[vtkusers] Recent Change to Handling Coincident Geometry

[Ken Martin]

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


-- 
Ken Martin PhD
Chairman & CFO
Kitware Inc.
28 Corporate Drive
Clifton Park NY 12065
518 371 3971

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