Error with addpoints when trying to animate a trajectory following a marker

I am trying to animate a trajectory following a marker.

Here is the problematic part of my code:

figure(1)
axis([0, ceil(max(x)), 0, ceil(max(y))])
daspect([1 1 1])
grid on
xlabel('x (m)'), ylabel('y (m)'), title('Bouncing Projectile'), subtitle(['{\theta} = ', num2str(theta), '°, u = ', num2str(u), ' ms^{-1}, h = ', num2str(h), ' m, g = ', num2str(g), ' ms^{-2}, C = ', num2str(C), ', N = ', num2str(N)])

h = animatedline('Color', 'b');
p = plot(x(1), y(1), 'o', 'MarkerEdgeColor', 'red', 'MarkerFaceColor', 'red');

for k = 1:length(x)
    p.XData = x(k);
    p.YData = y(k);
    addpoints(h, x(k), y(k));
    drawnow limitrate
    pause(dt)
end

Whithout the code for the marker, the program runs well. However, for some strange reason, adding the code for the marker seems to create an error:

Error using matlab.graphics.animation.AnimatedLine/addpoints

Value must be a handle.

Error in file (line 54)

addpoints(h, x(k), y(k));

A graph is produced when I run the program, however the axes are not labeled, there is no title, and only one point is plotted.

What mistake have I made?

[Umar]

Hi Felix,

The error "Value must be a handle" suggests that the handle used in the addpoints function is incorrect. Ensure that h is a valid handle to the graph object. To resolve this issue and improve the animation of the trajectory with a marker, I made some adjustments to the code.

function [x, y] = calculate_trajectory(theta, u, h, g, C, N, dt)
  % Initialize arrays to store x and y coordinates
  x = zeros(1, N);
  y = zeros(1, N);
  % Convert launch angle from degrees to radians
  theta = deg2rad(theta);
  % Calculate the trajectory using projectile motion equations
  for i = 1:N
      x(i) = u * cos(theta) * i * dt;
      y(i) = h + u * sin(theta) * i * dt - 0.5 * g * (i * dt)^2;
      % Check for ground collision and update y position accordingly
      if y(i) < 0
          y(i) = 0;
          break;
      end
  end
end
% Define initial parameters
theta = 45; % Launch angle in degrees
u = 10; % Initial velocity in m/s
h = 0; % Initial height in meters
g = 9.81; % Acceleration due to gravity in m/s^2
C = 0; % Air resistance constant
N = 100; % Number of time steps
dt = 0.1; % Time step

% Calculate trajectory
[x, y] = calculate_trajectory(theta, u, h, g, C, N, dt);

% Add red marker at the starting point
p = plot(x(1), y(1), 'o', 'MarkerEdgeColor', 'red', 'MarkerFaceColor', 'red');

% Initialize the animated line
h = animatedline('Color', 'b');
addpoints(h, x(1), y(1));

for k = 2:length(x)
  % Update the position of the red marker
  p.XData = x(k);
  p.YData = y(k);
  % Add points to the animated line for the trajectory
  addpoints(h, x(k), y(k));
  drawnow limitrate
  pause(dt)
end
% Plot the bouncing projectile trajectory at the end of animation
h = animatedline('Color', 'b', 'MaximumNumPoints', N);
addpoints(h, x(k), y(k));
drawnow limitrate

Feel free to customize the above code based on your preferences. Now, you should be able to figure out to resolve your problem.

Please see attached code snippet with plot.

[Felix Oliver]

Thank you very much for your help.

[Umar]

Hi Felix,

No problem. I hope this answers your question.

[Walter Roberson]

@Umar

Please fix the formatting of your code.

When you are accessing using a mobile service, then the start of code is marked by an empty line followed by one or more spaces (preferably two spaces.) The code block then follows until the next empty line (even if the line has no leading spaces.)

I cannot fix it for you without losing the image you included.

[Umar]

Hi Walter,

I tried following your instructions and it did not help. I remembered talking about this issue to one of the moderators and he couldn’t fix it. So, can you please help out, I will appreciate your help.

[Walter Roberson]

@Umar

I went onto mobile and fixed it. I inserted two spaces at the beginning of the function line, and two spaces at the begining of each block of code following an empty line -- for example two spaces at the beginning of % Calculate trajectory

[Umar]

@Walter,

Could you please do me a favor, the code that you edited above, could you please execute this code on your mobile and provide a screenshot of the plot to see what results are you getting.

[Walter Roberson]

% Define initial parameters
theta = 45; % Launch angle in degrees
u = 10; % Initial velocity in m/s
h = 0; % Initial height in meters
g = 9.81; % Acceleration due to gravity in m/s^2
C = 0; % Air resistance constant
N = 100; % Number of time steps
dt = 0.1; % Time step

% Calculate trajectory
[x, y] = calculate_trajectory(theta, u, h, g, C, N, dt);

% Add red marker at the starting point
p = plot(x(1), y(1), 'o', 'MarkerEdgeColor', 'red', 'MarkerFaceColor', 'red');

% Initialize the animated line
h = animatedline('Color', 'b');
addpoints(h, x(1), y(1));

for k = 2:length(x)
  % Update the position of the red marker
  p.XData = x(k);
  p.YData = y(k);
  % Add points to the animated line for the trajectory
  addpoints(h, x(k), y(k));
  drawnow limitrate
  pause(dt)
end
% Plot the bouncing projectile trajectory at the end of animation
h = animatedline('Color', 'b', 'MaximumNumPoints', N);
addpoints(h, x(k), y(k));

drawnow limitrate

function [x, y] = calculate_trajectory(theta, u, h, g, C, N, dt)
  % Initialize arrays to store x and y coordinates
  x = zeros(1, N);
  y = zeros(1, N);
  % Convert launch angle from degrees to radians
  theta = deg2rad(theta);
  % Calculate the trajectory using projectile motion equations
  for i = 1:N
      x(i) = u * cos(theta) * i * dt;
      y(i) = h + u * sin(theta) * i * dt - 0.5 * g * (i * dt)^2;
      % Check for ground collision and update y position accordingly
      if y(i) < 0
          y(i) = 0;
          break;
      end
  end
end

Answer 1

[Walter Roberson]

h = animatedline('Color', 'b');
p = plot(x(1), y(1), 'o', 'MarkerEdgeColor', 'red', 'MarkerFaceColor', 'red');

You do not have hold turned on. The call to plot() is a "high level" plotting call. "high level" plotting calls erase the current axes unless "hold" is turned on.

[Felix Oliver]

Thank you for this.

[Umar]

Hi Felix,

Walter and I answered your question. Do you have any further questions for us, please let us know. Also, please give a vote to Walter to go above and beyond. He is one of the most respected person on Mathworks platform.