If you have tried to create models for your 3D games, you probably know that many modeling programs out there (like Max) are quite expensive, and have so many features that using them feels like drinking out of a fire hose. One of the simplest modeling programs to use (in my opinion) is Google Sketchup (
http://sketchup.google.com/). It doesn't have a lot of the features of some of the other modeling programs available, but it is more than sufficient for most models you will want to create. There is even a "3D Warehouse" full of Sketchup models available for download (
http://sketchup.google.com/3dwarehouse/).
Only problem is, the free version does not export your model into a common format (like 3ds). So I figured out a way to do the conversion.
The first important piece of information I found, is that the .kmz file format is actually just a .zip file. If you export your sketchup model to "Google Earth 4 (*.kmz)", you can change the name of your file from filename.kmz to filename.zip and look inside. You will find a model file in there, of the Collada "Digital Asset Exchange" format. It has the extension ".dae".
Once you have your .dae file, you will, of course need a converter to change it to a .3ds file. I found a free converter that will do this for you. It is called the FBX converter tool, on the Autodesk website.
You can download the windows version of the FBX converter from:
http://images.autodesk.com/adsk/files/fbx200611_2_converter_win.exeThe program will install to the following location (unless you specify otherwise):
Program Files\Autodesk\FBX\FBXConverter\2006.11.2\FbxConverterUI.exe
I also saw Mac and Linux versions of the converter (I forgot to grab the links while I was there - you'll have to search the Autodesk site if you aren't running windows).
Now the FBX converter will convert your .dae file into a .fbx file. From there, the FBX converter will let you convert the .fbx file into a .3ds file.
This converter also supports a few other formats, so it could be pretty handy to have around.
Here is the process for converting from .skp to .3ds:
1) Download or create a .skp model in Sketchup
2) In Sketchup, export your model to "Google Earth 4 (*.kmz)
(note: be sure to choose version 4, because the other one is different)
3) Change the extention of the .kmz file to .zip
4) Pull out the .dae file
5) In FBX Converter, convert the .dae file to .fbx
6) In FBX Converter, convert the .fbx file to .3ds
The main problem with this conversion method is that the Autodesk FBX converter is often not able to convert many .dae files (possibly due to large poly counts)
. Fortunately, I found another more complicated method which generally works for cases when the method above doesn't work. I will outline this other method in my next post.
Another issue I sometimes have when converting models is the materials not being quite right. When necessary, I use a free program called LithUnwrap (
http://www.sharecg.com/v/5169/software-and-tools/LithUnwrap---Free-UV-Mapper-for-Windows) to edit the materials. Lithunwrap works great for this, but it does have a maximum number or polys it will load, so there are probably better programs out there for editing materials.
I made an applet that loads a .skp model I converted:
http://www.paulscode.com/source/LegoCar/(Use the mouse to rotate the object)
You can get the source code at:
http://www.paulscode.com/source/LegoCar/LegoCar.javahttp://www.paulscode.com/source/LegoCar/LegoCar.3ds(In this demo applet, the .3ds file needs to be compiled into the jar)
And to show that I actually converted that model, here is the original Sketchup file:
http://www.paulscode.com/source/LegoCar/LegoCar.skp 
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