I use blender to render patent images, which require "black and white" drawings. Hemi is very handy because 1 hemi lamp is enough and no shadows. But this time I got this: After using gimp Edge detect -> Revert colors -> Auto-crop image -> Threshold to 239 -> 1 bit color index, I got this: It's still usable, but it's ugly! So what happened? Hemi lamp used to be simple and works great! All I need is to put 1 single hemi lamp at origin (0,0,0)!
There is no edges showing on the planes because they have no thickness. Add a slight thickness to each plane and you should see detectable edges.