solid meshes with edges enabled show all triangle edges when semi-opaque
Under vtk 8.1.1, a semi-opaque plane with solid appearance and edges enabled would be shown like this:
in vtk 9.2.6, it is shown like this, with the inner triangle edge visible from some angles:
The same behavior occurs with all meshes. The inner edge gets thicker as general line thickness is increased, but is not as thick as the "true" edges. It is generally visible from oblique angles, but not acute ones.
Further discussion at this support topic: https://discourse.vtk.org/t/change-in-rendering-of-semi-opaque-solid-meshes-with-edges-enabled/11120/4
See the following minimum reproducible example
import vtk
polydata = vtk.vtkPolyData()
points = vtk.vtkPoints()
points.InsertNextPoint(0, 0, 0)
points.InsertNextPoint(1, 0, 0)
points.InsertNextPoint(1, 1, 0)
points.InsertNextPoint(0, 1, 0)
polys = vtk.vtkCellArray()
polys.InsertNextCell(4, [0, 1, 2, 3])
colors = vtk.vtkUnsignedCharArray()
colors.SetNumberOfComponents(4)
colors.InsertNextTuple([200, 200, 200, 255])
polydata.SetPoints(points)
polydata.SetPolys(polys)
polydata.GetCellData().SetScalars(colors)
mapper = vtk.vtkPolyDataMapper()
mapper.SetInputData(polydata)
actor = vtk.vtkActor()
actor.SetMapper(mapper)
actor.GetProperty().SetEdgeColor(1, 0, 0)
actor.GetProperty().SetEdgeVisibility(True)
actor.GetProperty().SetLineWidth(2)
renderer = vtk.vtkRenderer()
renderer.AddActor(actor)
renWin = vtk.vtkRenderWindow()
renWin.AddRenderer(renderer)
renWin.Render()
renderer.ResetCamera()
interactor = vtk.vtkRenderWindowInteractor()
interactor.SetRenderWindow(renWin)
interactor.Start()