function  [Xj,S] = jcalc( jtyp, q )

% jcalc  joint transform and motion subspace matrices.
% [Xj,S]=jcalc(type,q)  returns the joint transform and motion subspace
% matrices for a joint of the given type.  jtyp can be either a string or a
% structure containing a string-valued field called 'code'.  Either way,
% the string contains the joint type code.  For joints that take
% parameters (e.g. helical), jtyp must be a structure, and it must contain
% a field called 'pars', which in turn is a structure containing one or
% more parameters.  (For a helical joint, pars contains a parameter called
% 'pitch'.)  q is the joint's position variable.

if ischar( jtyp )
  code = jtyp;
else
  code = jtyp.code;
end

switch code
  case 'Rx'				% revolute X axis
    Xj = rotx(q);
    S = [1;0;0;0;0;0];
  case 'Ry'				% revolute Y axis
    Xj = roty(q);
    S = [0;1;0;0;0;0];
  case {'R','Rz'}			% revolute Z axis
    Xj = rotz(q);
    S = [0;0;1;0;0;0];
  case 'Px'				% prismatic X axis
    Xj = xlt([q 0 0]);
    S = [0;0;0;1;0;0];
  case 'Py'				% prismatic Y axis
    Xj = xlt([0 q 0]);
    S = [0;0;0;0;1;0];
  case {'P','Pz'}			% prismatic Z axis
    Xj = xlt([0 0 q]);
    S = [0;0;0;0;0;1];
  case 'H'				% helical (Z axis)
    Xj = rotz(q) * xlt([0 0 q*jtyp.pars.pitch]);
    S = [0;0;1;0;0;jtyp.pars.pitch];
  case 'r'				% planar revolute
    Xj = plnr( q, [0 0] );
    S = [1;0;0];
  case 'px'				% planar prismatic X axis
    Xj = plnr( 0, [q 0] );
    S = [0;1;0];
  case 'py'				% planar prismatic Y axis
    Xj = plnr( 0, [0 q] );
    S = [0;0;1];
  otherwise
    error( 'unrecognised joint code ''%s''', code );
end