[vtkusers] Line smoothing and parallel rendering

Chris Kruszynski Chris.Kruszynski at cwi.nl
Thu Aug 10 03:15:17 EDT 2006

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

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