Matlab for loop animations - matlab

Question

I'm trying to animate the graph of a function but I cant get the program to graph the correct points. I want to plot points between time 0 and 10 and animate this graph. How do I get the plot as a function of time?
clear;
w = 2*pi;
t = 0:.01:10;
y = sin(w*t);
x = cos(w*t);
for j=1:10
plot(x(6*j),y(6*j),'*');
axis square;
grid on;
F(j) = getframe;
end
movie(F,1,1);
I refined the code to:
clear;
w = 2*pi;
for j=2:11
t=j-1;
y = sin(w*t);
x = cos(w*t);
plot(x(t),y(t),'*');
axis square;
grid on;
F(j) = getframe;
end
movie(F);
This should do what I'm trying however now I'm getting an "Index exceeds matrix dimension." How can I solve this?

Answer 1

Here is an example that show an animated point along a circular path, while recording an AVI movie.
To learn more about doing animations and recording movies in MATLAB, check out this guide.
%# some parameters
ERASEMODE = 'normal'; %# normal,xor
RENDERER = 'painters'; %# painters,zbuffer,opengl
%# data
t = linspace(0,2*pi,100)'; %'# adjust number of points here
D = [cos(t) -sin(t)];
%# plot circluar path
figure('DoubleBuffer','on', 'Renderer',RENDERER)
plot(D(:,1), D(:,2), 'Color','b', 'LineWidth',2)
grid on, axis([-1.5 1.5 -1.5 1.5]), axis square
xlabel('x'), ylabel('y'), title('Circle Animation')
%#set(gca, 'DrawMode','fast')
%# moving point
hPoint = line('XData',D(1,1), 'YData',D(1,2), 'EraseMode',ERASEMODE, ...
'Color','r', 'Marker','.', 'MarkerSize',30);
%# moving coordinates text
hTxtCoords = text(D(1,1), D(1,2), sprintf('(%.2f,%.2f)',D(1,:)), ...
'Color',[0.2 0.2 0.2], 'FontSize',8, 'EraseMode',ERASEMODE, ...
'HorizontalAlignment','left', 'VerticalAlignment','top');
%# angle text
hTxtAngle = text(0, 0, num2str(t(1),'%.02f'), ...
'FontSize',15, 'EraseMode',ERASEMODE, ...
'HorizontalAlignment','center', 'VerticalAlignment','middle');
%# prepare video output
useVideoWriter = ~verLessThan('matlab','7.11');
if useVideoWriter
vid = VideoWriter('vid.avi');
vidObj.Quality = 100;
vid.FrameRate = 30;
open(vid);
else
vid = avifile('vid.avi', 'fps',30, 'quality',100);
end
%# loop
for i=1:numel(t)
set(hPoint, 'XData',D(i,1), 'YData',D(i,2)) %# update point location
set(hTxtAngle, 'String',num2str(t(i),'%.02f')) %# update angle text
set(hTxtCoords, 'Position',D(i,:), ... %# update angle text
'String',sprintf('(%.3f,%.3f)',D(i,:)))
drawnow %# force refresh
if ~ishandle(hPoint), break; end %# if you close the figure
%# capture frame
if useVideoWriter
writeVideo(vid,getframe);
else
vid = addframe(vid, getframe(gcf));
end
end
%# close and save video output
if useVideoWriter
close(vid);
else
vid = close(vid);
end
%# open AVI file using system default player
winopen('vid.avi')

Answer 2

It's doing exactly what you ask it to do. You're subsampling the x and y, so it looks kind of funny.
Try
plot(x,y);
axis square;
ax = axis;
for jx = 1 : length(t),
plot(x(ix), y(ix), '*');
axis(ax); grid on;
F(ix) = getframe;
end
movie(F, 1, 1/(t(2)-t(1)))
I would also use t = 1 : 0.1 : 10; so that it plots at 10 FPS instead of 100. Slowing the frequency down to, say, w = pi; will be smoother as well.
At the end of the day, Matlab is just not a great animation solution.
Answer to refined code question
You'd need to use plot(x,y);, but this will reveal another error - your frame index does not start at 1. It will choke on F(j) in the first iteration, where j = 2. Why not just loop over t = 1 : 10?