Animated Plots (MATLAB Movies)

· Animations are formed by a sequence of plot statements (each with a slightly different curve). The command getframe is used to capture each plot and store it. The movie( ) command plays back the frames as a movie.

>> help getframe

GETFRAME Get movie frame.

GETFRAME returns a movie frame. The frame is a snapshot

of the current axis. GETFRAME is usually used in a FOR loop

to assemble an array of movie frames for playback using MOVIE.

For example:

for j=1:n

< plot_commands goes here >

M(j) = getframe;

end

movie(M)

· M is a special type of array, which holds all the frames. Note that like any array index, j must be an integer or the command M(j)=getframe; would produce an error.

Projectile Animation

· Copy and Paste the following code into an m-file (projectile.m)

% Script file (projectile.m) to plot an animation of a projectile trajectory

%

% Author…

% Date…

clear; close all; clc

% Get the constants we need

angd=input('What angle do you want (degrees):');

v0=input('What is the initial velocity (ft/sec):');

numpts=input('How many points to evaluate (I suggest 50):');

% Run our trajecrtory function

[x,y,t]=trajectory(angd,v0,numpts);

% display results as animation

figure

for n=1:length(x)

plot(x(n),y(n),'ro');

axis equal;

axis([0,max(x),0,max(y)+10]);

xlabel('x (ft)');

ylabel('y (ft)');

title('Projectile Trajectory');

M(n)=getframe;

end

% play as smooth movie 3 times at 10 frames per second

% note that it goes through the frames 2x initially to get ready for

% full speed play. So it will actually play 2x slower and 3x at full speed.

numtimes=3;

fps=10;

movie(M,numtimes,fps)

· Execute the command by typing >> projectile and enjoy…

Sinusoidal Animation

· Let’s plot a sinusoid with the frequency changing before our eyes. Copy the following code into an m-file (sinmovie.m).

% Script file (sinmovie.m) to plot an animation of a sin function with increasing

% frequency.

%

% Author…

% Date…

clear;close all;clc

t=linspace(0,2*pi,1000);

count=1;

figure

for freqrps=.1:.1:2*pi

y=sin(freqrps*t);

plot(t,y);

xlabel('x');

ylabel('y');

axis([0,2*pi,-1,1])

S1=sprintf('y(t)=sin(%.2f t)',freqrps);

text(2,.6,S1)

freqcps=freqrps/(2*pi);

S2=sprintf('frequency=%.2f rads/sec (%.2f cyc/sec)',freqrps,freqcps);

text(2,.4,S2)

title('Sinusoidal Function');

M(count)=getframe;

count=count+1;

end

movie(M,2,10);

· Execute the command by typing >> sinmovie and enjoy…

Saving MATLAB as . avi’s

· Let’s say you want to save your movie in a file format that can be played on just about any PC (without MATLAB installed) or you want to include your movie on a web page or in a Power Point Presentation.

· You can save your movie (usually M) as an .avi file (see below) using the command movie2avi( ).

· AVI (.avi) Audio Video Interleave – Microsoft AVI was developed by Microsoft to add Multi-Media capabilities to Windows. It is the most common Multi-Media format if for no other reason than it is included on every PC running any version of Windows. It is capable of combining both Audio and Video into a relatively high quality file.

· Here is an example using your sin function movie we created above:

movie2avi(M, 'sinusoid.avi');

· You may get a few warnings, that’s typical. It will still work. Now use ls to verify that your .avi file is in your current directly. Use pwd to see the full path. Outside of MATLAB, work you way down to this directory and double click on your new file. It should play using windows media player (independent from MATLAB).

· To avoid codec issues you may wish to save with no compression

movie2avi(M, 'sinusoid.avi', 'compression', 'none');

· Sometimes the axes will not appear correctly in the .avi file. To fix this, use a figure handle with the getframe command like so:

clear;close all;clc

t=linspace(0,2*pi,1000);

count=1;

h = figure(1);

set(h,'Position',[100 678 560 420])

for freqrps=.1:.1:2*pi

y=sin(freqrps*t);

plot(t,y);

xlabel('x');

ylabel('y');

axis([0,2*pi,-1,1])

S1=sprintf('y(t)=sin(%.2f t)',freqrps);

text(2,.6,S1)

freqcps=freqrps/(2*pi);

S2=sprintf('frequency=%.2f rads/sec (%.2f cyc/sec)',freqrps,freqcps);

text(2,.4,S2)

title('Sinusoidal Function');

M(count)=getframe(h);

count=count+1;

end

movie2avi(M, 'sinusoid.avi', 'compression', 'none');