[Paraview-developers] Backface Styling bug?

[Ken Martin]

Unfortunately this is a real issue and the fix is sort of ugly/painful
unless I am missing something. The issue is that the new backend converts
quads/polygons into triangles as quads and polygons are deprecated in later
versions of OpenGL. Plus triangles are fast as they each have just 3 verts
and can be counted on to have three verts etc. :-) But when you render
wireframe you do not want to use the same IBO because if your original data
was a quad, showing triangle wireframes would be misleading. So we build a
different IBO that constructs the boundary edges as lines.  Lines do not
get front/back culled by OpenGL. The reason we do not use triangle_fans is
that without the primitive restart index support (not available in OpenGL
2.1 or ES 2.0/3.0/3.1) you (or the driver) have to essentially make a draw
call per primitive which is slow. Plus glPolygonMode is not in OpenGL ES so
it would not work on iOS/Android anyhow. So ... edge visibility or
wireframe when combined with front/back culling is not working and off the
top of my head I cannot think of a universal solution other than performing
the culling on the CPU and sending a new IBO up every frame, yuck.



Ken



Ken Martin PhD

Chairman & CFO

Kitware Inc.

28 Corporate Drive

Clifton Park NY 12065

ken.martin at kitware.com

919 869-8871 (w)





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*From:* Paraview-developers [mailto:paraview-developers-bounces at paraview.org]
*On Behalf Of *Joachim Pouderoux
*Sent:* Friday, September 11, 2015 5:52 AM
*To:* Felipe Bordeu
*Cc:* ParaView Developers
*Subject:* Re: [Paraview-developers] Backface Styling bug?



Not totally sure but it could be related to Ken's current merge request:
https://gitlab.kitware.com/vtk/vtk/merge_requests/643

Support of OpenGL2 backend in PV is not yet complete by the way. PV4.4 is
still base on the legacy backend.

Regards,




*Joachim Pouderoux*

*PhD, Technical Expert*
*Kitware SAS <http://www.kitware.fr>*



2015-09-11 11:24 GMT+02:00 Felipe Bordeu <felipe.bordeu at ec-nantes.fr>:


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hi I don't know if this is a bug of paraview or vtk.

1) make a simple Arrow source
2) change the view to Surface With Edges.
3) change the "Backface Representation" to "Cull Frontface"

only the back surfaces are rendered (as expected ), but ALL the edge are
rendered.

Using the 4.4.0-RC2 64bit on linux and using "
VTK_RENDERING_BACKEND            OpenGL2"

In Paraview 4.3.1 (binary from web) only the back edges and surface are
rendered (as expected).

Felipe



- --
Felipe Bordeu Weldt
Ingénieur de Recherche
- -------------------------------------
Tél. : 33 (0)2 40 37 16 57
Fax. : 33 (0)2 40 74 74 06
Felipe.Bordeu at ec-nantes.fr
Institut GeM - UMR CNRS 6183
École Centrale Nantes
1 Rue de La Noë, 44321 Nantes, FRANCE
- -------------------------------------
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