GEOSBufferWithStyle produce invalid geometry when quadsegs = 0

[kalenik]

GEOSBufferWithStyle doesn't produce valid geometry when quadsegs = 0

13:00:35 [gis] > select ST_IsValid(ST_Buffer(ST_GeomFromEWKT('SRID=3857;POLYGON((-6523441.741981709 -3639625.53883344,-6520995.75707658 -3639625.53883344,-6520995.75707658 -3637179.55392831,-6518549.77217145 -3637179.55392831,-6516103.78726632 -3637179.55392831,-6513657.802361189 -3637179.55392831,-6513657.802361189 -3634733.56902318,-6511211.81745606 -3634733.56902318,-6511211.81745606 -3632287.58411805,-6513657.802361189 -3632287.58411805,-6513657.802361189 -3629841.59921292,-6511211.81745606 -3629841.59921292,-6511211.81745606 -3627395.61430779,-6511211.81745606 -3624949.62940266,-6508765.83255093 -3624949.62940266,-6508765.83255093 -3622503.64449753,-6508765.83255093 -3620057.6595924,-6506319.8476458 -3620057.6595924,-6506319.8476458 -3617611.67468727,-6506319.8476458 -3615165.68978214,-6503873.862740669 -3615165.68978214,-6503873.862740669 -3612719.70487701,-6503873.862740669 -3610273.71997188,-6501427.87783554 -3610273.71997188,-6498981.89293041 -3610273.71997188,-6496535.90802528 -3610273.71997188,-6496535.90802528 -3612719.70487701,-6494089.923120149 -3612719.70487701,-6491643.93821502 -3612719.70487701,-6489197.95330989 -3612719.70487701,-6486751.96840476 -3612719.70487701,-6484305.983499629 -3612719.70487701,-6484305.983499629 -3615165.68978214,-6486751.96840476 -3615165.68978214,-6486751.96840476 -3617611.67468727,-6486751.96840476 -3620057.6595924,-6486751.96840476 -3622503.64449753,-6484305.983499629 -3622503.64449753,-6484305.983499629 -3624949.62940266,-6484305.983499629 -3627395.61430779,-6484305.983499629 -3629841.59921292,-6484305.983499629 -3632287.58411805,-6481859.9985945 -3632287.58411805,-6481859.9985945 -3634733.56902318,-6481859.9985945 -3637179.55392831,-6481859.9985945 -3639625.53883344,-6479414.01368937 -3639625.53883344,-6476968.02878424 -3639625.53883344,-6476968.02878424 -3642071.52373857,-6474522.043879109 -3642071.52373857,-6472076.05897398 -3642071.52373857,-6472076.05897398 -3644517.5086437,-6469630.07406885 -3644517.5086437,-6467184.08916372 -3644517.5086437,-6464738.104258589 -3644517.5086437,-6464738.104258589 -3646963.49354883,-6462292.11935346 -3646963.49354883,-6459846.13444833 -3646963.49354883,-6459846.13444833 -3649409.47845396,-6457400.1495432 -3649409.47845396,-6457400.1495432 -3651855.46335909,-6454954.164638069 -3654301.44826422,-6454954.164638069 -3656747.43316935,-6452508.17973294 -3656747.43316935,-6452508.17973294 -3659193.41807448,-6452508.17973294 -3661639.40297961,-6450062.19482781 -3661639.40297961,-6450062.19482781 -3664085.38788474,-6447616.20992268 -3664085.38788474,-6447616.20992268 -3666531.37278987,-6450062.19482781 -3666531.37278987,-6450062.19482781 -3668977.357695,-6450062.19482781 -3671423.34260013,-6452508.17973294 -3671423.34260013,-6452508.17973294 -3673869.32750526,-6454954.164638069 -3673869.32750526,-6454954.164638069 -3676315.31241039,-6454954.164638069 -3678761.29731552,-6457400.1495432 -3678761.29731552,-6454954.164638069 -3681207.28222065,-6454954.164638069 -3683653.2671257798,-6457400.1495432 -3686099.25203091,-6459846.13444833 -3686099.25203091,-6459846.13444833 -3688545.2369360398,-6462292.11935346 -3688545.2369360398,-6464738.104258589 -3688545.2369360398,-6464738.104258589 -3690991.22184117,-6464738.104258589 -3693437.2067462998,-6464738.104258589 -3695883.19165143,-6464738.104258589 -3698329.1765565597,-6462292.11935346 -3698329.1765565597,-6459846.13444833 -3698329.1765565597,-6459846.13444833 -3700775.16146169,-6457400.1495432 -3700775.16146169,-6457400.1495432 -3703221.1463668197,-6457400.1495432 -3705667.13127195,-6454954.164638069 -3705667.13127195,-6454954.164638069 -3708113.1161770797,-6454954.164638069 -3710559.10108221,-6454954.164638069 -3713005.0859873397,-6454954.164638069 -3715451.07089247,-6457400.1495432 -3715451.07089247,-6457400.1495432 -3717897.0557975997,-6457400.1495432 -3720343.04070273,-6457400.1495432 -3722789.0256078597,-6459846.13444833 -3722789.0256078597,-6459846.13444833 -3725235.01051299,-6459846.13444833 -3727680.9954181197,-6462292.11935346 -3727680.9954181197,-6462292.11935346 -3730126.98032325,-6464738.104258589 -3730126.98032325,-6464738.104258589 -3727680.9954181197,-6467184.08916372 -3727680.9954181197,-6469630.07406885 -3727680.9954181197,-6472076.05897398 -3727680.9954181197,-6472076.05897398 -3730126.98032325,-6474522.043879109 -3730126.98032325,-6476968.02878424 -3730126.98032325,-6479414.01368937 -3730126.98032325,-6479414.01368937 -3727680.9954181197,-6481859.9985945 -3727680.9954181197,-6484305.983499629 -3727680.9954181197,-6484305.983499629 -3725235.01051299,-6486751.96840476 -3725235.01051299,-6486751.96840476 -3722789.0256078597,-6489197.95330989 -3722789.0256078597,-6489197.95330989 -3720343.04070273,-6489197.95330989 -3717897.0557975997,-6489197.95330989 -3713005.0859873397,-6486751.96840476 -3710559.10108221,-6489197.95330989 -3710559.10108221,-6489197.95330989 -3703221.1463668197,-6491643.93821502 -3703221.1463668197,-6494089.923120149 -3703221.1463668197,-6494089.923120149 -3700775.16146169,-6494089.923120149 -3698329.1765565597,-6494089.923120149 -3695883.19165143,-6496535.90802528 -3695883.19165143,-6498981.89293041 -3693437.2067462998,-6506319.8476458 -3688545.2369360398,-6506319.8476458 -3686099.25203091,-6508765.83255093 -3683653.2671257798,-6506319.8476458 -3681207.28222065,-6503873.862740669 -3681207.28222065,-6503873.862740669 -3678761.29731552,-6503873.862740669 -3676315.31241039,-6503873.862740669 -3673869.32750526,-6506319.8476458 -3671423.34260013,-6506319.8476458 -3668977.357695,-6508765.83255093 -3668977.357695,-6508765.83255093 -3666531.37278987,-6506319.8476458 -3666531.37278987,-6506319.8476458 -3664085.38788474,-6503873.862740669 -3664085.38788474,-6501427.87783554 -3664085.38788474,-6501427.87783554 -3661639.40297961,-6498981.89293041 -3661639.40297961,-6498981.89293041 -3659193.41807448,-6501427.87783554 -3659193.41807448,-6503873.862740669 -3659193.41807448,-6503873.862740669 -3656747.43316935,-6506319.8476458 -3656747.43316935,-6506319.8476458 -3654301.44826422,-6508765.83255093 -3654301.44826422,-6511211.81745606 -3654301.44826422,-6513657.802361189 -3654301.44826422,-6513657.802361189 -3651855.46335909,-6516103.78726632 -3651855.46335909,-6516103.78726632 -3649409.47845396,-6518549.77217145 -3649409.47845396,-6518549.77217145 -3646963.49354883,-6520995.75707658 -3646963.49354883,-6520995.75707658 -3644517.5086437,-6520995.75707658 -3642071.52373857,-6523441.741981709 -3642071.52373857,-6523441.741981709 -3639625.53883344))'), 2445.98490513, 0));
NOTICE:  00000: Ring Self-intersection at or near point -6503873.8627406703 -3661639.4029796096
LOCATION:  pg_notice, lwgeom_pg.c:364

