comp.lang.idl-pvwave archive: archive » questionable display of an IDLgrPolygon Object with some transparency

Home » Public Forums » archive » questionable display of an IDLgrPolygon Object with some transparency

Show: Today's Messages :: Show Polls :: Message Navigator
E-mail to friend

Re: questionable display of an IDLgrPolygon Object with some transparency [message #42146 is a reply to message #42052]

Tue, 21 December 2004 09:11

Rick Towler
Messages: 821
Registered: August 1998

Senior Member

Karsten Rodenacker wrote:
>
> I have a bit surprising display of an IDLgrPolygon object with some
> transparency using xobjview, which shows under certain directions some
> rippling (Windows XP SP 2, IDL 6.1.1). A reduced screenshot is stored
> under http://ibb.gsf.de/homepage/karsten.rodenacker/Misc_WWW/Examp le.jpg.
>
> If there is some experience in the group any hints are appreciated to
> avoid this rippling.

This looks like the nefarious polygon z-order rendering issue that has
been discussed many times before. Google the group for "pimento
problems". Robert Schaefer had a similar problem recently so you may
want to search for that thread too.

The rippling occurs during rotation as polygon drawing order and their
position relative to others in Z get out of whack. IDL uses a simple
painters algorithm to render to the screen. For each polygon thrown at
it, it looks at the z-buffer to see what is already there and if this
poly is semi-transparent, takes that value and uses it to calculate the
new z value at that pixel. This works wonderfully if you always draw
your polys from back to front. But if you don't, the near polygon is
drawn with nothing but space behind it and the far polygon is never
drawn since it fails the z-test.

In your case, the banding is caused when some of the triangles are in
the correct order and some are not. The areas that are lighter show the
far side of the object as well as the near while the darker bands do not
show the far side since the near side triangles are rendered before the
far side.

There are no easy fixes for your complex object. Karl Shultz played
around with some BSP code which re-orders triangles properly in Z for
every transformation. I played around with it a bit but found it wasn't
robust enough to handle my data. It would be worth a shot though.
Check the RSI user contrib site for IDLbspPolygon or something like that.

Also, and I can't stress this enough. Put in a feature request for a
new renderer that doesn't have these limitations. Your case is a prime
example why this needs to be addressed. The competition doesn't suffer
from this limitation...

-Rick

P.S. My news server has been acting up. If this isn't posted to the
group, can you forward it on?

Report message to a moderator

[Message index]

		questionable display of an IDLgrPolygon Object with some transparency By: Karsten Rodenacker on Tue, 21 December 2004 00:55
		Re: questionable display of an IDLgrPolygon Object with some transparency By: Karsten Rodenacker on Wed, 22 December 2004 00:14
		Re: questionable display of an IDLgrPolygon Object with some transparency By: Rick Towler on Tue, 21 December 2004 09:11
		Re: questionable display of an IDLgrPolygon Object with some transparency By: Karsten Rodenacker on Tue, 21 December 2004 07:30

Previous Topic:	Re: reading f77_unformatted and writing it as ascii file
Next Topic:	Problems reading ASCII data

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Sat Oct 11 12:56:40 PDT 2025

Total time taken to generate the page: 1.75658 seconds