[vtkusers] Line smoothing and parallel rendering

Francois Bertel francois.bertel at kitware.com
Thu Aug 10 10:21:00 EDT 2006


I think the answer is supersampling is on by default if your GPU support it.  Follow this thread for detailed explanations:


Chris Kruszynski wrote:
> Gregory D Abram wrote:
>> I'm experimenting with the vtkCompositeRenderManager and am running
>> into a problem.  If I add lines (for example, the bounding box of a
>> data partition) to the rendering on a node, I get terrible streaks all
>> over the composited image. 
>> After much poking around, I've found that (even outside of the
>> compositing parallel rendering environment) OpenGL line rendering is
>> anti-aliased (even after setting RenderWindow->LineSmoothingOff()
>> immediately after creating the render window) and, further, when a
>> pixel is only partially affected by a line, it may or may not update
>> the Z depth buffer - based, I suppose, on whether the proportion of
>> coverage exceeds some threshold.  But it *always* affects the color
>> buffer, so you can have a pixel thats not the background color but is
>> at the background depth.  (No, I'm not hallucinating.   Yes, I have a
>> little program that demonstrates this.)
>> Then, in the parallel compositing code, when it tries to run-length
>> encode the background away, it looks for runs of constant Z = 1.  When
>> it reaches the end of such a run, it uses the color of the final pixel
>> of that run as the color of the whole run.  Unfortunately, that pixel
>> may be partially covered by an antialiased line, enough to change the
>> color, but not enough so that it updates the depth.  So the subsequent
>> run of "background" receives a  color that isn't the background color,
>> resulting in a streak of color extending either to the next
>> non-background pixel or the end of the buffer.
>> Near as I can tell, the run-length encoding code needs to match color
>> as well as Z = 1.  Or we can get the color buffer and Z buffer updates
>> to match, but I don't understand why they *aren't* matching as it is
>> now. 
>> Linux RHEL3,  Quadro FX 3400/4400 driver 7676.
>> Greg
> Quadro cards always use hardware anti-aliased lines (probably points as
> well), and I am not sure you can actually turn this feature off.
> - Chris

François Bertel, PhD  | Kitware Inc. Suite 204
1 (518) 371 3971 x113 | 28 Corporate Drive
                      | Clifton Park NY 12065, USA

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