High Fidelity supports a general object placement facility called “metavoxels” which allows a brush shape (sphere or cube, etc) to be used to place a voxel of a given size and shape with a given texture on a plane (such as X/Y for a vertical plane or X/Z for a horizontal plane. This allows 3D forms to be built. Holes can then be dug through these shapes to make caves, etc.
This is an example of a simple hedge or arch created on the X/Y place with a 1m diameter sphere brush and using a foliage texture… http://vue.ed.ac.uk/hifi/img/hedge.png.
Ai Austin Blog 
