery is blazing fast for shape files)
that can be done that one does not suffer such big performance hits when
using a compressed image as a background...

As a work around I recommended to the client to use non-compressed imagery
such as tiff - which when it has overviews is performing really well.
However that means bigger storage space needed and the need to decompress
the imagery and add overview, which we can use some gdal scripts for ....but
still we would like to avoid these steps if possible. 

Thus, this is just to see if there options to optimize anything or there is
nothing at this point that can be done to have better performance with
compressed imagery.

Also is there anybody that could report on how the speed for other formats
such as JPEG2000 compares to ecw  to MrSID (other formats)? 
Anyone ?

Thanks a lot 
Cheers


Karsten Vennemann
Principal

Terra GIS LTD
www.terragis.net