[postgis-users] order of points in ST_ConvexHull

[Martin Davis]

Whoops, yeah, you're right - that was a typo bug.  Good that it works 
for you....

Armin Burger wrote:
> Thanks, I tried your suggestion and it looks like the following works:
>
>  Max(x + y) => upper right
>  Min(x + y) => Lower Left
>  Max(abs(x - y)) => Upper Left
>  Min(abs(x - y)) => Lower Right
>
> Armin
>
> On 26/02/2009 22:11, Martin Davis wrote:
>> Or perhaps more simply, you can test the points using the following 
>> criteria:
>>
>> Max(x + y) => upper right
>> Min(x + y) => Lower Left
>> Max(abs(x - y)) => Lower Right
>> Min(abs(x - y)) => Lower Left
>>
>> Sufficool, Stanley wrote:
>>> Pull the centroid of the geometry and use generate_series() / pointn()
>>> to "explode" the points and test each point for above & right of
>>> centroid, above and left of centroid, etc...
>>>
>>>  
>>>> -----Original Message-----
>>>> From: postgis-users-bounces at postgis.refractions.net 
>>>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of 
>>>> Armin Burger
>>>> Sent: Thursday, February 26, 2009 11:57 AM
>>>> To: PostGIS Users Discussion
>>>> Subject: [postgis-users] order of points in ST_ConvexHull
>>>>
>>>>
>>>> Hello
>>>>
>>>> I need to find a possibility to identify upper-left, upper-right, 
>>>> etc. corners of +/- rectangular polygons. I.e. polygons with 
>>>> guranteed just 4 corners, but with a shape that is typically 
>>>> between a rectangle and a rhomb. The polygons define the geometry 
>>>> of image boundaries ("image footprints"). But it cannot be 
>>>> guaranteed which point in the polygon corresponds to which corner 
>>>> since the order of points during geometry creation is unknown.
>>>>
>>>> One idea was to use ST_ConvexHull(geometry) since for this very 
>>>> simple polygons the convex hull seems to be identical with the 
>>>> geometry. It looked to me that the order in this convex hull was:
>>>>    lower-right, lower-left, upper-left, upper-right
>>>> Does anybody know if this order is always like that or can this 
>>>> order change? Would anybody know another method to identify which 
>>>> point of the polygon corresponds to which corner?
>>>>
>>>> Regards
>>>>
>>>> Armin
>>>> _______________________________________________
>>>> postgis-users mailing list postgis-users at postgis.refractions.net
>>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>>
>>>>     
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>>
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-- 
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022