Live Script animation 2025a

The folowing code produces a nice on-time animation when run from the command line. Run as a Live Script, it produces an animation with a scroll bar and an export to movie button but the animation appears broken, having just a few frames and so does the mp4 if one exports it. How can I get this and similar codes to work in Live Scripts?

%% SIMPLE_PENDULUM_ANIM.m
% Standalone, command-line animation of a simple pendulum
% Emulates the same frame-stepped loop pattern as your working animation:
%   - Construct t_anim = 0:dt:tf
%   - Pre-create graphics objects
%   - In a for-loop: compute state -> set(XData/YData) -> drawnow -> pause(dt)
% Includes diagnostics to confirm that many distinct frames are produced.

%% Housekeeping
clearvars;
close all hidden;
clc;

%% Parameters
% Pendulum physical parameters
L = 1.0;                  % length in meters
g = 9.81;                 % gravity in m/s^2

% Display amplitude and motion
amp_deg    = 10;          % small angle amplitude in degrees
A          = deg2rad(amp_deg);       % radians
theta0     = A;                          % initial angle amplitude
phi        = 0;                          % phase shift

% Timing
frame_rate = 30;                           % frames per second
dt         = 1/frame_rate;                 % step between frames
T          = 2*pi*sqrt(L/g);              % period of small oscillations
nPeriods   = 4;                            % number of periods to show
t_final    = nPeriods * T;                 % total animation time
t_anim     = 0:dt:t_final;                 % time vector for animation
Nframes    = numel(t_anim);                % number of frames

% Print timing diagnostics
fprintf('Simple pendulum small-angle animation\n');
fprintf('  L = %.3f m, g = %.3f m/s^2\n', L, g);
fprintf('  Small-angle period T = %.4f s\n', T);
fprintf('  frame_rate = %d fps, dt = %.6f s\n', frame_rate, dt);
fprintf('  nPeriods = %d, t_final = %.4f s, Nframes = %d\n', nPeriods, t_final, Nframes);

%% Prescribed motion (small-angle)
% theta(t) = A * cos(omega * t + phi)
omega      = sqrt(g/L);                   % small-angle angular frequency
theta_anim = theta0 * cos(omega * t_anim + phi);

% Position of bob
x_anim =  L * sin(theta_anim);
y_anim = -L * cos(theta_anim);

% Diagnostics: max per-step changes and distinct-frame count
dx = [0, diff(x_anim)];
dy = [0, diff(y_anim)];
dmax = max(abs([dx(:), dy(:)]), [], 2).';
tol_distinct = 1e-9;                      % anything larger than this counts as a new frame
nDistinct = 1 + sum(dmax(2:end) > tol_distinct);
fprintf('  Max |Δx| per step = %.6g, Max |Δy| per step = %.6g\n', max(abs(dx)), max(abs(dy)));
fprintf('  Distinct frames (|Δ| > %.1e): %d / %d\n', tol_distinct, nDistinct, Nframes);

%% Figure and axes
fig = figure('Position',[100,100,600,600],'Name','Simple Pendulum Animation');
axis equal;
grid on;
hold on;

% Axis limits with padding
pad = 0.2;
xlim([-L - pad, L + pad]);
ylim([-L - pad, pad]);

xlabel('X (m)');
ylabel('Y (m)');
title(sprintf('Simple Pendulum — A = %.1f°; %.0f fps; %d periods', amp_deg, frame_rate, nPeriods));

%% Graphics objects (pre-create; update via set)
% Pivot
pivot_plot = plot(0, 0, 'ko', 'MarkerSize', 6, 'MarkerFaceColor', 'k');

% Rod as a line from pivot to bob
rod_plot = plot([0, x_anim(1)], [0, y_anim(1)], 'k-', 'LineWidth', 3);

% Bob as a filled circle
bob_size = 14;  % marker size
bob_plot = plot(x_anim(1), y_anim(1), 'ro', 'MarkerSize', bob_size, 'MarkerFaceColor', 'r', 'LineWidth', 1.5);

% Optional trail to visualize motion
trail_len = max(1, round(0.5*frame_rate));    % 0.5 s trail
trail_plot = plot(nan, nan, '-', 'Color', [0.2 0.6 1.0], 'LineWidth', 1);

% Time text
time_text = text(-L - 0.15, 0.05, sprintf('t = %.3f s', 0), 'FontSize', 12);

drawnow;

%% Animation loop
fprintf('Animating...\n');
for k = 1:Nframes
    % Current tip coordinates
    xk = x_anim(k);
    yk = y_anim(k);
    
    % Update rod
    set(rod_plot, 'XData', [0, xk], 'YData', [0, yk]);
    
    % Update bob
    set(bob_plot, 'XData', xk, 'YData', yk);
    
    % Update trail
    i0 = max(1, k - trail_len + 1);
    set(trail_plot, 'XData', x_anim(i0:k), 'YData', y_anim(i0:k));
    
    % Update time text
    set(time_text, 'String', sprintf('t = %.3f s', t_anim(k)));
    
    % Draw and pace
    drawnow;
    if k < Nframes
        pause(dt);
    end
    
    % Optional per-frame diagnostic printing (sparse)
    if mod(k, round(frame_rate)) == 0 || k == 1 || k == Nframes
        fprintf('  Frame %4d / %4d: (x,y) = (%.5f, %.5f), |Δ|_max = %.3g\n', ...
            k, Nframes, xk, yk, dmax(k));
    end
end

fprintf('Done.\n');

Answer 1

[Tridib]

Hi @Duncan Carlsmith,

The code actually works fine for me both in the Command Window and in Live Scripts.

However, from what I understand, the problem in Live Scripts seems to be that for-loops with "drawnow" or "pause" do not run in real time. Instead, the Live Editor only takes a few snapshots during the loop, so just a handful of frames are displayed. As a result, when using the "Export to Video" button in Live Scripts, the exported video contains only those few captured frames, making the animation appear choppy or incomplete.

A possible workaround could be to try capturing each frame manually using "getframe" within the loop and then saving them to a video file with "VideoWriter". For smoother playback in Live Scripts, it might help to store all frames first and then play them back using tools like "implay" or by adding a slider to move through the animation.

For more information, refer to these documentations:

getframe: https://www.mathworks.com/help/matlab/ref/getframe.html

VideoWriter: https://www.mathworks.com/help/matlab/ref/videowriter.html

implay: https://www.mathworks.com/help/medical-imaging/ref/medicalimage.implay.html

Hope this helps!

[Duncan Carlsmith]

Thanks Tridib. I'm aware of other methods. I like the "new" autodetection in Live Scripts of loops making animations to produce a scrollbar-able animation in the Live Script output with an export to video button, WHEN IT WORKS! It's great to let the user choose or not to let videos pile up and to manage their names etc. Sometimes this embedded video output work great but I get inconsistent behavior. In this post, I was referring to native (Macbook pro M1, v2025a) operation. I've seen this work on my laptop natively, and then the same code fail in the cloud. When running in the cloud, a video is a pain and a video embedded the LiveScript output highly preferable, but I can't get that to work consistenly either.