Re: [Gnuplot-info] maxima + gnupolot: 3D rendering bug?

[Ethan M. <merritt@u.washington.edu>]

On Thursday, December 09, 2010 07:53:12 am Stanislav Maslovski wrote:
> Hello,
> 
> On Tue, 2010-12-07 at 15:42 +0000, Stanislav Maslovski wrote:
> > On Tue, 2010-12-07 at 11:51 +0000, Stanislav Maslovski wrote:
> > > Hello,
> > > 
> > > Can anyone advise me on how can I workaround the problem with 3D
> > > rendering in the plot [1]? The problem is that the surfaces that must be
> > > hidden are seen in this plot (to clearly see them,
> > > load "maxout.gnuplot" and try rotating the plot).
> > > 
> > > The plot was generated in maxima, using the draw3d command. Any help
> > > will be very much appreciated!
> > > 
> > > [1] http://zalil.ru/30094104 (wait 10 secs, and download will start)
> > 
> > Replying to myself: actually, after reading some docs (and the code) I
> > realized that it was a limitation of the rendering algorithm used by the
> > gnuplot (a variant of painter's algorithm). So, I solved this issue by
> > patching pm3d.c:pm3d_depth_queue_flush() so that sorting of quadrangles
> > was done based on the average depth of 4 corners.

I don't think this is the best way to approach the problem.
As Hans-Bernhard pointed out, _any_ simple-minded sorting on a 
single Z value is going to fail for some configuration of triangles.
The bigger the triangle, the more likely that any single Z value is not
right for the whole thing.

I suggest that a better approach would be to add a pass that splits large
triangles into smaller pieces before sorting and rendering.
The only tricky part is how to define "large".   
[OK, there's potentially another tricky part if you are coloring by
something other than Z since you have to decide how to assign colors
to the newly created vertices.]

You can probably find sample code for this kind of divide-and-conquer
approach in a textbook or article, but it should be simple enough to
work it out from scratch.

	Ethan


> > 
> > It would be nice if this could be made configurable. I will probably
> > come out with a patch in the next couple of days to enable this feature.
> 
> Just in case: the patch with an example is available from here
> http://sourceforge.net/tracker/?func=detail&aid=3132649&group_id=2055&atid=302055
> 
> Cheers,
> --
> Stanislav