[postgis-devel] ST_Buffer - single sided option

[Stephen Knox]

Hi Darafei,

Thanks for that good feedback.

Actually I think thinking it through I wonder if it would be better to have
the option as single_side=left or single_side=right as I think that is much
more intuitive. All the tests I've done on geometries indicate that geos
honours the distance parameter as expected (positive is left hand side of
the line and negative right - see
https://geos.osgeo.org/doxygen/classgeos_1_1operation_1_1buffer_1_1BufferParameters.html#adc2afac5dacc613a39a72542bdb8bf21).
The only issue is by default it gives right hand buffers round joins and
left hand buffers mitre joins, but this can be overridden.

Really this seems to be of greatest use for linestrings, as the buffer
function already had a negative distance parameter for polygons, and
whether single_side is true or not does not make any difference. With
points again it ignores the parameter and undertakes a normal buffer, which
is the only sensible thing to do.

I'll work on that basis and add in the code, tests and docs for the revised
function and also submit a pull request from Github.

Thanks again

Stephen

On Thu, Jan 18, 2018 at 10:23 AM, Darafei "Komяpa" Praliaskouski <
me at komzpa.net> wrote:

> Hi,
>
> Thoughts that I have:
>
>  - this is a cool thing that I've needed multiple times. I worked around
> its absence by fiddling with ST_MakeLine(ST_Collect(line,
> ST_OffsetCurve(line, 1))) and then trying to make result valid, which
> sometimes worked;
>  - how do I remember which way is it being offset? I expect
> ST_Buffer(ST_Boundary(ST_ForceRHR(polygon)), 10, 'single_side') to be
> buffered "outside". Does it hold true?
>  - if there are no better ideas on ST_Buffer signature I'd say your option
> is good enough for me;
>  - there's no error reporting mentioning `single_side` as valid option;
>  - can `single_side=1` also work for `single_sided=1` and just
> `single_side`/`single_sided`?
>  - what does it do for line and for point?
>  - does it work on EMPTY, LINESTRING(0 0, 0 0), MULTILINESTRING, and
> self-intersecting one?
>  - how do I reverse the direction of buffering? I understand there's
> negative buffer parameter, does it work? I see you parse parameter to be
> single_side=1/0, maybe -1 will also make sense?
>  - out of curiosity, will ST_Buffer(geom,0,'single_side') fix invalid
> geometries? :)
>
> As someone responsible for github in PostGIS, I encourage you to put it to
> GitHub as a PR: that'll get you test coverage report, travis CI greenlight,
> a way to comment on your patch inline, and usual PR merge time on it
> recently improved. :)
>
> чт, 18 янв. 2018 г. в 12:56, Stephen Knox <stephenknox73 at gmail.com>:
>
>> Hi List,
>>
>> I noticed that PostGIS didn't implement the single-sided buffer
>> functionality of Geos (NB. not the Offset Curve functionality, but a
>> polygon), so have submitted a patch to implement that:
>> https://trac.osgeo.org/postgis/ticket/3989
>>
>> I have refrained from adding tests and documentation until I get some
>> agreement that the function signature is acceptable, as I am a new
>> contributor, and I think the ST_Buffer signature is already quit "unusual"
>> in that it implements text parameters.
>>
>> I was hoping that post the recent patch releases, a more experienced
>> contributor might be able to look at my (small) patch and give some
>> feedback.
>>
>> I can put it up on Github as well if that would help.
>>
>> Many thanks
>>
>> Stephen
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>
>
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