[pgrouting-dev] Question about improving alpha shapes

[Steve Horn]

For what it's worth:

Wouldn't get too excited about the PostGIS 2.0 implementation of
st_concavehull. It does allow for more flexibility but I found it to be a
couple orders of magnitude slower than the pgRouting (CGAL) implementation.

On Wed, Aug 22, 2012 at 4:03 PM, Stephen Woodbridge <woodbri at swoodbridge.com
> wrote:

> Hello T,
>
> Lots of good questions.
>
> In general, as you found out, "One size fits all" solutions tend to not
> work well for everyone. So I think have multiple solvers for various
> problems is a good idea. In an ideal world, we would build things such that
> they can be chained together, such that the output of one part can be fed
> as input into the next part.
>
> So if we had something like:
>
> solve graph -> points with costs -> makealpha -> geometries
>
> Where makealpha could be any number of solutions like convex hulls,
> concave hulls, your_code, etc
>
> In fact I have code the takes points with costs and makes a triangulated
> surface and then slices that at z-levels and extracts geometries from that,
> but not integrated into pgRouting at the moment.
>
> So, yes, contributions are welcome. Don't think of them as needing to be
> exclusive of other existing solutions.
>
> Thanks,
>   -Steve
>
>
> On 8/22/2012 3:29 PM, T S wrote:
>
>> Hello,
>>
>> For some projects I've been working on, I've had to make some
>> modifications to the alphashape method -
>> namely, allowing a custom alpha instead of just picking the optimal
>> regularly connected alpha, and allowing for disconnected multipolygons
>> instead of only returning a single polygon.
>>
>> The use case that comes up is that some networks have holes or gaps in
>> the 2d plane. I.e., driving over a bridge, you can get to the
>> landmasses on either side of the bridge, but you can't really get to
>> all the water around the bridge. This water shouldn't be included in
>> the polygon.
>>
>> I have an initial implementation of how this works, I have a few
>> questions about trying to contribute it in -
>> 1.) Is there any interest in this feature? Or does pgRouting prefer
>> the current behavior?
>> 2.) I noticed an earlier email thread discussing moving to
>> st_concavehull in postgis 2.0. While this would remove the dependency
>> on CGAL, st_concavehull doesn't appear to support returning multiple
>> disconnected geometries, and it's not really a true alpha shape.
>> 3.) If there is interest in moving this in, where should I look to for
>> coding standards, and which parts of the API are 'stable' and which
>> aren't? in specific -
>> 4) why does points_as_polygon return a setof geoms instead of just a
>> geometry?
>> 5) is it important for there to be both a points_as_polygon() and an
>> alphashape() function, or could the functionality be rolled up?
>>
>> Thank you,
>> T
>>
>>
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-- 
Steve Horn
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