How can I create a “cracked glass” material?

Question

I'm trying to figure out how the cracked and chipped glass effect in a Bioshock Infinite Burial at Sea Episode 2 works.

My current guess is that it is essentially a transparent shader with gloss. It would have a map defining the direction of reflections from the environment, with the cracks being significantly different from the mesh's normals. It would also include some kind of model of the angular dependence of the amount of refraction/transmission to reflection so that it can roughly approximate Fesnel's equations.

It doesn't appear to be a full refractive model, so I'm wondering how exactly this is implemented? Am I right with what I have said above?

Answer 1

You could achieve a good approximation of this effect with regular environment mapping (to sample the reflective data) and a normal map (to simulate the surface discontinuities in the glass at the cracks), plus a simple decal-like texture modulated in (such as the one glampert linked in the comments:

If you have the artistic talent, you could probably hand-author this yourself. If not, you may be able to generate a reasonable facsimile using procedural generation techniques similar to those used to generate lighting bolts.

Note that the environment map likely need not be an accurate representation of the surrounding area, just enough to suggest similarity. This is basically how the windows in Assassin's Creed 4 were done (or at least, a part of how they were done).