Hi Songkun, welcome to MoI!

Only surface objects can be exported to .3ds or .obj files.

If you only draw curve objects such as lines and circles, those will not be exported - .3ds and .obj files do not have a way to contain that type of information, instead they contain polygon mesh data. When you export an object made up of surfaces (for example, spheres, extrusions, lofts), MoI will convert those surfaces into polygons and write the polygon data to the .3ds or .obj file.

If you are still having problems exporting, could you please post your .3dm model file here as an attachment? That will help if I can take a look at it and see what is happening.

- Michael