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## -*- texinfo -*-
## @deftypefn  {} {} ostreamtube (@var{x}, @var{y}, @var{z}, @var{u}, @var{v}, @var{w}, @var{sx}, @var{sy}, @var{sz})
## @deftypefnx {} {} ostreamtube (@var{u}, @var{v}, @var{w}, @var{sx}, @var{sy}, @var{sz})
## @deftypefnx {} {} ostreamtube (@var{xyz}, @var{x}, @var{y}, @var{z}, @var{u}, @var{v}, @var{w})
## @deftypefnx {} {} ostreamtube (@dots{}, @var{options})
## @deftypefnx {} {} ostreamtube (@var{hax}, @dots{})
## @deftypefnx {} {@var{h} =} ostreamtube (@dots{})
## Calculate and display streamtubes.
##
## Streamtubes are approximated by connecting circular crossflow areas
## along a streamline.  The expansion of the flow is determined by the local
## crossflow divergence.
##
## The vector field is given by @code{[@var{u}, @var{v}, @var{w}]} and is
## defined over a rectangular grid given by @code{[@var{x}, @var{y}, @var{z}]}.
## The streamtubes start at the seed points
## @code{[@var{sx}, @var{sy}, @var{sz}]}.
##
## The tubes are colored based on the local vector field strength.
##
## The input parameter @var{options} is a 2-D vector of the form
## @code{[@var{scale}, @var{n}]}.  The first parameter scales the start radius
## of the streamtubes (default 1).  The second parameter specifies the number
## of vertices that are used to construct the tube circumference (default 20).
##
## @code{ostreamtube} can be called with a cell array containing pre-computed
## streamline data.  To do this, @var{xyz} must be created with the
## @code{stream3} function.  This option is useful if you need to alter the
## integrator step size or the maximum number of vertices of the streamline.
##
## If the first argument @var{hax} is an axes handle, then plot into this axes,
## rather than the current axes returned by @code{gca}.
##
## The optional return value @var{h} is a graphics handle to the plot
## objects created for each streamtube.
##
## Example:
##
## @example
## @group
## [x, y, z] = meshgrid (-1:0.1:1, -1:0.1:1, -3:0.1:0);
## u = -x / 10 - y;
## v = x - y / 10;
## w = - ones (size (x)) / 10;
## ostreamtube (x, y, z, u, v, w, 1, 0, 0);
## @end group
## @end example
##
## @seealso{stream3, streamline, streamtube}
## @end deftypefn

## References:
##
## @inproceedings{
##    title = {Visualization of 3-D vector fields - Variations on a stream},
##    author = {Dave Darmofal and Robert Haimes},
##    year = {1992}
## }
##
## @article{
##    title = {Efficient streamline, streamribbon, and streamtube constructions on unstructured grids},
##    author = {Ueng, Shyh-Kuang and Sikorski, C. and Ma, Kwan-Liu},
##    year = {1996},
##    month = {June},
##    publisher = {IEEE Transactions on Visualization and Computer Graphics},
## }

function h = ostreamtube (varargin)

  [hax, varargin, nargin] = __plt_get_axis_arg__ ("ostreamtube", varargin{:});

  options = [];
  xyz = [];
  switch (nargin)
    case 0
      print_usage ();
    case 6
      [u, v, w, spx, spy, spz] = varargin{:};
      [m, n, p] = size (u);
      [x, y, z] = meshgrid (1:n, 1:m, 1:p);
    case 7
      if (iscell (varargin{1}))
        [xyz, x, y, z, u, v, w] = varargin{:};
      else
        [u, v, w, spx, spy, spz, options] = varargin{:};
        [m, n, p] = size (u);
        [x, y, z] = meshgrid (1:n, 1:m, 1:p);
      endif
    case 8
      [xyz, x, y, z, u, v, w, options] = varargin{:};
    case 9
      [x, y, z, u, v, w, spx, spy, spz] = varargin{:};
    case 10
      [x, y, z, u, v, w, spx, spy, spz, options] = varargin{:};
    otherwise
      error ("ostreamtube: invalid number of inputs");
  endswitch

  scale = 1;
  num_circum = 20;
  if (! isempty (options))
    switch (numel (options))
      case 1
        scale = options(1);
      case 2
        scale = options(1);
        num_circum = options(2);
      otherwise
        error ("ostreamtube: invalid number of OPTIONS elements");
    endswitch

