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View Full Version : Why is overlapping Geometry bad?


synergy11
03-22-2007, 03:34 AM
With the obvious reason being the extra polygons that are covered are wasted and cause longer rendering times.

Here in this picture everything was modeled starting from one piece of geometry..it's all the same mesh.

However modeling those cylinders would be impossible without making them seperate and somehow attaching them.

So I just made them and duplicated them around the Y axis from the center of the building.
It took no time at all.

Am I missing something here or is this the way to go.

Any comments would be great help.
Thanks

Sinew
03-22-2007, 02:45 PM
as youve stated, overlapping geometry can generate higher poly counts, but this is up to your preference and the project needs.

overlapping geometry only realy becomes a problem when two faces occupy the exact same space. for example, if you dupilcate an object, but dont move it, then you'll have overlapping geometry. this can effect rendering, as a renderer doesnt know which face to render.

hope that helps.

kirigoi
03-26-2007, 04:01 AM
One other problem that might come up is that by cloning geometry you can't get a nice smooth join between the two parts; there'll always be a hard edge. It's not too necessary on architectural-type shots as the curvature on edges is very small compared to the size of the object, but on some models having an artificially sharp edge can really stand out.

synergy11
03-26-2007, 10:18 PM
If you were modeling something like this how would you attach those cylinders? or Would you extrude them out of the original mesh somehow.

Any input appreciated.

pauljs75
03-29-2007, 03:47 PM
Two ways to do it that I could think of...

Chop off the backsides of the cylinders. (Anything beyond where it pokes through.) Leave as separate objects. Still "lazy", but there's less background junk to calculate or eat memory that won't be seen anyways.

Or Chop off back of cylinders short of where it will pass through the edge, do some edge/vert matching up with the surface behind and bridge or weld. (I know this works in Wings with a bit more effort than the previous method. No reason it shouldn't in other similar apps.) Or if your modeling app is fancy enough, you could boolean. (Software does the same thing to some degree, but with varying results.)

kirigoi
03-29-2007, 11:32 PM
However modeling those cylinders would be impossible without making them seperate and somehow attaching them.Really? ;)

I don't have time to write a full tutorial, but here's a few clues:

http://www.skylight3d.com/images/CGTalk/screens/columns_01.png


Using an octagon as a base, I've scaled and cut a 2D segment to become the eaves of the model.

http://www.skylight3d.com/images/CGTalk/screens/columns_02.png


Looking more closely at the segment, I've sliced half a circle out of the segment on each side (I've mirrored a single circle across the segment to do this).

http://www.skylight3d.com/images/CGTalk/screens/columns_03.png


I've now cloned the segment 8 times around the y-axis and merged points.

http://www.skylight3d.com/images/CGTalk/screens/columns_04.png


Now I've extruded the shape up along y, made a polygon out of the hole at the bottom, and merged this polygon with the polys at the corners to make a single large polygon.

http://www.skylight3d.com/images/CGTalk/screens/columns_05.png


Extrude this polygon downwards and you're done.

http://www.skylight3d.com/images/CGTalk/screens/columns_06.png


This is what it looks like with rounded edges and a roof.

To be honest, you don't really need to round the edges given the scale the building's at, but it would be slightly more efficient in terms of polygon use to make it all one mesh, and depending on your renderer it might make for slightly shorter render times to not have multiple intersecting meshes to make the object.

For a model like this it really doesn't matter. Just do it however you want to get the results you need. M

synergy11
03-30-2007, 12:32 AM
Thank Kirigoi, I'm seeing lots more possibilities with that method.

You help has been invaluable. And your pictures are easy to follow.

I bet your a busy guy. But if you have time could you post some "clues" as to how YOU would go about doing this type of thing:

The pictures of what I'm going for are on my 3rd post in this thread

http://forums.cgsociety.org/showthread.php?f=24&t=478877

The method I used, as you can see in the first post pic, produced some undesirable results.

Basically I'm trying to create a dome with curved arches attached to it. It's common in the world. I'm sure you've came across it.

I'm interested in how you would approach a similar model.

Any clues would be great!

Thanks again Kirigoi

Sinew
03-30-2007, 12:27 PM
were on the lowest package, as we thought theres no point in buying the most expensive incase you find it isnt what we were after. you can upgrade later too if you need the added functions.

ysvry
03-30-2007, 10:06 PM
realy nice way of working.

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