Animation in Matlab

For Summer 1997 Envision-It! Workshop

One of the uses for Matlab is to make a series of calculations and plots.  For example, in the
concurrent/countercurrent flow model, we wanted to calculate the amount of material after each time iteration and plot this on the screen.  To do this, we had the program difflow.m

% DIFFLOW.M   Concurrent Flow model
% This script models the flow of material from a "source" stream to a
% "sink" stream which are separated by a semi-permeable membrane.
% In the concurrent flow case, these two streams are traveling in
% the same direction, and in the countercurrent case, they
% are traveling in opposite direction.
%
% Tom Huber, July 1997
%

ncells = 25;        % Number of Cells in the Array
NTimes = 50;        % Number of Time steps to perform

MSrce = zeros(ncells,1);  % Initialize the Array to have zero concentration
MSink = MSrce;             % Set up second array
VMax = 100;                % Maximum Concentration
VSrce = 1;                 % Volume Flow rate of Source Stream
VSink = 1;                 % Volume Flow rate of Sink Stream
k = .1;                    % Diffusion Constant

MSrce(1) = VMax;           % Set initial cell for source stream

for i=1:NTimes  % Perform a total of NTimes time steps
dm = k*(MSrce/VSrce - MSink/VSink);
MSrce = MSrce - dm;
MSink = MSink + dm;
MSrce = [VMax; MSrce(1:ncells-1)];
MSink = [0 ; MSink(1:ncells-1)];  % Concurrent flow case
clf
plot (MSrce,'r-')
hold on
plot (MSink,'g-')
legend('Source Stream','Sink Stream')
xlabel('Location')
ylabel('Mass Content')
title(['Time Step ' num2str(i)])
drawnow
end  % for i=1...

The following code performs the same calculation, but updates the graphs much faster than the original version.

%<pre>
% DIFFANIM.M   Concurrent Flow model
% This script models the flow of material from a "source" stream to a
% "sink" stream which are separated by a semi-permeable membrane.
% In the concurrent flow case, these two streams are traveling in
% the same direction, and in the countercurrent case, they
% are traveling in opposite direction.
%
% Tom Huber, July 1997
%
% Demonstrates how to do Animation in Matlab

ncells = 25;        % Number of Cells in the Array
NTimes = 50;        % Number of Time steps to perform

MSrce = zeros(ncells,1);  % Initialize the Array to have zero concentration
MSink = MSrce;             % Set up second array
VMax = 100;                % Maximum Concentration
VSrce = 1;                 % Volume Flow rate of Source Stream
VSink = 1;                 % Volume Flow rate of Sink Stream
k = .1;                    % Diffusion Constant

MSrce(1) = VMax;           % Set initial cell for source stream

clf
HandleSrce = plot (MSrce,'r-');  % Keep the handle for this plot
hold on
HandleSink = plot (MSink,'g-');
HandleTitle = title('Time Step 0');  % Keep the handle for the title
legend('Source Stream','Sink Stream')
xlabel('Location')
ylabel('Mass Content')
set (HandleSrce,'Erase','xor')  % Set these so it updates
set (HandleSink,'Erase','xor')
set (HandleTitle,'Erase','xor')

for i=1:NTimes  % Perform a total of NTimes time steps
dm = k*(MSrce/VSrce - MSink/VSink);
MSrce = MSrce - dm;
MSink = MSink + dm;
MSrce = [VMax; MSrce(1:ncells-1)];
%  MSink = [0 ; MSink(1:ncells-1)];  % Concurrent flow case
MSink = [MSink(2:ncells) ; 0];  % Countercurrent flow case
set(HandleSrce,'YData',MSrce)   % Change just the Y Data for plot
set(HandleSink,'YData',MSink)
titlestring = ['Time Step ' num2str(i)]
set(HandleTitle,'String',titlestring)  % Change just the title
drawnow
end  % for i=1...

In Matlab, all graphics objects have a "Handle" which uniquely identifies the object.  The statement
HandleSrce = plot (MSrce,'r-');
not only draws the plot, but stores the handle for this graphics object in the variable HandleSrce. At a later point in the script file, we can use this handle to modify other properties of this graph.  To allow us to update just the Y data on the graph, we need to execute the command
set (HandleSrce,'Erase','xor')
which instructs Matlab how to erase the line.  Now the real trick in animation comes with the
statement
set(HandleSrce,'YData',MSrce)
This changes just the Y values for the graph with handle HandleSrce.  Using this method, the
program just redraws the line without redrawing the coordinate system, title, legend, etc.  This is why this method is so much faster than doing a plot each time.

Electronic Copy: http://physics.gac.edu/~huber/envision/matgui/animate.html
Created: 7-JUL-97 by Tom Huber, Physics Department, Gustavus Adolphus College.