The dx,dy,dz parameters, tell how far in each direction it should go. You could normalize the vector and then multiply each component by the distance you want to pick, or subtract the starting point from the ending point.
I think the reason it's not just a starting point, and an ending point is that it uses a delta vector internally...
I think the reason it's not just a starting point, and an ending point is that it uses a delta vector internally...