fem-post

A Python API for post-processing fem-core results with VTK. You create views of a mesh and the fields solved on it, update them, and export them to .vtu (for ParaView) or .png, or open them in an interactive window.

fem-post lives in the fem-core repository for now (packages/fem-post/). It is self-contained (its own pyproject.toml, sources and tests, no fem_core import), so it can move to its own repository unchanged.

Installation

pip install ./packages/fem-post       # standalone
uv sync --extra viz                   # from the fem-core repository root (uv workspace member)

Dependencies: ngsolve, numpy, vtk. fem-post does not depend on fem-core. Solver results are accepted by shape (see fem_post.results), so fem-core's StaticResult, ModalResult and FrequencySweepResult work directly, and so does any object with the same attributes.

Usage

from fem_post import PostProcessor

post = PostProcessor(mesh)  # samples the mesh once; every view shares it

mesh_view = post.add_mesh_view("mesh")  # colored by material region
mesh_view.export_png("out/mesh.png")

pressure = post.add_field_view("pressure", static.result)  # GridFunction, CoefficientFunction or StaticResult
pressure.export_vtu("out/pressure")  # out/pressure.vtu
pressure.export_png("out/pressure.png")

modes = post.add_modal_view("modes", modal.result, deformed=True)
modes.export_vtu("out/modes")  # every mode in one file
modes.export_pngs("out")  # out/mode1.png, out/mode2.png, ...

post.add_frequency_sweep_view("pressure_sweep", sweep.result)
post.export_vtu("out/everything")  # several views' fields in one .vtu

Views

View names are unique within a PostProcessor: post["modes"], "modes" in post, post.views, post.remove_view("modes").

Step labels are built from indices, never from frequency values, so code can rebuild them exactly. A sweep view named pressure has steps pressure_0, pressure_1, … in the same order as result.frequencies (0-based, like select_step(i)). A modal view has steps mode1, mode2, … (1-based, like mode numbers). A step's label is also its .vtu array name and its PNG file name. The frequencies are in FrequencySweepView.frequencies, ModalView.frequencies_hz and the colorbar titles (e.g. "Mode 1 (41.93 Hz)").

Updating a view

Every view can be changed after it is created, and every change method returns the view, so calls chain:

• Field data. view.update(new_field_or_result) re-evaluates the view with new data (e.g. after a re-solve) and keeps its settings. On a MeshView, set_color_by_material(bool) plays this role.
• Display settings. view.configure(**settings) changes how the view is drawn. See the settings below.
• Current step. view.select_step(i) or view.select_step("mode2") sets the step that export_png() and show() use. view.steps lists the step labels and view.step gives the current index.

modes.update(new_modal.result).configure(part="real", warp_scale=50.0).select_step(2).export_png("out/mode3.png")

Display settings

Pass these to any add_*_view() call or to configure(). They are stored as an immutable DisplaySettings, available as view.settings.

Surfaces are shaded with smooth normals, split at sharp edges (folds, box corners), and lit on both sides by a key/fill/back light kit.

Shells and plates

A shell model is a surface mesh: surface elements in 3D, with no volume elements. fem-post detects one (is_surface_mesh(mesh)) and samples its surface elements instead of volume elements. MeshView colors it by face (mesh.GetBoundaries()), and its cells follow ngsolve's surface normal, so VTK's normals point the same way as a shell's pressure.

Shell and plate solutions bundle several fields in one GridFunction (a product space), so they have no single value to color by. Pass a field extractor as fields= to split each solution into named arrays:

from fem_post import PostProcessor, plate_fields, shell_fields

post = PostProcessor(shell_mesh, subdivision=3)  # 3-4 keeps a coarse curved shell smooth
roof = post.add_field_view("roof", static.result, fields=shell_fields, deformed=True)
modes = post.add_modal_view("modes", modal.result, fields=shell_fields, deformed=True)

plate = PostProcessor(plate_mesh).add_field_view("w", static.result, fields=plate_fields, deformed=True)

Every extractor entry is stored, so ParaView can show the rotation from the .vtu too. The first entry is stored under the step label and colored by, every other entry as <label>_<entry>, and deformed=True warps by the entry named displacement (for a complex field, by its real part). An extractor is any callable gridfunction -> {name: field}, so you can write your own. The two built-in ones select components by position, so fem-post still does not import fem-core.

