Ticket #4387 - One redundant constraint prevents me from moving elements

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padawan
Posts: 13
Joined: Sat Jun 13, 2020 7:16 pm

Ticket #4387 - One redundant constraint prevents me from moving elements

Postby padawan » Sun Jun 21, 2020 2:16 pm

I think I don't understand how constraints work.
Situation is as follows:

arcWith9Constraints.png
arcWith9Constraints.png (7.93 KiB) Viewed 282 times

There are 9 constraints:
2 fix the 2 points on the arc
5 coincidences (the 2 extremes of the arc with the 2 extremes of the lines, the other 2 extremes of the lines with the origin, and the center of the arc with the origin)
2 symmetries (the 2 extremes of the arc with the y axis, and the 2 points on the arc with the y axis)

In this situation I can't move anything.
There is one redundant constraint, because I don't need both points to be fixed on the arc. One point fixed and the symmetry should be enough.
Indeed, if I remove one of the constraints fixing the points on the arc, then I can move things (obviously within the limits of my constraints).

Why is that? Why is a redundant constraint preventing me from moving anything?
Last edited by Kunda1 on Sun Jun 28, 2020 9:16 pm, edited 1 time in total.
Reason: Added ticket number to thread title
Bance
Posts: 464
Joined: Wed Feb 11, 2015 3:00 pm
Location: London

Re: One redundant constraint prevents me from moving elements

Postby Bance » Sun Jun 21, 2020 2:22 pm

padawan
Posts: 13
Joined: Sat Jun 13, 2020 7:16 pm

Re: One redundant constraint prevents me from moving elements

Postby padawan » Sun Jun 21, 2020 2:58 pm

Aaaaah, thank you. I don't get why redundant constraints should give an error, but at least it's an explanation.
But then, why don't I get an error? What I have is

noRedundantConstraintError.png
noRedundantConstraintError.png (16.01 KiB) Viewed 270 times

Which is what I would expect.
Clicking on update has no effect.
If it can help, I am using Linux (Debian testing).

By the way, @chrisb: thanks for the tutorial. I am slowly finding my way, but this kind of documentation is always helpful.
Bance
Posts: 464
Joined: Wed Feb 11, 2015 3:00 pm
Location: London

Re: One redundant constraint prevents me from moving elements

Postby Bance » Sun Jun 21, 2020 3:06 pm

padawan
Posts: 13
Joined: Sat Jun 13, 2020 7:16 pm

Re: One redundant constraint prevents me from moving elements

Postby padawan » Sun Jun 21, 2020 3:21 pm

I have to say that these answers are not exactly easy to understand.

Are you saying that I am not being nice? That would honestly surprise me.
Are you saying that I should search? I didn't find anything. If I missed something please enlighten me.
Are you saying that I should post some more information, like my FreeCAD version or my FreeCAD file? You can say that, I didn't do it because I thought I was doing something wrong, being a complete noob.
As to the FreeCAD version up to date, I am using what I have with my Debian testing: 0.18.4. I can install the latest manually, if needed.

Anyway, for completeness of information I am attaching my FCStd file. Don't know what to do more than that, sorry.

EDIT: sorry, i reattached the file. I noticed that I didn't save it. It shouldn't make a difference anyway
Attachments
test.FCStd
(3.5 KiB) Downloaded 10 times
Bance
Posts: 464
Joined: Wed Feb 11, 2015 3:00 pm
Location: London

Re: One redundant constraint prevents me from moving elements

Postby Bance » Sun Jun 21, 2020 4:01 pm

You should be nice, and you should search, but most importantly you should include all the information that someone might need to help you. There is a big banner at the top of the page that describes how to make a good post for this forum.
Now there is a file we can have a look and see what the problem is. :D

padawan wrote:
Sun Jun 21, 2020 3:21 pm
Are you saying that I should post some more information, like my FreeCAD version or my FreeCAD file? You can say that, I didn't do it because I thought I was doing something wrong, being a complete noob.
Bance
Posts: 464
Joined: Wed Feb 11, 2015 3:00 pm
Location: London

Re: One redundant constraint prevents me from moving elements

Postby Bance » Sun Jun 21, 2020 4:25 pm

It's locked because you've used point on point constraints rather than point on line.
Ignore this
Might be a bug, it depends on the order that constraints are applied, If the point on line constraint(9) is applied before the second symmetry an error is raised.
Also the sketch is not completely locked, if further constraints are added the sketch responds.(try to add a radius)
Last edited by Bance on Sun Jun 21, 2020 4:34 pm, edited 1 time in total.
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Roy_043
Posts: 1399
Joined: Thu Dec 27, 2018 12:28 pm

Re: One redundant constraint prevents me from moving elements

Postby Roy_043 » Sun Jun 21, 2020 4:31 pm

It seems to me that the analysis of the OP is spot on.
Apart from the original issue, there is the fact that the redundant constraint (point on arc) is not detected and automatically removed.
chrisb
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Joined: Tue Mar 17, 2015 9:14 am

Re: One redundant constraint prevents me from moving elements

Postby chrisb » Sun Jun 21, 2020 4:46 pm

Such issues still occur in 0.19 if so called partially redundant constraints are involved. This occurs if internally one constraint is split into several, which happens e.g. with point-to-point tangency or symmetry; especially symmetry is a recurring problem child.
If such a constraint can not completely be dropped, but only parts of it, then the redundant part is silently dropped. Sometimes this leads to problems as you encounter them now.
I am not a friend of such silent drops, but if it works all right it may help beginners.
padawan
Posts: 13
Joined: Sat Jun 13, 2020 7:16 pm

Re: One redundant constraint prevents me from moving elements

Postby padawan » Sun Jun 21, 2020 5:43 pm

Bance wrote:
Sun Jun 21, 2020 4:01 pm
You should be nice, and you should search, but most importantly you should include all the information that someone might need to help you.
I really didn't expect it to come to this point. I was convinced someone would come and say "you are doing this wrong"
chrisb wrote:
Sun Jun 21, 2020 4:46 pm
Such issues still occur in 0.19 if so called partially redundant constraints are involved. This occurs if internally one constraint is split into several, which happens e.g. with point-to-point tangency or symmetry; especially symmetry is a recurring problem child.
If such a constraint can not completely be dropped, but only parts of it, then the redundant part is silently dropped. Sometimes this leads to problems as you encounter them now.
I am not a friend of such silent drops, but if it works all right it may help beginners.
I am a little curious about the logic. I have the unfortunate habit of trying to understand things. People always say that it's good, until the moment that i start asking questions :-)
The way I see it there are 3 constraints that add some information: the 2 symmetries (or 1 symmetry and the center of the arc in the origin) and one of the points on arc. The other constraint for the second point on arc is totally redundant. One of them, constraint 9, is also last in the list.
Shouldn't this constraint be picked up as 100% redundant?