0004279: A sketch appears to be fully constrained but sometimes a constraint flips - FreeCAD Tracker

NOTICE: Migration to GitHub Issues

On Feb 7, 2022, the FreeCAD project migrated all issues from this site at tracker.freecad.org to our main GitHub repository. All new bugs must be submitted there.
This Mantis repository is in read-only mode and will be retained for reference as long as it is useful.

For details please see the announcement at the Forums: https://forum.freecadweb.org/viewtopic.php?p=555883#p555883

ID	Project	Category	View Status	Date Submitted	Last Update

0004279	Sketcher	Bug	public	2020-03-04 07:33	2020-07-14 18:09

Reporter	jurgen_g	Assigned To	chrisb
Priority	normal	Severity	minor	Reproducibility	unable to reproduce
Status	closed	Resolution	no change required
Platform	Ubuntu	OS	Linux	OS Version	19.10

Summary	0004279: A sketch appears to be fully constrained but sometimes a constraint flips
Description	In occasions I notice how a chain of horizontal or vertical constraints appear to be fully constrained. But after changing parametric values (spreadsheed), suddenly a point flips from left of the previous point to right of it. All constraints are still met (all the distances are correct) but one constraint changed direction/orientation. Maybe a horizontal/vertical constraint could also add a "fixed orientation" check-mark. Workarounds are available, making this non-critical. My basic frustration/bug is that a sketch appears to be fully constrained (green lines) while in reality it isn't. It's not possible to slide any element of the sketch to a different position but different positions are possible to reach this point.
Steps To Reproduce	Simplified example of the sketch (screenshot) are attached. Notice that I could not trigger the flip to happen predictably so this example is a mock-up.
Tags	No tags attached.

FreeCAD Information	OS: Ubuntu 19.10 (ubuntu:GNOME/ubuntu) Word size of OS: 64-bit Word size of FreeCAD: 64-bit Version: 0.19. Build type: Release Python version: 3.7.5 Qt version: 5.12.4 Coin version: 4.0.0a OCC version: 7.3.0 Locale: English/United Kingdom (en_GB)

jurgen_g 2020-03-04 07:33 reporter	Screenshot from 2020-03-03 08-21-31.png (230,205 bytes) Screenshot from 2020-03-03 08-21-31.png (230,205 bytes) Screenshot from 2020-03-03 08-23-02.png (230,363 bytes) Screenshot from 2020-03-03 08-23-02.png (230,363 bytes)

abdullah 2020-06-02 14:31 manager ~0014455	All unfortunate that this is from a user perspective, it is not something that is fixable in the software. The solver provides a solution that is valid. There are the same number of constraints as parameters. It fulfils the condition of a fully constraint. In order to try to avoid flipping, the selection of the constraints used in the sketch is very important. Experienced users in the help forum are an invaluable source of wisdom regarding this topic.

chrisb 2020-07-14 18:09 developer ~0014648	Fully constrained doesn't mean unique. There is no continuous transition from one solution to another though. Everything works as designed. If you would have followed the extra big banner asking to open a forum discussion before opening a ticket, we could have explained this immediately.

Date Modified	Username	Field	Change
2020-03-04 07:33	jurgen_g	New Issue
2020-03-04 07:33	jurgen_g	File Added: Screenshot from 2020-03-03 08-21-31.png
2020-03-04 07:33	jurgen_g	File Added: Screenshot from 2020-03-03 08-23-02.png
2020-06-02 14:31	abdullah	Note Added: 0014455
2020-07-14 18:09	chrisb	Assigned To	=> chrisb
2020-07-14 18:09	chrisb	Status	new => closed
2020-07-14 18:09	chrisb	Resolution	open => no change required
2020-07-14 18:09	chrisb	Note Added: 0014648