% illustration of a parabolic trajectory  function main()     L=0.8;    s=0.1;    q=-0.4;    N=100;     arrow_size = 0.1;    sharpness = 20;    arrow_type = 1;     arrlen = 0.3; % arrow length    tiny = 0.01;    ball_radius = 0.05;        X=linspace(-L, L, N);    Y =L^2 - X.^2;    Xl = linspace(-L-s, L+s, N);         % KSmrq's colors    red    = [0.867 0.06 0.14];    blue   = [0, 129, 205]/256;    green  = [0, 200,  70]/256;    yellow = [254, 194,   0]/256;    white = 0.99*[1, 1, 1];    black = [0, 0, 0];    gray = 0.5*white;    lw = 2.3;         figure(1); clf; hold on; axis equal; axis off;    plot(X, Y, 'linewidth', lw, 'linestyle', '--', 'color', blue);    arrow([q-tiny, L^2-q^2], [q+arrlen-tiny, L^2-q^2-2*q*arrlen], lw, arrow_size, sharpness, arrow_type, red);    ball(q, L^2 - q^2, ball_radius, gray)    plot(Xl, 0*Xl, 'linewidth', 2*lw, 'color', black);        %saveas(gcf, 'Parabolic_trajectory.eps', 'psc2')   plot2svg('Parabolic_trajectory.svg');  function ball(x, y, radius, color) % draw a ball of given uniform color     Theta=0:0.1:2*pi;    X=radius*cos(Theta)+x;    Y=radius*sin(Theta)+y;    H=fill(X, Y, color);    set(H, 'EdgeColor', [0, 0, 0]);  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 degrees % 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*pi*sharpness/180);   % 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(pi*sharpness/180)/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