% Illustration of gradient descent
function main()
 
% the ploting window
   figure(1);
   clf; hold on;
   set(gcf, 'color', 'white');
   set(gcf, 'InvertHardCopy', 'off');
   axis equal; axis off;
 
% the box 
   Lx1=-2; Lx2=2; Ly1=-2; Ly2=2;
 
% the function whose contours will be plotted
   N=60; h=1/N;
   XX=Lx1:h:Lx2;
   YY=Ly1:h:Ly2;
   [X, Y]=meshgrid(XX, YY);
   f=inline('-((y+1).^4/25+(x-1).^4/10+x.^2+y.^2-1)');
   Z=f(X, Y);
 
% the contours
   h=0.3; l0=-1; l1=20;
   l0=h*floor(l0/h);
   l1=h*floor(l1/h);
   v=[l0:1.5*h:0 0:h:l1 0.8 0.888];
   [c,h] = contour(X, Y, Z, v, 'b'); 
 
% graphing settings
   small=0.08;
   small_rad = 0.01;
   thickness=1; arrowsize=0.06; arrow_type=2;
   fontsize=13;
   red = [1, 0, 0];
   white = 0.99*[1, 1, 1];
 
% initial guess for gradient descent
   x=-0.6498; y=-1.0212;
 
   % run several iterations of gradient descent
   for i=0:4
      H=text(x-1.5*small, y+small/2, sprintf('x_%d', i));
      set(H, 'fontsize', fontsize, 'color', 0*[1 1 1]);
 
     % the derivatives in x and in y, the step size
      u=-2/5*(x-1)^3-2*x;
      v=-4/25*(y+1)^3-2*y;
      alpha=0.11;
 
      if i< 4
         plot([x, x+alpha*u], [y, y+alpha*v]);
         arrow([x, y], [x, y]+alpha*[u, v], thickness, arrowsize, pi/8, ...
               arrow_type, [1, 0, 0])
         x=x+alpha*u; y=y+alpha*v;
      end
 
   end
 
% some dummy text, to expand the saving window a bit
   text(-0.9721, -1.5101, '*', 'color', white);
   text(1.5235,   1.1824, '*', 'color', white);
 
% save to eps
   saveas(gcf, 'Gradient_descent.eps', 'psc2')
 
function arrow(start, stop, thickness, arrow_size, sharpness, arrow_type, color)
 
% Function arguments:
% start, stop:  start and end coordinates of arrow, vectors of size 2
% thickness:    thickness of arrow stick
% arrow_size:   the size of the two sides of the angle in this picture ->
% sharpness:    angle between the arrow stick and arrow side, in radians
% arrow_type:   1 for filled arrow, otherwise the arrow will be just two segments
% color:        arrow color, a vector of length three with values in [0, 1]
 
% convert to complex numbers
   i=sqrt(-1);
   start=start(1)+i*start(2); stop=stop(1)+i*stop(2);
   rotate_angle=exp(i*sharpness);
 
% points making up the arrow tip (besides the "stop" point)
   point1 = stop - (arrow_size*rotate_angle)*(stop-start)/abs(stop-start);
   point2 = stop - (arrow_size/rotate_angle)*(stop-start)/abs(stop-start);
 
   if arrow_type==1 % filled arrow
 
% plot the stick, but not till the end, looks bad
      t=0.5*arrow_size*cos(sharpness)/abs(stop-start); stop1=t*start+(1-t)*stop;
      plot(real([start, stop1]), imag([start, stop1]), 'LineWidth', thickness, 'Color', color);
 
% fill the arrow
      H=fill(real([stop, point1, point2]), imag([stop, point1, point2]), color);
      set(H, 'EdgeColor', 'none')
 
   else % two-segment arrow
      plot(real([start, stop]), imag([start, stop]),   'LineWidth', thickness, 'Color', color);
      plot(real([stop, point1]), imag([stop, point1]), 'LineWidth', thickness, 'Color', color);
      plot(real([stop, point2]), imag([stop, point2]), 'LineWidth', thickness, 'Color', color);
   end
 
function ball(x, y, r, color)
   Theta=0:0.1:2*pi;
   X=r*cos(Theta)+x;
   Y=r*sin(Theta)+y;
   H=fill(X, Y, color);
   set(H, 'EdgeColor', 'none');