[postgis-devel] [PostGIS] #413: ST_Single_Sided_Buffer added

[Pierre Racine]

Right. In Arc, you can apply the OUTSIDE_ONLY option only to polygons. It is not possible to apply it to polylines. For me the result of building parallele lines along polylines (returning polylines) and building a buffer around polylines (returning polygons) is so similar that I don't see the need to create another function.

ST_Buffer should in any case allow the possibility to get polylines, not polygons. You could always ST_LineFromGeom(ST_Buffer()) to convert your polygon buffers to lines but aparently there is no such function as LineFromGeom... How do we convert polygons to polylines?

Pierre

>-----Original Message-----
>From: postgis-devel-bounces at postgis.refractions.net [mailto:postgis-devel-
>bounces at postgis.refractions.net] On Behalf Of Martin Davis
>Sent: 19 février 2010 15:49
>To: PostGIS Development Discussion
>Subject: Re: [postgis-devel] [PostGIS] #413: ST_Single_Sided_Buffer added
>
>As I read it, the ArcGIS Buffer method is defined to return only
>polygons.  This is a different situation than ST_OffsetCurve, which
>returns lines.
>
>I think that having the ST_OffsetCurve name is good, to indicate that
>the function returns linework.  That allows users to conceptualize the
>simple rule that ST_Buffer always returns polygonal geometry.
>
>Pierre Racine wrote:
>> In ArcGIS they pass one of the four following options:
>>
>> {FULL | LEFT | RIGHT | OUTSIDE_ONLY}
>>
>> http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Buffer_%28Analysis%29
>>
>> Ex.:
>>
>> gp.buffer("roads.shp", "buffered_roads.shp", "Distance", "OUTSIDE_ONLY", "ROUND", "LIST",
>"Road_Type")
>>
>> The last two are of no interest for PostGIS. The ROUND is the same as ST_Buffer's endcap. Probably
>there should be a 4th option to ST_Buffer's buffer_style_parameters to indicate the surface occupied
>by the buffer:
>>
>> 'lineside=full|left|right|outside'
>>
>> You probably want to be ablt to do "right|outside" or "left|outside" at the same time.
>>
>> Probably, to completely fulfill rafalmag's needs, it would be nice to add "none" as endcap, so we
>don't get any end cap at all.
>>
>> Pierre
>>
>>
>>> -----Original Message-----
>>> From: postgis-devel-bounces at postgis.refractions.net [mailto:postgis-devel-
>>> bounces at postgis.refractions.net] On Behalf Of strk
>>> Sent: 19 février 2010 14:42
>>> To: PostGIS Development Discussion
>>> Subject: Re: [postgis-devel] [PostGIS] #413: ST_Single_Sided_Buffer added
>>>
>>> On Fri, Feb 19, 2010 at 01:52:03AM -0200, George Silva wrote:
>>>
>>>> I'm sorry to give my 2 cents, but in most softwares everything it's just
>>>> called buffer, with different parameters.
>>>>
>>> Could you please provide a full call example of a few of these softwares
>>> to obtain what the ST_Single_Sided_Buffer would give ?
>>> What it does is giving a linestring offsetted from the original
>>> (no area returned).
>>>
>>> --strk;
>>>
>>>  ()   Free GIS & Flash consultant/developer
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>
>--
>Martin Davis
>Senior Technical Architect
>Refractions Research, Inc.
>(250) 383-3022
>
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