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In some specific cases, vtkImplicitPolyDataDistance calculates the wrong sign when calling EvaluateFunction. It appears to only occur when the query point sits directly above the edge of a face as shown in the image below.

The code to reproduce this issue is as follows.

const double length = 1.0;
vtkNew<vtkCubeSource> cube;
cube->SetXLength(length);
cube->SetYLength(length);
cube->SetZLength(length);
cube->Update();

vtkNew<vtkImplicitPolyDataDistance> signed_distance;

signed_distance->SetInput(cube->GetOutput());

double test_distance = signed_distance->EvaluateFunction(-0.5, 0.7, 0.);

With this code, the value of test_distance is -0.2 instead of 0.2.