View Full Version : Effecient way of modeling spherical arch object.

03 March 2007, 07:41 PM
Hello I'm trying to model a arch segment (arcade?) around a sphere top like in many examples in old architecure.

However I'm not sure that i am going a bout it the right way.

When I do it in the example pic below ..the duplicate around center is either to close or to far away, to do a proper merge...even then I'm not sure about attaching the top sphere.

Any insight into this problem would be great. There are no doubt many ways to go about this.
I'm using XSI 4, but examples in other programs would help me out.


03 March 2007, 01:41 PM
Dunno if XSI would let you do something like extruding by 0 and then scaling radial... But that might give an idea of how to approach the arched portion.:shrug:

05 May 2007, 01:32 PM
Hello again Synergy... ;)

I've included this in the general modelling section as I'm using Lightwave not XSI, but all techniques should be comparable. I've just done it a fairly simple way, but you could also cut arch shapes out of the sphere, which would reduce the amount of vertex pushing you need to do. Make sure you have decent edgeloops around the arches though as that's vital to having good subdivision surfaces.

First I've created a sphere around the origin. The number of sections and spans you use to make it depends on the size of and how many arches you want around the circumference. I've gone for six wide arches, so that means I've used 24 sections (3 for each arch, one for the gap between = 4 *6 arches = 24). I've used 10 spans, which you can tweak depending on how high you want the arches.

I've deleted the bottom half of the sphere, then selected the polygons that will make up the arches.

Now I've beveled these polygons inwards, and also outwards slightly to maintain the spherical shape. This will make an edge loop around your arches.

Now I've deleted the bottom polygons of the bevel, and set all the bottom vertices to y=0.

Now I've shifted the polygons outwards again to create another edgeloop. This operation may be tricky as it's not just a simple bevel as not all the polygons have moved radially; they've moved along their shared normals, constrained to the xz plane. You should be able to do this in XSI by using your tool settings, otherwise just move one individually along an axis (or a pair using mirror settings), then rotate the entire arch by 60 degrees and do the next one etc. If you do it this way, you should do the next step at the same time to save on repetition.

I've moved each section out further, flattened it, then added an extra edgeloop. I've then deleted the polygons at the ends of each arch, and added an extra edgeloop just above y=0 to stop the bottoms of the arches smoothing when I convert to SubDs.

I've now moved a few vertices around to make arches out of the bevelled polygons from above. All the arches can be done at the same time by moving up/down along y or scaling together in the xz plane to move radially, but I've marked the vertices you'll need to move on just one of the arches as follows:
1) Top of arch needs to be moved up. Where it joins the sphere move inwards (the two highlighted points will need to be moved in further to maintain the shape of the sphere.
2) Corners of arch down. Where they join the sphere move outwards.
3) To maintain parameterisation these will need to be moved inwards and upwards slightly so as not to make a lump on the surface of the sphere.

I've just done this by hand as I've got some experience with shaping SubDs, but if you're having trouble you might find it easier to place an identical smoothed sphere in the background (and arches if necessary) to either determine how to move the vertices by hand, or by conforming to the background. You might also find it easier to just move the arch outline and not bother with extruding the arches out till the end.

When converted into SubDs this is the result. I've also added some trim to the edges of the arches by making a curve from the end points.

This is what it looks like from below if you wanted to create a dome with arched windows.

Let me know if you have any questions. M

05 May 2007, 03:29 PM
The way I described does work too. Just tested it. But only works if you have the same or similar selection scaling options as Wings.
(Or you can just use Wings3D since it's free, and then export and bring it into your preferred program for further detail.)

05 May 2007, 08:15 PM
Thanks for the informative post.

I read it over a few times and I'm going to attempt it in XSI.

One thing pops out (if I have to right to be picky in this situation). If one wanted to add more thickness to the sphere how would you go about doing it.

Thanks again for your time kirigoi.
I'll post my pics as soon as I go through the tutorial.

05 May 2007, 08:17 PM
THe arches in your pic don't curve back like in kirigoi's

05 May 2007, 10:49 PM
I was looking at your first pic, thought you wanted circular arches, but not a sperical cutout. The same can be appled to the sphere w/o the cylindrical extension. It also saves having to scale radial for the cylindrical section, and instead one can just scale uniform from the center of where the starting sphere is cut in half. (probably easier to explain with pics... Might do that later when not rendering in background.)

05 May 2007, 04:29 PM
Hi Paul, you're right; your pic is much more accurate to Synergy's first pic, but I'd read another duplicate thread of his where he gave much more detail on what he wanted.

I can't really tell you exactly how to add thickness as XSI's tools are different to Lightwave. However, I've created a little vid of how I'd do it in LW, and you might be able to get some pointers.
33second avi, Techsmith Screen Capture codec, 1.33MB (

What I've done is copy the object, and paste it into a second layer. I've flipped all the polygons as this will be the interior of the object. Then I use a tool called smooth shift which is like an advanced bevel tool; it moves vertices out along their individual normals. You should be able to do this in XSI, but I'm not sure how. It's important to scale inwards rather than outwards because it is much more suitable for the geometry; scaling outwards along normals will result in pinching around the arches as the normals face each other. I've then copied this back onto the original object and merged vertices to join them together. I've then added extra edgeloops around the edge to give it sharper edges.


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