Render a 3D image as a 2D vector image

Is there any software that will take a 3d model (in any format) and allow you to render it as a 2D vector image (preferable as either an .SVG or .PDF)?

My intention is to render LEGO building instructions this way. While there are many tools that let you view them or generate nice, rasterized output, I'd really like to be able to generate vectorized output. Textures are not needed, and hidden line removal may not be needed.

I could use a tool that works on any platform (although my preference is OS X, Linux, then Windows). Open source is preferred.

If no one knows of a tool that does this, does anyone have a good recommendation of something to hack on and add a feature to output via Cairo?

Google Sketchup has an option to print images using 2D vectors. If you have a "print to PDF" option on your computer, you can then have a nice .pdf. If necessary, one can import the .pdf into Inkscape to generate an .SVG file.

You don't find vector output for renderers since they do a lot of work on calculating the right light on the surfaces. This is a hard problem already, but outputting that in a vector format will be as hard.

If you require simpler line art and are fine with some manual tweaking give PantoGraph for Blender a try.

If you have a wallet, there is a package (I believe windows only) called 'Deep Exploration' that will open most 3D CAD and model formats. You can easily output to 2d vector formats including pdf and AI.

Price is a little steep, yet this package is specifically designed to generate assembly instructions.