# Preview an SG Mesh with PyVista

## Problem Description

Once a Structure Gene (SG) is read into sgio, it is useful to *see* the mesh —
the geometry, the cell blocks, and per-element data such as `property_id` —
before running an analysis. This example reads a VABS cross-section and renders
its mesh with [PyVista](https://pyvista.org/).

## Solution

{class}`sgio.SGMesh` is a backend-neutral mesh container, and sgio provides a
bridge to PyVista so any SG mesh can be handed to VTK's renderer:

1. Read the cross-section into a {class}`sgio.StructureGene` with
   {func}`sgio.read`.
2. Call `sg.mesh.to_pyvista()` to get a `pyvista.UnstructuredGrid`. Geometry,
   `point_data`, and `cell_data` (e.g. `property_id`) cross the bridge.
3. Render it — here a headless screenshot; drop `off_screen` / `screenshot` for
   an interactive window.
4. `SGMesh.from_pyvista(grid)` shows the reverse bridge (grid back to `SGMesh`).

```{literalinclude} ../../../examples/preview_sg_mesh/run.py
:language: python
```

```{note}
PyVista is an **optional dependency** (it pulls in `vtk`). The core sgio IR does
not require it; `to_pyvista` imports PyVista lazily and only fails if you call it
without PyVista installed. Install it with `pip install sgio[pyvista]`.
```

## Result

The script prints the grid summary and the reverse-bridge cell blocks, and
writes `preview.png` — the airfoil cross-section colored by `property_id` with
mesh edges shown. The 2D section lies in the Y-Z plane, so the camera looks down
the X axis (`cpos="yz"`).

```{figure} ../../../examples/preview_sg_mesh/preview.png
:align: center
:width: 70%
```

## File List

- [run.py](../../../examples/preview_sg_mesh/run.py): Main Python script
- [sg21t_tri3.sg](../../../examples/preview_sg_mesh/sg21t_tri3.sg): Input VABS cross-section (triangular mesh)
- [preview.png](../../../examples/preview_sg_mesh/preview.png): Generated preview image