Segmentation of half transparent material, e.g. glass
Why not just use a simple 2D FFT (guassian) high pass filer?

I did this real quick using MATLAB

Shard #1 using high pass FFT:



The same thing is done on #2.

Shard #2 using high pass FFT:



As you can see, the background and glass area is wiped out, and only the edges are traced.  I did not spend any time on it, but you can threshold the HP filtered output to have more crisp edges, or push the HP cut off higher.

Is this more the results you are looking to get?