ST_CurveToLine not behaving as expected on 3 point circle

[zimmicz]

Compare results of this query

SELECT ST_GeomFromText('CIRCULARSTRING( -1 0, 0 0, -1 0)'),
(SELECT ST_CurveToLine(ST_GeomFromText('CIRCULARSTRING( -1 0, 0 0, -1 0)'))),
(SELECT ST_GeomFromText('CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0)')),
(SELECT ST_CurveToLine(ST_GeomFromText('CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0)')));

The second statement returns a line with half a radius of original circularstring.

[dbaston]

Are you using QGIS by chance? It looks to me like QGIS is incorrectly rendering CIRCULARSTRING ( -1 0, 0 0, -1 0 ) . It should look like the ST_CurveToLine result but instead passes through the point (1,0).

Interestingly, Microsoft considers the use of a three-point CIRCULARSTRING to define a circle invalid: ​https://msdn.microsoft.com/en-us/library/ff929141.aspx

[pramsey]

When we look at this in another renderer (JTS test builder), the linearized version is as expected (circle w/ one edge on 0,0 and the other on -1,0)