# 2D slice from 3D solution **URL:** \ **Category:** Uncategorized\ **Created:** [March 9, 2022, 10:06am UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808 "2022-03-09T10:06:10Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Ingineering\_DjouFEA](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/ingineering_djoufea/32/3507_2.png) [@Ingineering\_DjouFEA](https://fenicsproject.discourse.group/u/Ingineering_DjouFEA)\ **Post date:** [March 9, 2022, 10:06am UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/1 "2022-03-09T10:06:10Z")
Hello everyone, I need your help. Indeed I solved a transient problem of soft tissue mechanics on a 3D slab. However, I would like to have the solution on an xy plane of the domain for z=0 and in the middle (z=0.5). I have used some solutions that I have found on the internet but, I get no result. I have mainly tested the solutions in the links: [https://fenicsproject.org/qa/3257/building-2d-function-g-x-y-from-a-3d-function-f-x-y-z-constant/](https://fenicsproject.org/qa/3257/building-2d-function-g-x-y-from-a-3d-function-f-x-y-z-constant/) > **[Interpolate nonmatching mesh · mikaem/fenicstools Wiki](https://github.com/mikaem/fenicstools/wiki/Interpolate-nonmatching-mesh)** > > Contribute to mikaem/fenicstools development by creating an account on GitHub. > **[Interpolation nonmatching mesh in parallel · mikaem/fenicstools Wiki](https://github.com/mikaem/fenicstools/wiki/Interpolation-nonmatching-mesh-in-parallel)** > > Contribute to mikaem/fenicstools development by creating an account on GitHub. Thank you in advance. ```auto # Create a 2D unit square mesh from the boundary of mesh3D mesh2 = BoundaryMesh(mesh3D, "exterior") side = MeshFunction("size_t", mesh2, 0) xyplane = AutoSubDomain(lambda x: x[2] < 1e-8) xyplane.mark(side, 1) mesh3 = SubMesh(mesh2, side, 1) x = mesh3.coordinates() x[:, 2] += 0.5 VH2 = FunctionSpace(mesh3, "DG", 1) # solution obtained from mechanical problem u,p=up.split() # Interpolation for 2D slice from 3D u1 = interpolate(u, VH2) ``` ---
**Author:** ![DGarciaLahuerta](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dgarcialahuerta/32/2673_2.png) [@DGarciaLahuerta](https://fenicsproject.discourse.group/u/DGarciaLahuerta)\ **Post date:** [March 9, 2022, 2:22pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/2 "2022-03-09T14:22:13Z")
You can try defining an expression that you can interpolate to get rid of your third axis. Something like this: ```auto #Dummy problem from fenics import * import matplotlib.pyplot as plt # Create mesh and define function space mesh = UnitCubeMesh(8, 8, 8) V = FunctionSpace(mesh, 'CG', 1) # Define boundary condition u_D = Constant(0) def boundary(x, on_boundary): return on_boundary bc = DirichletBC(V, u_D, boundary) # Define variational problem u = TrialFunction(V) v = TestFunction(V) f = Constant(100) a = dot(grad(u), grad(v))*dx L = f*v*dx # Compute solution u = Function(V) solve(a == L, u, bc) # Plot solution and mesh plt.colorbar(plot(u)) plt.title('u') ``` ![full_sol](https://global.discourse-cdn.com/free1/uploads/fenicsproject1/original/2X/3/3eccd80556e45be306428b2f6582331929a08b53.png) Define an expression to evaluate `u` at some value of z: ```auto class u_slice(UserExpression): def eval(self, value, x): value[0] = u(x[0],x[1],0.5) def value_shape(self): return () mesh_2D = UnitSquareMesh(8, 8) V_2D = FunctionSpace(mesh_2D, 'CG', 1) u_sl = interpolate(u_slice(), V_2D) plt.colorbar(plot(u_sl)) plt.title('u in the middle plane') ``` ![plane_sol](https://global.discourse-cdn.com/free1/uploads/fenicsproject1/original/2X/7/7c29445d59f009a588f4dc73bcd2bbaffd98421d.png) ---
**Author:** ![Ingineering\_DjouFEA](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/ingineering_djoufea/32/3507_2.png) [@Ingineering\_DjouFEA](https://fenicsproject.discourse.group/u/Ingineering_DjouFEA)\ **Post date:** [March 9, 2022, 3:02pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/3 "2022-03-09T15:02:21Z")
Hi Thank you very much for your reply. I managed to do it differently. And it and your code work perfectly. Thank you. ---