    if (! isreal (scale) || scale <= 0)
      error ("ostreamtube: SCALE must be a real scalar > 0");
    endif
    if (! isreal (num_circum) || num_circum < 3)
      error ("ostreamtube: number of tube vertices N must be greater than 2");
    endif
    num_circum = fix (num_circum);
  endif

  if (isempty (hax))
    hax = gca ();
  else
    hax = hax(1);
  endif

  if (isempty (xyz))
    xyz = stream3 (x, y, z, u, v, w, spx, spy, spz, 0.2);
  endif

  div = divergence (x, y, z, u, v, w);

  ## Use the bounding box diagonal to determine the starting radius
  mxx = mnx = mxy = mny = mxz = mnz = [];
  j = 1;
  for i = 1 : length (xyz)
    sl = xyz{i};
    if (! isempty (sl))
      slx = sl(:, 1); sly = sl(:, 2); slz = sl(:, 3);
      mxx(j) = max (slx); mnx(j) = min (slx);
      mxy(j) = max (sly); mny(j) = min (sly);
      mxz(j) = max (slz); mnz(j) = min (slz);
      j += 1;
    endif
  endfor
  dx = max (mxx) - min (mnx);
  dy = max (mxy) - min (mny);
  dz = max (mxz) - min (mnz);
  rstart = scale * sqrt (dx*dx + dy*dy + dz*dz) / 25;

  h = [];
  for i = 1 : length (xyz)
    sl = xyz{i};
    num_vertices = rows (sl);
    if (! isempty (sl) && num_vertices > 2)

      usl = interp3 (x, y, z, u, sl(:, 1), sl(:, 2), sl(:, 3));
      vsl = interp3 (x, y, z, v, sl(:, 1), sl(:, 2), sl(:, 3));
      wsl = interp3 (x, y, z, w, sl(:, 1), sl(:, 2), sl(:, 3));
      vv = sqrt (usl.*usl + vsl.*vsl + wsl.*wsl);

      div_sl = interp3 (x, y, z, div, sl(:, 1), sl(:, 2), sl(:, 3));
      is_singular_div = find (isnan (div_sl), 1, "first");

      if (! isempty (is_singular_div))
        max_vertices = is_singular_div - 1;
      else
        max_vertices = num_vertices;
      endif

      if (max_vertices > 2)

        htmp = plottube (hax, sl, div_sl, vv, max_vertices, ...
                         rstart, num_circum);
        h = [h; htmp];

      endif
    endif
  endfor

endfunction

function h = plottube (hax, sl, div_sl, vv, max_vertices, rstart, num_circum)

  phi = linspace (0, 2*pi, num_circum);
  cp = cos (phi);
  sp = sin (phi);

  X0 = sl(1, :);
  X1 = sl(2, :);

  ## 1st rotation axis
  R = X1 - X0;
  RE = R / norm (R);

  ## Initial radius
  vold = vv(1);
  vact = vv(2);
  ract = rstart * exp (0.5 * div_sl(2) * norm (R) / vact) * sqrt (vold / vact);
  vold = vact;
  rold = ract;

  ## Guide point and its rotation to create a segment
  N = get_normal1 (R);
  K = ract * N;
  XS = rotation (K, RE, cp, sp) + repmat (X1.', 1, num_circum);

  px = zeros (num_circum, max_vertices - 1);
  py = zeros (num_circum, max_vertices - 1);
  pz = zeros (num_circum, max_vertices - 1);
  pc = zeros (num_circum, max_vertices - 1);

  px(:, 1) = XS(1, :).';
  py(:, 1) = XS(2, :).';
  pz(:, 1) = XS(3, :).';
  pc(:, 1) = vact * ones (num_circum, 1);

  for i = 3 : max_vertices

    KK = K;
    X0 = X1;
    X1 = sl(i, :);
    R = X1 - X0;
    RE = R / norm (R);

    ## Tube radius
    vact = vv(i);
    ract = rold * exp (0.5 * div_sl(i) * norm (R) / vact) * sqrt (vold / vact);
    vold = vact;
    rold = ract;

