MySQL Bugs: #46372: buffer of linestring returns multipolygon in certain cases

Bug #46372	buffer of linestring returns multipolygon in certain cases
Submitted:	24 Jul 2009 15:31	Modified:	7 Aug 2012 12:27
Reporter:	John Powell	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: GIS	Severity:	S3 (Non-critical)
Version:	mysql-5.1-wl1326	OS:	Any
Assigned to:	Assigned Account	CPU Architecture:	Any
Tags:	buffer, gis, linestring, multipolygon

Description:
For some linestrings, the buffer function returns a multipolygon, when the output should always be a single polygon, possibly with inner rings. The 2nd geometry in the multipolygon is a single, repeated point. It is easy to work around this bug, but it could cause confusion.

How to repeat:
create geometry from linestring -- attached below, as too long for initial report

buffer by 100m and select geometry type of output

mysql>set @buff=buffer(@geom,100); select geometrytype(@buff);
+---------------------+
| geometrytype(@buff) |
+---------------------+
| MULTIPOLYGON        | 
+---------------------+

select the 2nd geometry of this multipolygon

mysql> select astext(geometryn(@buff,2));
+------------------------------------------------------------------------+
| astext(geometryn(@buff,2))                                             |
+------------------------------------------------------------------------+
| POLYGON((447495.42 1201570.5,447495.42 1201570.5,447495.42 1201570.5)) | 
+------------------------------------------------------------------------+

The output is a single, repeated point.

linestring to create geometry for buffering

Attachment: ls.csv (text/csv), 40.12 KiB.

Thank you for the bug report. Verified as described:

mysql> set @buff=buffer(@geom,100); select geometrytype(@buff);
Query OK, 0 rows affected (1 min 8.05 sec)                     

+---------------------+
| geometrytype(@buff) |
+---------------------+
| MULTIPOLYGON        |
+---------------------+
1 row in set (0.00 sec)

mysql> select astext(geometryn(@buff,2));
+------------------------------------------------------------------------+
| astext(geometryn(@buff,2))                                             |
+------------------------------------------------------------------------+
| POLYGON((447495.42 1201570.5,447495.42 1201570.5,447495.42 1201570.5)) |
+------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> show variables like "%version%";
+-------------------------+---------------------+
| Variable_name           | Value               |
+-------------------------+---------------------+
| protocol_version        | 10                  |
| version                 | 5.1.35-debug        |
| version_comment         | Source distribution |
| version_compile_machine | x86_64              |
| version_compile_os      | unknown-linux-gnu   |
+-------------------------+---------------------+
5 rows in set (0.01 sec)

mysql>

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/91672

2663 Alexey Botchkov	2009-11-24
      Bug#46372 buffer of linestring returns multipolygon in certain cases 
         mistake in the algorithm - dx_dy parameter should be just dx if
         dy is 0.
      
      per-file comments:
        sql/gcalc_slicescan.cc
      Bug#46372 buffer of linestring returns multipolygon in certain cases 
           GET_DX_DY fixed
        sql/gcalc_slicescan.h
      Bug#46372 buffer of linestring returns multipolygon in certain cases 
           some debugging features added
        sql/item_geofunc.cc
      Bug#46372 buffer of linestring returns multipolygon in certain cases 
           debugging code added

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/92967

2664 Alexey Botchkov	2009-12-04
      Bug#46372      buffer of linestring returns multipolygon in certain cases
          there was a mistake in handling of 'holes' in the polygons.
          Fixed by adding the 'first_poly_node' pointer to the res_point
          structure to point to the first point of the surrounding polygon.
          Also a set of debugging functions was added. It doesn't affect the
          working fucntions, but greatly simplifies the debugging.
          This path fixes also bug#46498 and bug#45883 also.
      
      per-file comments:
        sql/gcalc_slicescan.cc
      Bug#46372      buffer of linestring returns multipolygon in certain cases
        sql/gcalc_slicescan.h
      Bug#46372      buffer of linestring returns multipolygon in certain cases
        sql/gcalc_tools.cc
      Bug#46372      buffer of linestring returns multipolygon in certain cases
        sql/gcalc_tools.h
      Bug#46372      buffer of linestring returns multipolygon in certain cases
        sql/item_geofunc.cc
      Bug#46372      buffer of linestring returns multipolygon in certain cases
        sql/spatial.cc
      Bug#46372      buffer of linestring returns multipolygon in certain cases

The bug as stated is fixed, however the problem of buffers producing mutlipolygons remains, if the points are very close together.

set @ls=geomfromtext('LINESTRING(1000 1000, 1000.01 1000.01)');

select geometrytype(buffer(@ls,2));
+-----------------------------+
| geometrytype(buffer(@ls,2)) |
+-----------------------------+
| MULTIPOLYGON                |
+-----------------------------+

mysql> select numgeometries(buffer(@ls,2));
+------------------------------+
| numgeometries(buffer(@ls,2)) |
+------------------------------+
|                            3 |
+------------------------------+

This bug was earlier fixed in mysql-gis tree (before mysql-5.6 release).