::New();
normalGen->SetInputData(cleaner->GetOutput());
normalGen->ComputePointNormalsOn();
normalGen->SplittingOn(); // I suspect this is re-splitting my welded vertices
normalGen->SetNonManifoldTraversal(true);
normalGen->ConsistencyOn();
// I used 30.0 degrees. Is this the cause?
normalGen->SetFeatureAngle(30.0);
if (isClosedMesh && IsWatertight(mesh)) {
normalGen->AutoOrientNormalsOn();
} else {
normalGen->AutoOrientNormalsOff();
}
normalGen->Update();
```
#### **Step-by-Step Verification via Exported Files**
I exported the model as a `.vtp` file after every processing step to compare the results.
1. **`04_Outside_cleaner_Computed.vtp`** : This is the model generated by the C++ code after passing through `vtkTriangleFilter` and `vtkCleanPolyData`.
2. **`04_Outside_triFilter_Computed.vtp`** : This is the model generated by the C++ code after `vtkTriangleFilter` (before cleaning). I loaded this file into ParaView and manually applied the **Clean** filter.
- **Observation:** Both of the above have the **exact same number of points and cells**. This confirms the C++ Clean step is working identically to ParaView’s Clean step regarding topology.
However, the divergence happens at the Normals step:
1. **`04_Outside_Normals_Computed.vtp`** : This is the final output from the full C++ pipeline (Triangulate → Clean → Normals).
2. **`GenerateSurfaceNormals1` (in ParaView)**: I loaded the code-cleaned file (`04_Outside_cleaner_Computed.vtp`) and manually applied “Generate Surface Normals” in ParaView.
- **Observation:** This object has the **same high point count** as the C++ output (`04_Outside_Normals_Computed.vtp`).
1. **`GenerateSurfaceNormals2` (in ParaView)**: I loaded the raw triangulated file (`04_Outside_triFilter_Computed.vtp`), then manually applied the **Clean** filter followed by the **Generate Surface Normals** filter entirely within ParaView.
- **Observation:** This object has significantly **fewer points** than the previous two cases.
Any advice on why the C++ `vtkPolyDataNormals` splits the edges while ParaView keeps them connected would be greatly appreciated.
---
### Author:  [@zhichaozheng322](https://discourse.vtk.org/u/zhichaozheng322)
#### Post date: [January 13, 2026, 10:54am UTC](https://discourse.vtk.org/t/help-discrepancy-between-paraview-and-vtk-c-vtkcleanpolydata-works-but-mesh-shatters-after-computing-normals/16227/2 "2026-01-13T10:54:25Z")
---
### Author:  [@zhichaozheng322](https://discourse.vtk.org/u/zhichaozheng322)
#### Post date: [January 14, 2026, 3:54am UTC](https://discourse.vtk.org/t/help-discrepancy-between-paraview-and-vtk-c-vtkcleanpolydata-works-but-mesh-shatters-after-computing-normals/16227/3 "2026-01-14T03:54:33Z")
I have managed to solve this issue. It turns out the root cause was **Non-Manifold Topology** hidden inside the mesh, which caused `vtkPolyDataNormals` to miscalculate orientations when Consistency checks were enabled.
Here is the breakdown of the problem and the solution:
**1. The Root Cause:** After processing the mesh with `vtkCleanPolyData` (or `vtkStaticCleanPolyData`), some internal vertices were merged, creating **Non-Manifold Edges** (edges shared by 3 or more faces). Specifically, there was an internal “coil” or “septum” structure connecting the inner walls.
When I used `normalGen->ConsistencyOn()`, the algorithm traversed the mesh to unify normal directions. However, upon hitting these non-manifold edges, the traversal logic propagated the wrong orientation (flipping normals inward) to large portions of the mesh, causing the “shattered” look with black artifacts.
**2. How I Diagnosed It:** I verified this using **ParaView** :
1. Applied `CleanPolyData` to the model.
2. Applied the **`Feature Edges`** filter.
3. Unchecked all options and selected **only `Non-Manifold Edges`**.
4. This revealed a disconnected loop of edges inside the cylinder, confirming the topology issue.
**3. The Solution (C++ Code):** To fix this for rendering, the key is to turn **OFF** Consistency (trusting the local winding order) while keeping Splitting **ON** (to handle the sharp 90-degree edges of the cylinder).
Here is the working configuration:
`C++auto normalGen = vtkSmartPointer::New();
normalGen-\>SetInputData(mesh);
// 1. Enable Splitting to handle sharp edges (e.g., the 90-degree edge of the cylinder cap)
normalGen-\>SplittingOn();
normalGen-\>SetFeatureAngle(60.0);
// 2. CRITICAL: Turn OFF Consistency.
// Since the mesh has non-manifold geometry, enforcing consistency causes
// the propagation to fail and flip normals incorrectly.
// We trust the original local winding order here.
normalGen-\>ConsistencyOff();
// 3. Ensure robustness
normalGen-\>ComputePointNormalsOn();
normalGen-\>SetNonManifoldTraversal(true);
normalGen-\>Update();`
Hope this helps anyone else encountering this issue.