Test failure with OrientedEnvelope

[pnorman]

not ok 30 - ST_AsText(OrientedEnvelope(ST_GeomFromText('LINESTRING(0 0, 10 10, 8 12)')))
    expected: 'POLYGON((8 12,10 10,0 0,-2 2,8 12))'
    actual: 'POLYGON((2.30769230769231 -1.53846153846154,1.77635683940025e-15 -8.88178419700125e-16,8 12,10.3076923076923 10.4615384615385,2.30769230769231 -1.53846153846154))'

PostGIS full version is POSTGIS="2.1.7 r13414" GEOS="3.5.0-CAPI-1.9.0 r4084" PROJ="Rel. 4.9.1, 04 March 2015" GDAL="GDAL 1.11.2, released 2015/02/10" LIBXML="2.9.2" LIBJSON="UNKNOWN" RASTER, PostgreSQL is 9.4.4 server and client

[ajashton]

Thanks, will try to replicate.

[pnorman]

I stepped through the function on my new server, and an old server running 2.0 r9605 from SVN, where the test passed.

Stepping through manually, I get the same answer on both - the answer I get on my new server. But, looking closely at all the steps, I can see what's going on. Both return values have the same ST_Area, showing that there is not a unique solution. Thinking about it, it's obvious that this is the case: consider a regular polygon with N sides, there will be N solutions, unless some are equal. e.g. an equilateral triangle has three OrientedEnvelopes, and a circle will have infinitely many OrientedEnvelopes.

This linestring happens to trigger that case. When I step through by hand, I get slightly different rounding behavior, so it picks a different solution. The problem is the comparison area_cur < area_min, which depends on the floating point behavior of

• ST_Area
• ST_Envelope
• ST_Rotate
• ST_Azimuth
• pi()
• Any underlying system calls

As to why does it trigger that case? I believe any right-angled triangle will, but haven't gone through a geometric proof. Any isosceles certainly will. This case happens to be a 1-5-sqrt(26) right-angled triangle.

We should probably change the test so that we test a shape with a unique solution.

There's also another issue - should we guarantee a particular solution when there are multiple valid ones?

[pnorman]

ping @ajashton