How to remove the track of a N point matlab animation

Question

I have animated the motion of N points in 1D. The problem is that I cannot find a way to get rid of the previous plots and remove the tracks created in the motion.

function sol=Draw(N)
    %N is the number of points
    PointsPos=1:N
    for s=1:1000
        for i=1:N
            PointsPos(i)=PointsPos(i)+ rand(1,1)
            %The position of the point is increased.
        end
        for i=1:N
            %loop to draw the points after changing their positions
            hold on
            plot(PointsPos,1,'.')
        end
        pause(0.005) 
        %here I want to delete the plots of the previous frame s
    end     
    end

Mingjing Zhang · Accepted Answer · 2016-08-18T15:30:07.727

1

A general guideline for MATLAB procedural animation is:

Avoid creating or deleting graphical objects as much as possible in the animation loop.

Therefore, if you invoke plot image surf or delete in the animation loop, then most likely you are not doing it optimally.

Here, the best practice is to create the plot BEFORE the animation loop, then use set(plot_handle, 'XData', ...) to update the x coordinates of the plot points.

Also you should add a rand(1, N) to PointsPos, as opposed to adding rand(1, 1) N times.

So you code should look somewhat similar to the following:

function sol=Draw(N)
    PointsPos=1:N
    h = plot(PointsPos, ones(1, N), '.');
    for s=1:1000
        PointsPos=PointsPos+ rand(1,N)
        set(h, 'XData', PointsPos);

        pause(0.005) 
    end
end

edited Aug 18 '16 at 15:30

answered Aug 17 '16 at 20:29

Mingjing Zhang

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11

You should put the pause inside the loop, if you want to see this as animation – EBH Aug 18 '16 at 10:00
@EBH Thank you for pointing that out. I edited directly on OP's code and deleted the wrong 'end'. Now corrected. – Mingjing Zhang Aug 18 '16 at 15:32

EBH · Answer 2 · 2017-07-21T13:23:25.220

If I understood your goal, then this should do what you want:

function sol = Draw(N)
steps = 1000;
% N is the number of points
PointsPos = cumsum([1:N; rand(steps-1,N)],1);
p = scatter(PointsPos(1,:),ones(N,1),[],(1:N).');
colormap lines
for s = 2:steps
    p.XData = PointsPos(s,:);
    drawnow
end
end

Note that:

There is no need to compute anything inside the loop, use vectorization and cumsum to calculate all the positions of the points (i.e. all the PointsPos matrix) at once.
For plotting unrelated circles, scatter would be a better choice.
Instead of using pause with some arbitrary time you can use drawnow to update your plot.

How to remove the track of a N point matlab animation

2 Answers2