Chamfer fails for bspline corner

[andershol]

Description

Applying a chamfer to an object fails, when the object has been created as an extrusion of a path having two bsplines meet in a concave corner. The path looks fairly well behaved i.e. the control points are evenly spaced, the curves are smooth and there do not seem to be some kind of knot or infinitesimal small or large values. Further more moving the corner just a little bit (relative to the size of the object) allows the chamfer to work fine (e.g. replace -76.6894 with -76.75 for the Y value below). This is annoying as it is hard to workout why it fails and what might be done to fix it.

This is a test-case reduced from this FreeCAD issue.

Expected Behavior

Nice chamfer

Actual Behavior

Chamfer fails

Sample Code or DRAW Tcl Script

pload ALL
vinit
dset Y -76.6894 # Makes blend fail
#dset Y -76.75   # Ok
bsplinecurve bc1 3 2 0 4 1 4 73.33 -78.324136 0 1 79.1083 -77.1377 0 1 86.4629 -76.9095 0 1 93.8515 Y 0 1
bsplinecurve bc2 3 2 0 4 1 4 93.8515 Y 0 1 107.14 -77.7741 0 1 117.599 -78.5754 0 1 125.89 -80.3085 0 1
polyline line 125.89 -80.3085 0 125.89 -68.760604 0 73.33 -68.760604 0 73.33 -78.324136 0
mkedge ebc1 bc1
mkedge ebc2 bc2
wire w ebc1 ebc2 line
mkplane f w
prism solid f 0 0 20
explode solid e
vdisplay solid solid_4 solid_7
vfit
blend result solid 1 solid_4 solid_7
vdisplay result

Operating System

Windows

Compiler

GCC

Bitness

64-bit

OCCT Version

7.9.3 - pre-compiled drawexe.exe from OCCT release. Also fails with 7.8.1 used by FreeCADs LibPack

Additional Files

No response