[vtkusers] Coincident topology questions

Thu Jan 11 14:41:33 EST 2018

Yes, that should work and -3, -3 is pretty reasonable for values. Something
odd is going on. Can you give a bit more detail or a snippet of code?

On Thu, Jan 11, 2018 at 2:30 PM, Langer, Stephen A. (Fed) <
stephen.langer at nist.gov> wrote:

> So if I use
>
>      vtkMapper::SetResolveCoincidentTopologyToPolygonOffset();
>
> and
>
>      mapper->SetRelativeCoincidentTopologyPolygonOffsetParameters(-3,- 3);
>
> then polygons drawn with that mapper should obscure coincident polygons
> drawn with other mappers?  That's not happening.    What am I missing?
>
>
>
> -- Steve
>
>
>
> *From: *Ken Martin <ken.martin at kitware.com>
> *Date: *Wednesday, January 10, 2018 at 7:52 PM
> *To: *"Langer, Stephen A. (Fed)" <stephen.langer at nist.gov>
> *Cc: *"vtkusers at vtk.org" <vtkusers at vtk.org>
> *Subject: *Re: [vtkusers] Coincident topology questions
>
>
>
> Here is part of an old email that has some description
>
>
>
> You can use the relative settings if the two coincident polygons are in
> two different mappers.
>
>
>
> * On Tue, Dec 1, 2015 at 1:29 PM, Ken Martin <ken.martin at kitware.com <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fpublic.kitware.com%2Fmailman%2Flistinfo%2Fvtkusers&data=02%7C01%7Cstephen.langer%40nist.gov%7Ceb611caace3b43fd388508d5588d7ed0%7C2ab5d82fd8fa4797a93e054655c61dec%7C1%7C0%7C636512287107516935&sdata=MOAz%2BaPGI75IQNOwc551Ke8i2lYFFFyY3aD3E4dq%2BuQ%3D&reserved=0>> wrote:*
>
> >
>
> >>
>
> >>* I wanted to take a second to write up a recent change I made to how VTK*
>
> >>* (on the OpenGL2 backend) can handle coincident geometry.  Previously VTK*
>
> >>* supported global values for specifying a polygonal offset to push surfaces*
>
> >>* forward or back in the zbuffer. This was often used to draw a surface back*
>
> >>* a bit in the zbuffer, and then draw the same surface in wireframe mode at*
>
> >>* the normal z location. The result was a wireframe overlaid on top of the*
>
> >>* surface. This approach faced a couple limitations. Every surface was offset*
>
> >>* the same amount and there was no way to offset points from lines.  To*
>
> >>* address this I have added a few methods to vtkMapper to specify global line*
>
> >>* and point offsets as well as ivars to hold local offsets per mapper. The*
>
> >>* global methods look like:*
>
> >>
>
> >>*   static void SetResolveCoincidentTopologyLineOffsetParameters(*
>
> >>*     double factor, double units);*
>
> >>
>
> >>* while the instance methods look like:*
>
> >>
>
> >>*   void SetRelativeCoincidentTopologyLineOffsetParameters(*
>
> >>*     double factor, double units);*
>
> >>
>
> >>* Both signatures use two parameters which are worth mentioning. The first*
>
> >>* factor is how much to shift the zbuffer based on how quickly the z values*
>
> >>* of the surface or line changes. This may seem complicated but it addresses*
>
> >>* a simple issue.  Due to rasterization issues, surfaces and lines with sharp*
>
> >>* zvalue slopes are more prone to noise in the zvalues they produce. To*
>
> >>* combat that, we offset them by an amount that is based on their z slope. If*
>
> >>* the surface is perpendicular to the view direction the factor is*
>
> >>* irrelevant, but the more the surface is at a sharp angle to the view*
>
> >>* direction, the more the factor comes into play. A value of 1.0 to 2.0 is*
>
> >>* fairly typical for the factor. The second parameter is the units, a*
>
> >>* constant offset to the zbuffer. It is currently specified conservatively in*
>
> >>* terms of a 16bit zbuffer for the OpenGL backend so a value of 1.0 to 2.0 is*
>
> >>* a good place to start. For rendering points we do not have a slope so those*
>
> >>* methods only take the units argument.*
>
> >>
>
> >>* I have changed the global defaults so that surfaces are offset at*
>
> >>* 2.0,2.0, lines are offset at 1.0,1.0, and points are at 0.0. There is a new*
>
> >>* test in OpenGL2/Testing/CXX/TestCoincident.cxx that renders points on top*
>
> >>* of lines on top of a surface while rendering the points first and the*
>
> >>* surface last (the opposite order of their visibility).*
>
> >>
>
> >>* Thanks*
>
> >>* Ken*
>
>
>
>
>
> On Wed, Jan 10, 2018 at 5:44 PM, Langer, Stephen A. (Fed) <
> stephen.langer at nist.gov> wrote:
>
> Hi --
>
> How does one set the arguments to the various vtkMapper::SetRelativeCoincidentTopology
> methods?  I can't find an explanation for 'factor' and 'units'.
>
> The attached screenshot shows a surface representation of a grid of
> rectangular voxels and a wire-frame grid of tetrahedra.  Four of the
> tetrahedral elements are also drawn with a surface representation in red
> and are supposed to obscure the voxels, but a mix of tex and voxel is drawn
> instead.  The surfaces of the voxels and the tetrahedral are coplanar.
> They don't share nodes and edges, but the points on the x=0 face all have
> x=0 exactly.  Is the coincident topology resolution machinery expected to
> work in this case?
>
> I'm using vtk 8.1.0.
>
> Thanks.
>
>       -- Steve
>
>
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>
> --
>
> Ken Martin PhD
>
> Distinguished Engineer
> Kitware Inc.
>
> 28 Corporate Drive
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-- 
Ken Martin PhD
Distinguished Engineer
Kitware Inc.
28 Corporate Drive
Clifton Park NY 12065

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