I finally figured out how to make STL files for 3D printing using a variety of sources for drawings. I am not good at freehand drawing, especially on the computer with a mouse or a crappy digitizer which I have. So I am stuck with tracing art from other sources. I have screwed around with 123D design and it is so terrible in windows 8.1 that I finally gave up and swore to never open that program again until i hear that the many many bugs have been fixed.
So I have been looking for a way to make an occasional 3D print without paying hundreds or thousands of dollars to buy software that I might use a few times a year. Here is what I want to do and what I finally succeeded at doing:
Create a 2D drawing using ordinary image files as the source and to extrude these into 3D models and edit then in the normal ways. 123D documentation says it will do this but the PC version does not allow import and if you have win 8.1, it will not connect to the cloud based program. It works with my win 7 computer but I hate that computer for other reasons and 123D has such crappy documentation that I am probably best avoiding it for a while anyway. OReilly is supposed to publish a book in February 2014 or so which might change the documentation aspect of things but I am afraid that the reason the book's release date has been pushed forward a few times is that 123D is still in development and is sort of a mess of features that don't really work. O'Reilly published really good books and probably does not want to be associated with a product that doesn't work.
Here is the procedure that actually works. I have tried several others that don't work for a variety of reasons:
Google Sketchup is now a for sale program sold by Trimble but they have a trial version that seems to work. I forget what the limits are or how much a full version costs. Hopefully I can use the trial version indefinitely. It may be possible to use an old Google free version of Sketchup but I don't know where to find one or if it does export .dae files which is necessary for this method. Sketchup allows the import of a variety of standard image files, jpg, png, gif, etc etc and allows the export of COLLADA (.dae) files. You can import almost any image file, trace it with the freehand draw tool (not available from the toolbar icons, you open it from the draw dropdown and selecting freehand) and then export as .dae which can them be converted with another program, MeshLab http://meshlab.sourceforge.net/ This is a great program that allows the import and export a a variety of file formats and will produce STL files that can then be used in your 3D printing program such as Repetier Host.
If you have a recent 64 bit version of windows, don't bother with the 64 bit version of MeshLab. It requires an old version of the Microsoft Visual C++ whatever they call it. MeshLab will not install if you have a newer version of the C++ plus on your computer. Fortunately you can just install the 32 bit version.
I tested this method and it does actually work and produces an STL file that is correctly opened in Repetier Host. Will make a real test file soon and print it to make sure there are no errors in the file that I don't yet see.