GEOS Version: 3.10.0dev

[mdavis]

Odd, this behaviour is different to JTS - it produces a valid buffer geometry.

A quadSegs value of 0 is meaningless. So the code should either limit the valid to >= 1, or raise an error. In JTS the value is limited to 1, and has the effect of creating mitred joins.

In any case, the upstream process should avoid using quadsegs = 0.

[kalenik]

I tried to use quadsegs = 1 for geometry in my example and still received an error about ring self-intersection but with quadsegs = 2 valid geometry is produced. If JTS produced valid buffer geometry with quadsegs = 1 it looks like GEOS bug.

[robe]

Retargeting in prep for GEOS 3.10.0 release

[komzpa]

quadsegs=0 historically created bevel-like joins in GEOS.

[sergeish]

Simplified test case:

# SELECT ST_IsValid(ST_Buffer(ST_GeomFromEWKT('SRID=3857;POLYGON((-6503873.862740669 -3656747.43316935, -6481859.9985945 -3656747.43316935, -6481859.9985945 -3688545.2369360398, -6506319.8476458 -3688545.2369360398, -6506319.8476458 -3664085.38788474, -6501427.87783554 -3664085.38788474, -6501427.87783554 -3661639.40297961, -6498981.89293041 -3661639.40297961, -6498981.89293041 -3659193.41807448, -6503873.862740669 -3659193.41807448, -6503873.862740669 -3656747.43316935))'), 2445.98490513, 0));
NOTICE:  Ring Self-intersection at or near point -6503873.8627406703 -3661639.4029796096
 st_isvalid 
------------
 f
(1 row)

The result depends on exact coordinate values, the error goes away when input polygon is scaled so that coordinates and buffer width are integer (divided by buffer width).

Result:

Expected result, generated with geometry precision model type changed from PrecisionModel::FLOATING to PrecisionModel::FIXED, scale=10^10:

[mdavis]

JTS recently had the buffer quadSegs behaviour improved/clarified: ​JTS-778.

So the first thing to do is to port that fix, and see if that solves this problem. If not can then investigate further.