# [postgis-users] Problem with 3D polygon intersection test

Nicolas Ribot nicolas.ribot at gmail.com
Tue Oct 30 02:44:44 PDT 2012

```Hi,

st_intersection is only suitable for 2D operations. Third dimension is not
taken into account.

Nicolas

On 30 October 2012 09:44, Ed Linde <edolinde at gmail.com> wrote:

> Hi All,
> Is there a simpler test I can do? I am quite sure that I am probably not
> using the right
> function in my SQL, or not doing the intersection test correctly. Is there
> any simpler
> way to test this 3D intersection? Am waiting on the postgis gurus to come
> along and
> give me some ideas, coz I am at my wits ends on this one! :) The polygons
> look good
> to me when I visualize them, but don't get how all triangles are found as
> intersecting?
> Anyway will wait.
>
> Cheers,
> Ed
>
> On Tue, Oct 30, 2012 at 9:00 AM, Ed Linde <edolinde at gmail.com> wrote:
>
>> Hi All,
>> I am having some troubles getting the *ST_Intersect* function to work
>> with 3D polygons.
>> I have a table to hold triangles as polygons in *"small_tris"*
>> Another table for holding the polygonal faces of a 3D voronoi diagram in
>> *"vfaces"*
>> So now I am trying to do an intersection between the triangles and the
>> voronoi faces
>> to get the triangles that intersect with the faces. And it seems that
>> *ALL* triangles
>> are intersecting. I have attached a PDF where I have plotted the
>> triangles and the
>> voronoi cells. so the box around in red and blue with a major face going
>> through is
>> the voronoi diagram. The Voronoi partition in 3D is shown at the bottom
>> too for clarity
>> Is the select assuming something? So the polygons are flat and the
>> intersections
>> should just be lines or points. I am under some pressure to get this to
>> work, so if
>>
>> Cheers,
>> Ed
>>
>> *-- TABLE SCHEMAS*
>> bounds=# \d vfaces;
>>       Table "public.vfaces"
>>   Column  |   Type   | Modifiers
>> ----------+----------+-----------
>>  gen_id   | integer  |
>>  geomtext | geometry |
>>
>> bounds=# \d small_tris;
>>     Table "public.small_tris"
>>   Column  |   Type   | Modifiers
>> ----------+----------+-----------
>>  id       | integer  | not null
>>  v0       | integer  |
>>  v1       | integer  |
>>  v2       | integer  |
>>  geomtext | geometry |
>> Indexes:
>>     "small_tris_pkey" PRIMARY KEY, btree (id)
>>     "small_idx" gist (geomtext)
>>
>>
>> *
>> -- TABLE CONTENT*
>> bounds=# select id, st_astext( geomtext ) from small_tris;
>>  id |                       st_astext
>> ----+-------------------------------------------------------
>>   0 | POLYGON Z ((10 10 805,20 10 809,10 20 814,10 10 805))
>>   1 | POLYGON Z ((10 20 814,20 10 809,20 20 819,10 20 814))
>>   2 | POLYGON Z ((20 20 819,30 10 809,30 20 812,20 20 819))
>>   3 | POLYGON Z ((30 10 809,20 20 819,20 10 809,30 10 809))
>>   4 | POLYGON Z ((30 20 812,30 10 809,40 10 826,30 20 812))
>>   5 | POLYGON Z ((20 20 819,30 20 812,20 30 827,20 20 819))
>>   6 | POLYGON Z ((10 20 814,20 20 819,20 30 827,10 20 814))
>>   7 | POLYGON Z ((10 40 805,10 30 814,20 30 827,10 40 805))
>>   8 | POLYGON Z ((10 50 804,10 40 805,20 40 807,10 50 804))
>>   9 | POLYGON Z ((20 30 827,20 40 807,10 40 805,20 30 827))
>>  10 | POLYGON Z ((30 30 817,20 40 807,20 30 827,30 30 817))
>>  11 | POLYGON Z ((30 40 816,20 50 809,20 40 807,30 40 816))
>>  12 | POLYGON Z ((20 40 807,20 50 809,10 50 804,20 40 807))
>>  13 | POLYGON Z ((10 30 814,10 20 814,20 30 827,10 30 814))
>>  14 | POLYGON Z ((30 30 817,20 30 827,30 20 812,30 30 817))
>>  15 | POLYGON Z ((30 20 812,40 20 823,30 30 817,30 20 812))
>>  16 | POLYGON Z ((40 20 823,30 20 812,40 10 826,40 20 823))
>>  17 | POLYGON Z ((40 20 823,40 10 826,50 10 803,40 20 823))
>>  18 | POLYGON Z ((30 30 817,40 20 823,40 30 814,30 30 817))
>>  19 | POLYGON Z ((40 20 823,50 20 808,40 30 814,40 20 823))
>>  20 | POLYGON Z ((50 20 808,40 20 823,50 10 803,50 20 808))
>>  21 | POLYGON Z ((40 30 814,50 20 808,50 30 816,40 30 814))
>>  22 | POLYGON Z ((30 30 817,40 30 814,40 40 800,30 30 817))
>>  23 | POLYGON Z ((30 40 816,40 40 800,30 50 813,30 40 816))
>>  24 | POLYGON Z ((30 40 816,30 30 817,40 40 800,30 40 816))
>>  25 | POLYGON Z ((30 50 813,40 40 800,40 50 817,30 50 813))
>>  26 | POLYGON Z ((30 40 816,30 50 813,20 50 809,30 40 816))
>>  27 | POLYGON Z ((50 40 821,40 50 817,40 40 800,50 40 821))
>>  28 | POLYGON Z ((50 50 809,40 50 817,50 40 821,50 50 809))
>>  29 | POLYGON Z ((50 40 821,40 40 800,50 30 816,50 40 821))
>>  30 | POLYGON Z ((40 30 814,50 30 816,40 40 800,40 30 814))
>>  31 | POLYGON Z ((30 30 817,30 40 816,20 40 807,30 30 817))
>> (32 rows)
>>
>>  select st_astext(geomtext) from vfaces;
>>
>> st_astext
>>
>> -----------------------------------------------------------------------------------------------
>>  POLYGON Z ((51 17.8 799,9 38.8 799,9 51 819.33333333333,19.4 51 828,51
>> 35.2 828,51 17.8 799))
>>  POLYGON Z ((51 17.8 799,9 38.8 799,9 51 799,51 51 799,51 17.8 799))
>>  POLYGON Z ((51 35.2 828,19.4 51 828,51 51 828,51 35.2 828))
>>  POLYGON Z ((9 9 799,9 38.8 799,51 17.8 799,51 9 799,9 9 799))
>>  POLYGON Z ((51 17.8 799,9 38.8 799,9 51 819.33333333333,19.4 51 828,51
>> 35.2 828,51 17.8 799))
>>  POLYGON Z ((9 9 828,9 51 828,19.4 51 828,51 35.2 828,51 9 828,9 9 828))
>> (6 rows)
>>
>> *select distinct a.id
>> from small_tris a, vfaces b
>> where st_intersects (a.geomtext, b.geomtext)
>> order by a.id;*
>>  id
>> ----
>>   0
>>   1
>>   2
>>   3
>>   4
>>   5
>>   6
>>   7
>>   8
>>   9
>>  10
>>  11
>>  12
>>  13
>>  14
>>  15
>>  16
>>  17
>>  18
>>  19
>>  20
>>  21
>>  22
>>  23
>>  24
>>  25
>>  26
>>  27
>>  28
>>  29
>>  30
>>  31
>> (32 rows)
>>
>>
>>
>
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