Unimesh Plugin (Milkshape)
Excellent, supports full bone assignments, works for hair and body meshes. This is one of the most complete tools, supports fat/pregnancy, and bumpmaps.
Get the Unimesh Milkshape Plugin Here:
Useful Add-ons by Demon432:
Beginner Tutorial: http://www.modthesims2.com/showthread.php?t=142305
There are 2 more Unimesh Body Tutorials, and then a Hair Editing Tutorial. The Beginner tutorial barely shows what can be done, do all 3 to learn how to use the various features of the plugin.
Only -- Make SURE to get the newest version! While the download is free, the program is $25 to register after a month. Well worth it.
This plugin is powerful, and currently only available with Milkshape. It's wise to read the thread for the plugin to get tips on how to properly reassign the hair vertices, and how to add new objects to the mesh.
Before importing any meshes, make sure that 'autosmooth' is unchecked on the groups page.
All bone values must add up to 100%, or there will be slightly odd display issues in the game from time to time, like holes in the mesh.
Do NOT weld things, for those who have used other plugins. Leave the seams be and they will display fine in game. For combining new objects and getting the smoothing right without damaging your mesh, make the new part separately in a different edit, and smooth it before combining it with your mesh file. If you have an area that needs smoothing, you can also get some extra tools by demon432
that can help with that.
Some Nightlife and later meshes have a 4th bone assignment on some joints. If you want compatibilty with all games, you might need to reassign those joints. Wes has released extra plugins (currently in the thread) that will auto adjust all the bones to be 100% total, and will find all the bones with 4 assignments. Wes also wrote a little article about it here: http://www.modthesims2.com/article.php?t=128644
If you want your new mesh to have bumpmaps - start your base mesh project with a mesh that already has them, that is easiest.
Yes, you can use Unimesh to convert a mesh from one age to another - although this is not a 1 step thing, it involves getting in the proper skeleton and reworking the mesh.