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

 synergy1103 March 2007, 07:43 PMHello 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. Thanks!
kirigoi
05 May 2007, 01:14 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.

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_01.png

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

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_02.png

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.

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_03.png

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

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_04.png

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.

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_05.png

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.

kirigoi
05 May 2007, 01:26 PM
http://www.skylight3d.com/images/CGTalk/screens/dome_arches_06.png

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.

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_07.png

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.

http://www.skylight3d.com/images/CGTalk/screens/dome_arches_08.png

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

Update: I posted this on the wrong forum... Sorry if I've confused all you XSI users, I'll duplicate this in Synergy's post in the general modelling forum...

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05 May 2007, 01:26 PM
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