Go Back   AC3D Forums > Resources > AC3D Tutorials and How-To's
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Display Modes
Old 3rd July 2006, 05:28 AM   #1
coldby
Senior Member
Professional user
 
Join Date: Jun 2006
Posts: 102
Question funny results

Seeing is believing - just try:
1 -draw a square - add two vertices in the middle of the upper and lower sides - bend the surface on the (virtual) line connecting the new vertices by selecting and dragging both end vertices of the corresponding sides - say "ooooh"....
2 - now do the same but insert the new vertices in the right and left sides - say "what the heck???"
M. C. Escher would have loved it, but to me it looked more like a medium-heavy hangover

Can anyone offer a consistent theory on what happens to an originally plane surface having more than 3 vertices when some of them get displaced along the normal?
coldby is offline   Reply With Quote
Old 3rd July 2006, 10:31 AM   #2
Dennis
Senior Member
Professional user
 
Dennis's Avatar
 
Join Date: Jul 2003
Posts: 899
Default Re: funny results

For more fun, perform a Surface -> Change Vertex Order and the "escher-effect" happens on the opposite selection.

4+ sided polys allow you to "break some rules" in 3D that should keep things like this from happening.

My 3D programming skills are a few years old, so anyone feel free to correct me if I'm off, but when the 6-sided shape is drawn on-screen, it is still drawn as triangles. When you pull the vertices at odd angles like this, the triangles being drawn will start to appear where you may not have expected them, based on the vertex ordering and the vertices you move.

Any time you push vertices like this, it's simply best to break the geometry up to form a crease along that section so you get consistent, predictable results.
Dennis is offline   Reply With Quote
Old 3rd July 2006, 11:30 AM   #3
coldby
Senior Member
Professional user
 
Join Date: Jun 2006
Posts: 102
Default Your wise words

That's absolutely right, only triangles can guarantee that they'll always lay on a plane regardless of what you do with the vertices; any other polygon is easily distorted off-plane thus producing results depending on how it is calculated. In my next test I'ill try to raise two opposite vertices of a square - but I strongly doubt I'll get the saddle-shaped hyperboloid one would await.
There might be a law against using AC3D in the office, its a real drug! Good enough that there's still the excellent excuse "I wanted to see how it looked like before having it machined"

P.S.: congratulation for supercoldmilk.com, it's a fine a site if I ever saw one!
coldby is offline   Reply With Quote
Old 3rd July 2006, 12:21 PM   #4
coldby
Senior Member
Professional user
 
Join Date: Jun 2006
Posts: 102
Default It did!

I'll be damned if it didn't work! Not at once, of course, but after raising two opposite vertices of a square and performing some subdivisions, what I got looked exactly like the saddle-hyperboloid I DID'NT expect. Live and learn...
coldby is offline   Reply With Quote
Old 3rd July 2006, 04:22 PM   #5
Dennis
Senior Member
Professional user
 
Dennis's Avatar
 
Join Date: Jul 2003
Posts: 899
Default Re: funny results

Congrats - and thanks for the web page kudos

Subdivisions are great for making smooth shapes out of simple geometry like this --- and that's actually a pretty neat way to make a saddle using only 4 verts in the base mesh.

I always model with an eye toward using quads. Subdivision operations tend to come out more favorably with quads, plus there are tools like select-loop and divide-loop that work only/best with quads. I make a point of removing all 5+ sided surfaces from a doc (in the odd event that any slip in there). While most apps handle 5+ sided geometry much better nowadays than they used to, I still feel safer sticking with quads/tris...
Dennis is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -4. The time now is 04:17 AM.


AC3D Forum
(C) Inivis Limited 2020