How to avoid dark/black area in blender when using hemi? 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)!

After I set 0.01 thickness(using extrude/z/.01) to those "black planes", they're all dark/black now...


It seems to have something to do with camera direction. There would be no dark/black planes if you looked from downside of the planes, just like this:
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.

This seems an odd way of doing this though, I'd look into blender's edge detection functionality or freestyle as an alternative.