sketcher: changing angle results in unresolvable sketch

[uwestoehr]

I stumble frequently over cases when I suddenly are in an unresolvable sketch state. I found now a reproducible case:

- open the attached test file:

Sketch-Array-bug-FC-018-result.FCStd

test case

(10.98 KiB) Downloaded 18 times

- there is only one angle constraint. Change it to -45°

result: your sketch became unresolvable.

The point is that if the solver fails, the input should be ignored and the former (last resolvable state should be kept).
(I don't understand why the solver fails because it should be geometrically possible. However, this is not the point.)

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16117 (Git)
Build type: Release
Branch: releases/FreeCAD-0-18
Hash: dbb4cc6415bac848a294f03b80f65e888d531742
Python version: 3.6.6
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: German/Germany (de_DE)

[chrisb]

This seems to be indeed a problem. If I change the unconstraint arcs as shown in the image, I can change the angle to a value of -45. I will try to simplify the sketch. It seems too complicated to me to help Abdullah to comfortably find the problem.

[chrisb]

Bildschirmfoto 2019-06-28 um 22.57.33.png (11.48 KiB) Viewed 763 times

The problem seems to come from the circles getting negative radius values. In the attached example the same issue occurs if you change the length constraint to 50mm.

The error does not occur if I change one of the slots to a circle.

ping

[chrisb]

issue #4042

[chrisb]

I could simplify it even further. This sketch contains only 4 elements.

- Load file
- edit sketch
- change dimension to 50

Bildschirmfoto 2019-06-28 um 23.26.36.png (11.01 KiB) Viewed 751 times

[uwestoehr]

I could simplify it even further.

Many thanks Chris for having a look and providing a simple test case.