Matlab Simulation: Point (symbol) Moving from start point to end point and back

Question

Hi,

I would like to create an animation to demonstrate LDPC coding which is based on Sum-Product Algorithm

So far I have created a graph which shows the connections between symbol nodes (left) and parity nodes (right) and would like to animate points travelling from symbol to parity nodes and back.

The figure is drawn by executing the following method:

function drawVertices(H)
hold on;
nodesCount = size(H);
parityNodesCount = nodesCount(1);
symbolNodesCount = nodesCount(2);
symbolPoints = zeros(symbolNodesCount, 2);
symbolPoints(:, 1) = 0;
for i = 0 : symbolNodesCount - 1
    ji = symbolNodesCount - i;
    scatter(0, ji)
    symbolPoints(i + 1, 2) = ji;

end;
parityPoints = zeros(parityNodesCount, 2);
parityPoints(:, 1) = 10;
for i = 0 : parityNodesCount - 1
    ji = parityNodesCount - i;
    y0 = symbolNodesCount/2 - parityNodesCount/2;
    scatter(10, y0 + ji)
    parityPoints(i + 1, 2) = y0 + ji;
end;
axis([-1 11 -1 symbolNodesCount + 2]);
axis off

%connect vertices
d = size(H);
for i = 1 : d(1)
    for j = 1 : d(2)
        if(H(i, j) == 1)
            plot([parityPoints(i, 1) symbolPoints(j, 1)], [parityPoints(i, 2) symbolPoints(j, 2)]);
        end;
    end;
end;

So what I would like to do here is to add another method which takes start point (x and y) and end point as arguments and animates a travelling circle (dot) from start to end and back along the displayed lines.

I would appreciate if anyone of you could show the solution or suggest any useful tutorial about matlab simulations.

Thank you!

Answer 1

I believe the best way to learn is by example. So I suggest you look at the demo lorenz which comes with MATLAB:

edit lorenz

For other cool animations, look for orbits.m and swinger.m demos part of Cleve Moler's book: Experiments with MATLAB

---

I show here a simple animation of a point moving along a circular path. The hold idea boils down to using EraseMode set to xor, and updating XData and YData of the point for each iteration:

%# coordinates
t = (0:.01:2*pi)';         %# 'fix SO syntax highlight
D = [cos(t) -sin(t)];

%# setup a figure and axis
hFig = figure('Backingstore','off', 'DoubleBuffer','on');
hAx = axes('Parent',hFig, 'XLim',[-1 1], 'YLim',[-1 1], ...
          'Drawmode','fast', 'NextPlot','add');
axis(hAx, 'off','square')

%# draw circular path
line(D(:,1), D(:,2), 'Color',[.3 .3 .3], 'LineWidth',1);

%# initialize point
h = line('XData',D(1,1), 'YData',D(1,2), 'EraseMode','xor',  ...
        'Color','r', 'marker','.', 'MarkerSize',50);
%# init text
hTxt = text(0, 0, num2str(t(1)), 'FontSize',12, 'EraseMode','xor');

i=0;
while true
    i = rem(i+1,numel(t)) + 1;               %# circular increment
    set(h,'XData',D(i,1), 'YData',D(i,2))    %# update X/Y data
    set(hTxt,'String',num2str(t(i)))         %# update angle text
    drawnow                                  %# force refresh
    if ~ishandle(h), return; end             %# in case you close the figure
end

For a detailed explanation of the parameters used (EraseMode, Backingstore, DoubleBuffer, ..), refer to this animation guide

Answer 2

From the documentation for comet.m

t = 0:.01:2*pi;
x = cos(2*t).*(cos(t).^2);
y = sin(2*t).*(sin(t).^2);
comet(x,y);