**Author:** ![RemDelaporteMathurin](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/remdelaportemathurin/32/4543_2.png) [@RemDelaporteMathurin](https://fenicsproject.discourse.group/u/RemDelaporteMathurin)\ **Post date:** [June 30, 2023, 7:33pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/4 "2023-06-30T19:33:48Z")
Hi! Thanks for this! A few questions: 1. Plotting 3D functions with fenics doesn’t seem possible anymore Your code yields: ```auto --------------------------------------------------------------------------- TypeError Traceback (most recent call last) Cell In[2], line 30 27 solve(a == L, u, bc) 29 # Plot solution and mesh ---> 30 plt.colorbar(plot(u)) 31 plt.title('u') File /opt/conda/lib/python3.10/site-packages/dolfin/common/plotting.py:438, in plot(object, *args, **kwargs) 436 # Plot 437 if backend == "matplotlib": --> 438 return _plot_matplotlib(object, mesh, kwargs) 439 elif backend == "x3dom": 440 return _plot_x3dom(object, kwargs) File /opt/conda/lib/python3.10/site-packages/dolfin/common/plotting.py:279, in _plot_matplotlib(obj, mesh, kwargs) 277 from mpl_toolkits.mplot3d import axes3d # noqa 278 # Enabling the 3d toolbox requires some additional arguments --> 279 ax = plt.gca(projection='3d') 280 else: 281 ax = plt.gca() TypeError: gca() got an unexpected keyword argument 'projection' ``` 1. Is there a way to do what you did using a SubMesh of the 3D mesh? ---
**Author:** ![Ekrem\_Ekici](https://avatars.discourse-cdn.com/v4/letter/e/977dab/32.png) [@Ekrem\_Ekici](https://fenicsproject.discourse.group/u/Ekrem_Ekici)\ **Post date:** [July 2, 2023, 10:07am UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/5 "2023-07-02T10:07:54Z")
You can also save the solution as a .pvd or .xdmf file and read it back in paraview. Then you use the slice filter and export the 2D data as a .csv file. Then you can read and manipulate it with pandas. ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [July 2, 2023, 11:52am UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/6 "2023-07-02T11:52:21Z")
Try downgrading the matplotlib version. ---
**Author:** ![RemDelaporteMathurin](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/remdelaportemathurin/32/4543_2.png) [@RemDelaporteMathurin](https://fenicsproject.discourse.group/u/RemDelaporteMathurin)\ **Post date:** [July 2, 2023, 1:43pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/7 "2023-07-02T13:43:41Z")
Yep should work. However this should be downgrading to a very old version of matplotlib. Any chance this is another case where we should PR to dolfin to fix the deprecated syntax? ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [July 2, 2023, 5:59pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/8 "2023-07-02T17:59:29Z")
Its fine to make a PR to update it, we can get that merged rather quickly. Just note that only minor patches to external packages will be made. If you want 3D plotting I would recommend vedo or pyvista (not matplotlib). I’ve made a recent extension to interface with pyvista > **[GitHub - jorgensd/dolfin\_pyvista\_adapter: Adapter for Pyvista for arbitrary...](https://github.com/jorgensd/dolfin_pyvista_adapter)** > > Adapter for Pyvista for arbitrary order (discontinuous) Lagrange with Legacy Dolfin. - GitHub - jorgensd/dolfin\_pyvista\_adapter: Adapter for Pyvista for arbitrary order (discontinuous) Lagrange wit... [http://jsdokken.com/dolfin\_pyvista\_adapter/docs/demo.html](http://jsdokken.com/dolfin_pyvista_adapter/docs/demo.html) ---
**Author:** ![RemDelaporteMathurin](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/remdelaportemathurin/32/4543_2.png) [@RemDelaporteMathurin](https://fenicsproject.discourse.group/u/RemDelaporteMathurin)\ **Post date:** [July 3, 2023, 1:46pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/9 "2023-07-03T13:46:25Z")
Thanks for this Jorgen! I know matplotlib is not ideal in this situation. I’m trying to have 3D visualisations in a jupyter notebook in a Github Codespace for a workshop. I’ve given a go at vedo but couldn’t get it to work in Codespaces. Does dolfin\_pyvista\_adapter work in jupyter notebooks? ---
**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\ **Post date:** [July 3, 2023, 1:51pm UTC](https://fenicsproject.discourse.group/t/2d-slice-from-3d-solution/7808/10 "2023-07-03T13:51:25Z")
Yes, as shown in [dolfin\_pyvista\_adapter/docs/demo.ipynb at main · jorgensd/dolfin\_pyvista\_adapter · GitHub](https://github.com/jorgensd/dolfin_pyvista_adapter/blob/main/docs/demo.ipynb)