[Qgis-user] image rendering "degradation" when using overviews

[maning sambale]

Hi,

I have a set of digital paper maps.  See this example for original
jpeg maps [0].  Now the original maps are not really very nice anyway
when rectified because of the polygon hatched styling.

I then combined all the maps using both VRT [1] and one big GTiff [2].
 The rendering quality is just as bad especially when zooming to full
extent.

When I added external overviews using the Raster > Build Overview
tool, the overviews are much worst when zoomed to full extent both in
the VRT [3] and GTiff [4]

Any idea why?
I think the best option eventually is to digitize (by hand since its
hard to automaticaly extract polygons due to the hatching) rather than
using the images as underlay.  I'm puzzled by the bad result of the
overview rendering.

Using QGIS 1.7, GDAL 1.8

[0] http://dl.dropbox.com/u/2096185/qgi_overview_degradation/original_jpg.png
[1] http://dl.dropbox.com/u/2096185/qgi_overview_degradation/vrt.png
[2] http://dl.dropbox.com/u/2096185/qgi_overview_degradation/tif.png
[3] http://dl.dropbox.com/u/2096185/qgi_overview_degradation/vrt_ovrvw.png
[4] http://dl.dropbox.com/u/2096185/qgi_overview_degradation/tif_ovrvw.png

-- 
cheers,
maning
------------------------------------------------------
"Freedom is still the most radical idea of all" -N.Branden
wiki: http://esambale.wikispaces.com/
blog: http://epsg4253.wordpress.com/
------------------------------------------------------