[Qgis-user] constructing polygons based on raster values

[Eelke Folmer]

Dear all,
I'm looking for a way to construct a polygon file based on raster values 
(but not to make a multi-polygon for each raster cell). Similar to 
contourlines I suppose except that I want to end up with polygons. I 
tried to do it via contourlines but got messy (self-intersections) 
geometry when trying to construct polygons from the contourlines.
Any hints on how to do this?
Thanks,
Eelke