[postgis-users] Better way of removing pixelation with postgis raster

[Pierre Racine]

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-bounces at postgis.refractions.net [mailto:postgis-users-
> bounces at 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;
> ------------------------------------
> 
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