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## -*- texinfo -*-
## @deftypefn  {} {} quiver (@var{u}, @var{v})
## @deftypefnx {} {} quiver (@var{x}, @var{y}, @var{u}, @var{v})
## @deftypefnx {} {} quiver (@dots{}, @var{s})
## @deftypefnx {} {} quiver (@dots{}, @var{style})
## @deftypefnx {} {} quiver (@dots{}, "filled")
## @deftypefnx {} {} quiver (@var{hax}, @dots{})
## @deftypefnx {} {@var{h} =} quiver (@dots{})
##
## Plot a 2-D vector field with arrows.
##
## Plot the (@var{u}, @var{v}) components of a vector field at the grid points
## defined by (@var{x}, @var{y}).  If the grid is uniform then @var{x} and
## @var{y} can be specified as vectors and @code{meshgrid} is used to create
## the 2-D grid.
##
## If @var{x} and @var{y} are not given they are assumed to be
## @code{(1:@var{m}, 1:@var{n})} where
## @code{[@var{m}, @var{n}] = size (@var{u})}.
##
## The optional input @var{s} is a scalar defining a scaling factor to use for
## the arrows of the field relative to the mesh spacing.  A value of 1.0 will
## result in the longest vector exactly filling one grid square.  A value of 0
## disables all scaling.  The default value is 0.9.
##
## The style to use for the plot can be defined with a line style @var{style}
## of the same format as the @code{plot} command.  If a marker is specified
## then the markers are drawn at the origin of the vectors (which are the grid
## points defined by @var{x} and @var{y}).  When a marker is specified, the
## arrowhead is not drawn.  If the argument @qcode{"filled"} is given then the
## markers are filled.
##
## 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 a quiver object.
## A quiver object regroups the components of the quiver plot (body, arrow,
## and marker), and allows them to be changed together.
##
## Example:
##
## @example
## @group
## [x, y] = meshgrid (1:2:20);
## h = quiver (x, y, sin (2*pi*x/10), sin (2*pi*y/10));
## set (h, "maxheadsize", 0.33);
## @end group
## @end example
##
## @seealso{quiver3, compass, feather, plot}
## @end deftypefn

function h = quiver (varargin)

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

  if (nargin < 2)
    print_usage ();
  endif

  oldfig = [];
  if (! isempty (hax))
    oldfig = get (0, "currentfigure");
  endif
  unwind_protect
    hax = newplot (hax);
    htmp = __quiver__ (hax, false, varargin{:});

    ## FIXME: This should be moved into __quiver__ when problem with
    ##        re-initialization of title object is fixed.
    if (! ishold ())
      set (hax, "box", "on");
    endif

  unwind_protect_cleanup
    if (! isempty (oldfig))
      set (0, "currentfigure", oldfig);
    endif
  end_unwind_protect

  if (nargout > 0)
    h = htmp;
  endif

endfunction


%!demo
%! clf;
%! [x,y] = meshgrid (1:2:20);
%! h = quiver (x,y, sin (2*pi*x/10), sin (2*pi*y/10));
%! title ("quiver() plot w/arrowheads (default)");

%!demo
%! clf;
%! [x,y] = meshgrid (1:2:20);
%! h = quiver (x,y, sin (2*pi*x/10), sin (2*pi*y/10), "o");
%! title ("quiver() plot w/origin markers");

%!demo
%! clf;
%! [x,y] = meshgrid (1:2:20);
%! h = quiver (x,y, sin (2*pi*x/10), sin (2*pi*y/10));
%! set (h, "marker", "o");
%! title ("quiver() plot w/origin markers and arrowheads");

%!demo
%! clf;
%! x = linspace (0, 3, 80);
%! y = sin (2*pi*x);
%! theta = 2*pi*x + pi/2;
%! quiver (x, y, sin (theta)/10, cos (theta)/10, 0.4);
%! axis equal tight;
%! hold on; plot (x,y,"r"); hold off;
%! title ("quiver() with scaled arrows");


%!test <*39552> # Check arrow length, scale factor adjustment, one arrow.
%! hf = figure ("visible", "off");
%! hax = gca ();
%! unwind_protect
%!   [x,y] = meshgrid (1:2);
%!   u = [0 1; 2 3];
%!   v = [1 2; 3 4];
%!   numpts = numel (x);
%!   sf = 0.5;
%!
%!   ## Check single arrow.
%!   h = quiver (hax, x(4), y(4), u(4), v(4), 1);
%!   childxdata = get (get (h, "children"), "xdata");
%!   stemchild = find (cellfun (@numel, childxdata) == 3);
%!   xendpoint = childxdata{stemchild}(2);
%!   assert (xendpoint, x(4) + u(4), eps);
%!
%!   h = quiver (hax, x(4), y(4), u(4), v(4), sf);
%!   childxdata = get (get (h, "children"), "xdata");
%!   stemchild = find (cellfun (@numel, childxdata) == 3);
%!   xendpoint = childxdata{stemchild}(2);
%!   assert (xendpoint, x(4) + sf*u(4), eps);
%!
%! unwind_protect_cleanup
%!   close (hf);
%! end_unwind_protect

%!test <*39552> # Check arrow length, scale factor adjustment, multiple arrows.
%! hf = figure ("visible", "off");
%! hax = gca ();
%! unwind_protect
%!   [x,y] = meshgrid (1:2);
%!   u = [0 1; 2 3];
%!   v = [1 2; 3 4];
%!   numpts = numel (x);
%!   sf = 0.5;
%!
%!   ## Check multiple arrows.
%!   h = quiver (hax, x, y, u, v, 1);
%!   childxdata = get (get (h, "children"), "xdata");
%!   stemchild = find (cellfun (@numel, childxdata) == 3*numpts);
%!   xendpoint1 = childxdata{stemchild}(5);
%!   xendpoint2 = childxdata{stemchild}(11);
%!   assert (xendpoint1, x(2) + (sqrt(2)/10)*u(2), eps);
%!   assert (xendpoint2, x(4) + (sqrt(2)/10)*u(4), eps);
%!
%!   h = quiver (hax, x, y, u, v, sf);
%!   childxdata = get (get (h, "children"), "xdata");
%!   stemchild = find (cellfun (@numel, childxdata) == 3*numpts);
%!   xendpoint1 = childxdata{stemchild}(5);
%!   xendpoint2 = childxdata{stemchild}(11);
%!   assert (xendpoint1, x(2) + sf*(sqrt(2)/10)*u(2), eps);
%!   assert (xendpoint2, x(4) + sf*(sqrt(2)/10)*u(4), eps);
%!
%! unwind_protect_cleanup
%!   close (hf);
%! end_unwind_protect

## Test input validation
%!error <Invalid call> quiver()
%!error <Invalid call> quiver(1)