I have a postgis table containing polygons. I want to transform my polygons to 4 points polygons : retrieve the smallest MBR.

Here comes an illustration of what I want :

alt text

My original polygon is in pink color. I guess, it will be quite easy to get the MBR (with ST_Envelope and BBox function) But is there an easy way to get what I want?

Any ideas? Many thanks


For that simple example you have there, the ST_ConvexHull would give you that answer. For the more complex case, haven't thought much about it, but my guess is that you can

1) Take the ST_ConvexHull - http://www.postgis.org/documentation/manual-1.5SVN/ST_ConvexHull.html

If you are using PostGIS 1.5+ 2) Then get the ST_LongestLine of the convexhull ST_LongestLine(ST_ConvexHull(geom), ST_ConvexHull(geom)) That will give you two corners of the rectangle (though there is another part -- I'm missing) since those two corners may not be the diagonal) http://www.postgis.org/documentation/manual-1.5SVN/ST_LongestLine.html

You may also want to take a look at the code for ST_MinimumBoundingCircle -- its currently just implemented as a plpgsql function -- but what you are doing I think is a simplification of that.



You can find it described here: http://www.cgafaq.info/wiki/Minimum_Rectangle_Enclosing_Points

Which leads to: http://cgm.cs.mcgill.ca/~orm/maer.html


A complete answer can be found here however in summary:

  • [S] rotate the polygon to match one of its edges with X axis
  • compute minmax of X and Y for vertices
  • this gives you a candidate for minimum area rectangle (MAR) or MBR
  • store the area of (x2-x1)*(y2-y1)
  • repeat from [S] for all edges
  • find the minimum area which corresponds to MBR
  • rotate found [[x1,y1],[x2,y2]] by -angle applied in [S]


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