Odd bugs with Constructive Solid Geometry 2D
3 years 10 months ago #3468
by mcmcclur
Odd bugs with Constructive Solid Geometry 2D was created by mcmcclur
I've noticed some odd behavior when generating 2D meshes using Constructive Solid Geometry. Consider the following code, which should generate the intersection of two overlapping circles:
We instead get a single circle centered at at the origin. The same thing happens with a union. An intersection yields an empty mesh; it's like the two circles are the same. If we shift the center a small amount, the code works as expected.
I've done this using a conda install on my Macbook Pro as well as with an apt-get install on Ubuntu 20.04 LTS, both with the same result.
Code:
import netgen.gui
from netgen.geom2d import CSG2d, Circle
from ngsolve import Mesh
geo = CSG2d()
c0 = Circle(center=(0,0),radius=1, bc='a')
c1 = Circle(center=(1,0),radius=1, bc='b')
domain = c0 * c1
geo.Add(domain)
mesh = Mesh(geo.GenerateMesh())
We instead get a single circle centered at at the origin. The same thing happens with a union. An intersection yields an empty mesh; it's like the two circles are the same. If we shift the center a small amount, the code works as expected.
I've done this using a conda install on my Macbook Pro as well as with an apt-get install on Ubuntu 20.04 LTS, both with the same result.
3 years 10 months ago #3476
by matthiash
Replied by matthiash on topic Odd bugs with Constructive Solid Geometry 2D
Hello,
I cannot reproduce the issue with Netgen 6.2.2009-69-gea7f6c1e.
Which version are you running? If it's older than 6.2.2009, I suggest an update.
Best,
Matthias
I cannot reproduce the issue with Netgen 6.2.2009-69-gea7f6c1e.
Which version are you running? If it's older than 6.2.2009, I suggest an update.
Best,
Matthias
The following user(s) said Thank You: mcmcclur
3 years 10 months ago #3478
by mcmcclur
Thanks for the response. I am running 6.2.2009 on both my Mac and on my Ubuntu server. I've converted a Jupyter notebook to a webpage so you can see my version and full code here:
marksmath.org/ProblematicMeshIllustration.html
Replied by mcmcclur on topic Odd bugs with Constructive Solid Geometry 2D
matthiash wrote: I cannot reproduce the issue with Netgen 6.2.2009-69-gea7f6c1e.
Which version are you running? If it's older than 6.2.2009, I suggest an update.
Thanks for the response. I am running 6.2.2009 on both my Mac and on my Ubuntu server. I've converted a Jupyter notebook to a webpage so you can see my version and full code here:
marksmath.org/ProblematicMeshIllustration.html
3 years 10 months ago - 3 years 10 months ago #3479
by mcmcclur
Replied by mcmcclur on topic Odd bugs with Constructive Solid Geometry 2D
Here's a similar problem. To produce this, I set up a virtual machine running Ubuntu 20.04 LTS on Digital Ocean. Python version is 3.8. I then logged in as root and followed almost exactly the
Ubuntu instructions on this page.
- thus:
(Since I was root, I didn't need sudo.)
I then used my favorite, console based text editor to create a file named "ng_bad_script.py" with the following content:
If I run this script, meshing then fails:
The situation is worse if I delete the point (0.5,0.5) from the Solid2d object and store the program in "ng_really_bad_script.py", for then I enter an infinite loop:
Note that loclines.Size=1 printout repeats until I terminate.
Again, this is a totally fresh install and the exact same thing happens on my Mac with an Anaconda based installation.
To be clear, though, I'm generally quite impressed with the software and I'm looking forward to using it to generate examples for the PDE class that I'll be teaching this semester. Examples where the points are in generic position tend to work; thus, problems like the above tend to go away if the points are perturbed a bit.
Code:
apt-add-repository universe
add-apt-repository ppa:ngsolve/ngsolve
apt-get update
apt-get install ngsolve
(Since I was root, I didn't need sudo.)
I then used my favorite, console based text editor to create a file named "ng_bad_script.py" with the following content:
Code:
#!/usr/bin/python3
from ngsolve import Mesh
from netgen.geom2d import Solid2d, Circle, CSG2d
dom = Solid2d([(0,0),(1,0),(1,1),(0.5,0.5)])-Circle((0.5,0.5),0.2)
geo = CSG2d()
geo.Add(dom)
mesh = Mesh(geo.GenerateMesh())
If I run this script, meshing then fails:
Code:
root@netgen:~# python3 ng_bad_script.py
importing NGSolve-6.2.2009
Generate Mesh from spline geometry
Boundary mesh done, np = 30
CalcLocalH: 30 Points 0 Elements 0 Surface Elements
Meshing domain 1 / 1
load internal triangle rules
give up with qualclass 201
number of frontlines = 27
Surface meshing done
Traceback (most recent call last):
File "ng_script.py", line 9, in <module>
mesh = Mesh(geo.GenerateMesh())
netgen.libngpy._meshing.NgException: meshing failed
The situation is worse if I delete the point (0.5,0.5) from the Solid2d object and store the program in "ng_really_bad_script.py", for then I enter an infinite loop:
Code:
root@netgen:~# python3 ng_really_bad_script.py
importing NGSolve-6.2.2009
Generate Mesh from spline geometry
Boundary mesh done, np = 30
CalcLocalH: 30 Points 0 Elements 0 Surface Elements
Meshing domain 1 / 1
load internal triangle rules
loclines.Size = 1
loclines.Size = 1
loclines.Size = 1
...
...
Note that loclines.Size=1 printout repeats until I terminate.
Again, this is a totally fresh install and the exact same thing happens on my Mac with an Anaconda based installation.
To be clear, though, I'm generally quite impressed with the software and I'm looking forward to using it to generate examples for the PDE class that I'll be teaching this semester. Examples where the points are in generic position tend to work; thus, problems like the above tend to go away if the points are perturbed a bit.
Last edit: 3 years 10 months ago by mcmcclur.
3 years 10 months ago #3480
by matthiash
Replied by matthiash on topic Odd bugs with Constructive Solid Geometry 2D
Thanks for the detailed information. I discovered the reason for this bug in the source code. I will let you know, when it's fixed.
Best,
Matthias
Best,
Matthias
The following user(s) said Thank You: mcmcclur
3 years 9 months ago #3489
by matthiash
Replied by matthiash on topic Odd bugs with Constructive Solid Geometry 2D
The issues are fixed on the master branch. Nightly builds will be available tomorrow.
Best,
Matthias
Best,
Matthias
The following user(s) said Thank You: mcmcclur
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