[postgis-devel] Faster ST_Intersection

[Regina Obe]

We've discussed this before I've especially wanted a BOX 3D geometry primitive as that's a very big thing in 3D augmented reality

I think all the 3D toolkits (I know X3Dom has it, I believe three js has it too).

 

I think we tried to get at least the concept of a true BOX type in 2.0 and we got into arguments about moving forward on it.

Only one I can think of is SQL/MM specs don't support the thing so we'd be deviating from the specs.

 

I forget what other arguments were against it.  

 

From: postgis-devel [mailto:postgis-devel-bounces at lists.osgeo.org] On Behalf Of Darafei "Kom?pa" Praliaskouski
Sent: Friday, October 26, 2018 9:09 AM
To: PostGIS Development Discussion <postgis-devel at lists.osgeo.org>
Subject: Re: [postgis-devel] Faster ST_Intersection

 

It looks like for polygon / box intersection area can be calculated in O(n), by just clamping the ordinates (https://github.com/postgis/postgis/commit/3233ccd19cdb6958b53164262cd812d3f6d70fb2). 

 

It also may be adapted for any convex polygons, with O(n*m) though. Convex check also seems to be O(n). Building dataset tends to be mostly convex (and mostly 4-corner), so this may be a specialized fast path.

 

Is there a chance we may want BOX() as new geometry primitive to save a is_convex / is_box check? :)

 

Any other thoughts?

 

On Tue, Oct 23, 2018 at 8:04 PM Paul Ramsey <pramsey at cleverelephant.ca <mailto:pramsey at cleverelephant.ca> > wrote:

The first GIS system I ever used had an “intersection area” function that was distinct from, and faster than, the full-on “intersection” function that output a new geometry. I wonder if there is some possibility there…

 

P





On Oct 23, 2018, at 6:01 PM, Darafei Komяpa Praliaskouski <me at komzpa.net <mailto:me at komzpa.net> > wrote:

 

Hi,

 

I have two polygonal layers to intersect and need to measure area of intersections.

 

ST_Area(ST_Intersection(a.geom, b.geom)) works fine, but is rather slow. Typical polygon on first side has <10 edges, on second 200..1000 edges.

 

What are my options to make it faster? Any ideas, links, experiences to share are welcome.

 

For starters, this seems to work faster than vanilla ST_Intersection in my case:

create or replace function clipped_st_intersection(geom1 geometry, geom2 geometry)
  returns geometry
as $$select ST_Intersection(
              case
                when ST_MemSize(geom1) > 160 then ST_ClipByBox2D(geom1, geom2)
                else geom1 end, case
                when ST_MemSize(geom2) > 160 then ST_ClipByBox2D(geom2, geom1)
                else geom2 end)$$
language sql
immutable
strict
parallel safe;

-- 

Darafei Praliaskouski
Support me: http://patreon.com/komzpa

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Darafei Praliaskouski

Support me: http://patreon.com/komzpa

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