    ## Project KK onto RE and get the difference in order to calculate the next
    ## guiding point
    Kp = KK - RE * dot (KK, RE);
    K = ract * Kp / norm (Kp);

    ## Rotate around RE and collect surface patches
    XS = rotation (K, RE, cp, sp) + repmat (X1.', 1, num_circum);

    px(:, i - 1) = XS(1, :).';
    py(:, i - 1) = XS(2, :).';
    pz(:, i - 1) = XS(3, :).';
    pc(:, i - 1) = vact * ones (num_circum, 1);

  endfor

  h = surface (hax, px, py, pz, pc);

endfunction

## Arbitrary N normal to X
function N = get_normal1 (X)

  if ((X(3) == 0) && (X(1) == -X(2)))
    N = [- X(2) - X(3), X(1), X(1)];
  else
    N = [X(3), X(3), - X(1) - X(2)];
  endif

  N /= norm (N);

endfunction

## Rotate X around U where |U| = 1
## cp = cos (angle), sp = sin (angle)
function Y = rotation (X, U, cp, sp)

  ux = U(1);
  uy = U(2);
  uz = U(3);

  Y(1, :) = X(1) * (cp + ux * ux * (1 - cp)) + ...
            X(2) * (ux * uy * (1 - cp) - uz * sp) + ...
            X(3) * (ux * uz * (1 - cp) + uy * sp);

  Y(2, :) = X(1) * (uy * ux * (1 - cp) + uz * sp) + ...
            X(2) * (cp + uy * uy * (1 - cp)) + ...
            X(3) * (uy * uz * (1 - cp) - ux * sp);

  Y(3, :) = X(1) * (uz * ux * (1 - cp) - uy * sp) + ...
            X(2) * (uz * uy * (1 - cp) + ux * sp) + ...
            X(3) * (cp + uz * uz * (1 - cp));

endfunction

%!demo
%! clf;
%! [x, y, z] = meshgrid (-1:0.1:1, -1:0.1:1, -3.5:0.1:0);
%! a = 0.1;
%! b = 0.1;
%! u = - a * x - y;
%! v = x - a * y;
%! w = - b * ones (size (x));
%! sx = 1.0;
%! sy = 0.0;
%! sz = 0.0;
%! ostreamtube (x, y, z, u, v, w, sx, sy, sz, [1.2, 30]);
%! colormap (jet);
%! shading interp;
%! view ([-47, 24]);
%! camlight ();
%! lighting gouraud;
%! grid on;
%! view (3);
%! axis equal;
%! set (gca, "cameraviewanglemode", "manual");
%! title ("Spiral Sink");

%!demo
%! clf;
%! [x, y, z] = meshgrid (-2:0.5:2);
%! t = sqrt (1.0./(x.^2 + y.^2 + z.^2)).^3;
%! u = - x.*t;
%! v = - y.*t;
%! w = - z.*t;
%! [sx, sy, sz] = meshgrid (-2:4:2);
%! xyz = stream3 (x, y, z, u, v, w, sx, sy, sz, [0.1, 60]);
%! ostreamtube (xyz, x, y, z, u, v, w, [2, 50]);
%! colormap (jet);
%! shading interp;
%! view ([-47, 24]);
%! camlight ();
%! lighting gouraud;
%! grid on;
%! view (3);
%! axis equal;
%! set (gca, "cameraviewanglemode", "manual");
%! title ("Integration Towards Sink");

## Test input validation
%!error ostreamtube ()
%!error <invalid number of inputs> ostreamtube (1)
%!error <invalid number of inputs> ostreamtube (1,2)
%!error <invalid number of inputs> ostreamtube (1,2,3)
%!error <invalid number of inputs> ostreamtube (1,2,3,4)
%!error <invalid number of inputs> ostreamtube (1,2,3,4,5)
%!error <invalid number of OPTIONS> ostreamtube (1,2,3,4,5,6, [1,2,3])
%!error <SCALE must be a real scalar . 0> ostreamtube (1,2,3,4,5,6, [1i])
%!error <SCALE must be a real scalar . 0> ostreamtube (1,2,3,4,5,6, [0])
%!error <N must be greater than 2> ostreamtube (1,2,3,4,5,6, [1, 1i])
%!error <N must be greater than 2> ostreamtube (1,2,3,4,5,6, [1, 2])