how to define different shapes inside 3 d matrix and extract the points on boundary and its tangential direction?

i have a 3 D matrix(e.g: 521*161*521). now i need the trajectories of different shapes inside the matrix. for e.g: i need to draw the line, s-curve or circle inside the 3 D matrix. then i need to extract the boundary points of those shapes(x,y,z) and tangential direction of those points. if somebody can give suggestions that would be great.