A variation of the standard way to compute a look-at matrix works here as well: If

**m** denotes the movement direction (e.g. projected onto the x-z plane) and

**n** the terrain normal at the current position, then

**r** :=

**m** cross

**n**
denotes the side vector perpendicular to both

**m** and

**n**. Then

**f** :=

**n** cross

**r**
denotes the forward vector that can be used as look-along vector as requested. It usually need to be normalized, though.

If, on the other hand, the normal isn't available because you're working with a height map, then

this topic may help you. (Please notice that my first posts therein haven't considered triangulation correctly, but down from post #15 inclusive things are done well). After computing the height at the current position and the height a bit in direction of the movement vector, then the normalized difference vector is the result.

BTW: To be pedantic: A bi-normal is a construct that can be computed at locations on a line. In case of a surface the correct term is bi-tangent.

**Edited by haegarr, 21 December 2012 - 12:51 PM.**