Exporting and viewing

• view.export_vtu(path) writes the mesh and every step's arrays to one file. .vtu is appended if missing.
• post.export_vtu(path, views=None) writes the arrays of several views (default: all) to one file.
• view.export_png(path, step=None, width=1200, height=900) renders one step off-screen.
• view.export_pngs(directory) writes <step label>.png for every step.
• view.show() opens an interactive window. n/Right and p/Left flip between steps.
• view.point_array(step=None) returns the shown values as a numpy array, and view.grid is the underlying vtkUnstructuredGrid for custom VTK work.

PNG export needs an OpenGL context but no display. On headless Linux, run under xvfb-run -a. show() needs a real display. Do not run it under xvfb-run, which gives it an invisible display.

Embedding in your own window

show() opens a standalone window. To draw a view in a window you own, such as a Qt-embedded QVTKRenderWindowInteractor, build a Scene from the view's grid, attach it to your render window and draw a step:

from fem_post.scene import Scene

view = post.add_mesh_view(edges=True)
scene = Scene(view.grid)
scene.attach(render_window)  # adds the surface and colorbar renderers
view.draw(scene)  # current step; or view.draw(scene, "mode2")
render_window.Render()

view.draw(scene, 1, fit_camera=False)  # switch step, keep the camera
scene.detach(render_window)  # before attaching a different scene

A Scene is bound to one grid: after update() or set_color_by_material() replace view.grid, build a new Scene.

Tensor fields and stress

A matrix-valued field (dims == (d, d), e.g. a stress CoefficientFunction) is stored as a 9-component tensor array, row-major with components named xx, xy, … (ParaView reads it as a tensor). A 2D tensor is padded to 3 × 3 with zeros. Next to it fem-post stores <name>_von_mises, <name>_max_principal and <name>_min_principal, computed from the symmetric part. A tensor view is colored by von Mises on a sequential colormap by default. Use component= to choose another quantity:

stress = post.add_field_view("stress", sigma, smooth=True)  # sigma: a d x d CoefficientFunction
stress.export_png("out/von_mises.png")
stress.configure(component="xx").export_png("out/sigma_xx.png")  # diverging colormap
stress.configure(component="max_principal", colormap="sequential").export_png("out/sigma_1.png")

A complex tensor T = A + iB (a harmonic response) describes the stress Re(T e^{iφ}) = A cos φ − B sin φ over one cycle. With part="real" or part="imag" the derived values are those of A or B. With part="abs" (the default) they are the peak over the cycle:

• von Mises exactly, as the largest eigenvalue of a 2 × 2 quadratic form;
• the largest principal value from 36 phase samples, refined by a parabola;
• the smallest principal value is minus that peak, since σ(φ + π) = −σ(φ).

The _abs tensor itself holds each component's modulus.

fem-core computes the stress of its structural models (fem_core.services.derived_fields.stress_fields) in the field-extractor shape fields= takes, so a fem-core stress plot is post.add_field_view("stress", result, fields=stress_fields(model), smooth=True).

Smoothing. A stress comes from derivatives of the solution, so it jumps from element to element, and fem-post's per-element sampling shows those jumps as a patchwork. Pass smooth=True to add_field_view, add_modal_view or add_frequency_sweep_view to average every field over the sample points that coincide across elements. Averaging happens only within one material region (one face, for a shell), so real jumps at material interfaces and shell folds are kept. A tensor is averaged before its von Mises and principal values are computed. Smoothing is off by default, so the raw element values stay visible unless you ask for it.

How fields are sampled

Each volume element (each surface element of a surface mesh) is refined subdivision times on its reference element (2^subdivision cells per edge, default PostProcessor(mesh, subdivision=2)). The refined points are mapped through the element transformation with ngsolve.Mesh.MapToAllElements, and each field is evaluated there in one vectorized call. As a result:

• curved elements (curvature_order > 1) render with their true curved geometry;
• higher-order fields render smoothly rather than piecewise-linear on coarse elements;
• discontinuous fields (material index, L2 spaces) stay sharp, because points are per element.

The geometry is sampled once per PostProcessor and shared by all its views, so adding or updating a view only evaluates its fields. Supported element types are TRIG and QUAD (2D, and surface meshes in 3D) and TET, HEX and PRISM (3D). PYRAMID is supported but always unrefined.

A complex field name is stored as name_re, name_im and name_abs, and a 2D vector field is padded to 3 components.

The lower-level MeshSampler and mesh_to_grid() build grids directly, for custom VTK pipelines.

Development

xvfb-run -a uv run pytest packages/fem-post/tests    # from the fem-core repository root