function contour( x, y, z, levels )
# contour (x,y,z,levels): plot contour lines of a function
#
#  Plot contour lines by interpolating the values of a function
#  on a grid.  If x,y,z are matrices with the same dimensions
#  the the corresponding elements represent points on the function.
#  levels contains a vector of heights at which to draw level curves,
#  or a scalar containing the number of equally spaced curves to
#  draw.
#
#  If x is a vector and y is a vector, then z must be a matrix
#  with the same number of rows as x and the same number of
#  columns as y.  The corresponding points are x(i),y(j),z(i,y)
#  are points of the function.

if( nargin < 3 )
  usage( "contour" );
  return;
endif
 
if( nargin == 3 )
  levels = 5;
endif

if( is_scalar( levels ) )
  minz = min(min(z));
  maxz = max(max(z));
  diff = (maxz-minz)/(2*levels);

  levels = linspace(minz+diff,maxz-diff,levels );
endif

if( !is_vector( levels ))
  error( "levels must be a vector or a scalar" );
endif

if( is_vector(x) && is_vector(y))
  if( length(x) != rows(z) || length(y) != columns(z) )
    usage( "contour" );
    return;
  endif
  [x,y] = meshgrid(x,y);
endif

if( rows(x) != rows(z) || rows(y) != rows(z) ||
    columns(x) != columns(z) || columns(y) != columns(z) )
    usage( "contour" );
    return;
endif

__cntr__(x,y,z,levels);

endfunction