How to make previous inputs progressively fade out in a Matlab plot when I add new inputs

Question

Let's say I have this very simple loop

for i=1:10
    [xO, yO, xA, yA, xB, yB, xC, yC] = DoSomething(i);
    line([xO,xA,xB,xC],[yO,yA,yB,yC]);
    pause(0.1);
end

The coordinates that I am plotting correspond to the joints of a multibody system, and I am simulating their positions over time (please see a sample of the plot here):

Since some of the links move in a periodic way, it gets confusing to keep track visually of the movement. For this reason, now comes the question: how can I plot the lines in a way that, when a new line is plotted, the previous lines are faded progressively? In other words, so that I have a gradient from the most recently plotted data (most opaque) to the oldest data (increasingly transparent until it completely fades out).

This way when a new line is drawn in the same position as very old data, I will notice that it is a new one.

Answer 1

You can do this by modifying the 4th Color attribute of past lines.

Here's a demo resulting gif, where I faded out 10% of the transparency each frame, so only the most recent 10 lines are visible.

Here is the code, see my comments for details:

% Set up some demo values for plotting around a circle
a = 0:0.1:2*pi; n = numel(a);
[x,y] = pol2cart( a, ones(1,n) );

% Initialise the figure, set up axes etc
f = figure(1); clf; xlim([-1,1]); ylim([-1,1]);
% Array of graphics objects to store the lines. Could use a cell array.
lines = gobjects( 1, n );
% "Buffer" size, number of historic lines to keep, and governs the 
% corresponding fade increments.
nFade = 10;

% Main plotting loop
for ii = 1:n
    % Plot the line
    lines(ii) = line( [0,x(ii)], [0,y(ii)] );
    % Loop over past lines.
    % Note that we only need to go back as far as ii-nFade, earlier lines
    % will already by transparent with this method!
    for ip = max(1,ii-nFade):ii
        % Set the 4th Color attribute value (the alpha) as a percentage
        % from the current index. Could do this various ways.
        lines(ip).Color(4) = max( 0, 1 - (ii-ip)/nFade );
    end
    % Delay for animation
    pause(0.1);
end

You may want to do some plot/memory management if you've got many lines. You can delete transparent lines by adding something like

if lines(ii).Color(4) < 0.01
    delete(lines(ii));
end

Within the loop. This way your figure won't have loads of transparent remnants.

---

Notes:

• I generated the actual gif using imwrite in case that's of interest too.
• Apparently the 4th Color value 'feature' has been depreciated in R2018b (not sure it was ever officially documented).

Got enough upvotes to motivate making a slightly more fun demo...