While traditional Parallax mapping compute the offset based on one sample of the height map, the Occlusion version will make a loop to sample the height map many times in order to reach a more precise location of what the pixel to compute should reflect. This algorithm is really quick to perform, you can almost think of it as being free if you already are using Bump. The core algorithm which perform an offset computation for the texture UV coordinates, based on a height map. There are many techniques that are based on the Parallax Mapping principle, Babylon.js supports two of them. (A diffuse texture is required for using parallax mapping). The simple reason is that the height map needed to perform Parallax Mapping is most of the time encoded in the Alpha channel of the Normal Map texture. While this technique is independent from Normal Mapping (a.k.a Bump) it's often used in conjunction with it. Parallax Mapping is an algorithm which, based from a height map, apply an offset on the material's textures in order to accentuate the effect of relief in the geometry's surface.
Starting with Babylon.js v2.4, we introduced Parallax Mapping.
