What if you use ST_Mapalgebra to isolate the value you want to work on before vectorizing so that you reduce a lot the number of pixel to vectorize?
In your case you would produce two intermediate rasters (one for class 1, one for class 2), vectorize/buffer/union/rasterize them and then merge them using ST_Union(rast)? Pierre > -----Original Message----- > From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users- > boun...@postgis.refractions.net] On Behalf Of Tom van Tilburg > Sent: Friday, July 06, 2012 9:37 AM > To: PostGIS Users Discussion > Subject: [postgis-users] Better way of removing pixelation with postgis raster > > Hi list, > > Im trying to create a landuse map for a customer that requires the > removal of pixelation (isolated islands of pixels, possible larger than > 1 pixel). > To make things complicated, different pixelclasses have different > tresholds for removal. > For instance, class 1 is removed when the combined pixels are less than > 4, whereas class2 is removed when the combined pixels are less than 50. > Furthermore, some classes count diagonal neighbours as 'touching' > whereas other classes discard them. > > My solution is given below. > As you can see there is an expensive buffer-union-buffer trio to make > sure I connect the correct polygons before I can calculate their area. > This calculation is part of a big raster calculation. All of the > calculations are going very fast, except this one. > Anyone with an idea to make it faster? In the end all I need is removal > of isolated islands of pixels so any matrix calculation would be fine as > well. > > thanks, Tom > > ------------------------------------------ > WITH > --Dump to polygons > polygons As > ( > SELECT (ST_DumpAsPolygons(rast)).geom geom, > (ST_DumpAsPolygons(rast)).val val > FROM out_landuse_rast_30 > ) > --Merge polygons > ,polygons_dissolved As > ( > SELECT > ST_Buffer((ST_Dump(ST_Union(ST_Buffer(geom,1)))).geom,-1) geom, val > FROM polygons > GROUP BY val > ) > > --Select polygons that should go out > ,polygons_out As > ( > SELECT St_Collect(geom) geom FROM polygons_dissolved > WHERE > (ST_Area(geom) >= ((30*30) * 4) --4 pixels > AND val IN (1) > ) > OR > (ST_Area(geom) >= ((30*30) * 50) --50 pixels > AND val IN (2) > ) > ) > > --Create raster (mask) that should go out > SELECT 1 As rid, ST_AsRaster(a.geom, b.rast, '8BUI',1,0,false) rast > FROM polygons_out a, emptycanvas30 b; > ------------------------------------ > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
