Asked by Raldi
on 22 Mar 2018

I have the following code that will create a quiver plot (imagine a team of players in a football game) and I would like to update the position of the ball (a red dot).

x = rand(10);

y = rand(10);

direction = 3*rand(10);

u = sin(direction);

v = cos(direction);

H = quiver(x, y, u, v, 'filled', 'Marker', 'o', 'LineWidth', 1.8, 'AutoScaleFactor', .1);

for c = 0:100

% make a step

x = x + 5 * sin(direction);

y = y + 5 * cos(direction);

Xball = 10*rand();

Yball = 10*rand();

u = sin(direction);

v = cos(direction);

pause(0.5);

set(H, 'XData', x, 'YData', y, 'udata', u,'vdata', v, 'MarkerFaceColor', 'b');

%scatter(H, Xball, Yball, 12, 'm', 'filled'); % will complain about the axes

hold on;

scatter(Xball, Yball, 12, 'm', 'filled'); % will work but will keep showing all the balls

hold off;

drawnow;

end

The quiver plot will update correctly but I cannot add the ball, and make it change position on each iteration. What is the correct way of doing it?

Answer by Benjamin Kraus
on 22 Mar 2018

Accepted Answer

Regarding this line of code:

scatter(H, Xball, Yball, 12, 'm', 'filled'); % will work but will keep showing all the balls

Each call to scatter will create a new scatter object. What you want to do is to create a single scatter object and then update the data in that single object. Something closer to this:

s = scatter(H, Xball, Yball, 12, 'm', 'filled'); % Create onces

for c = 0:game

s.XData = Xball; % Update the data of the existing object in every loop

s.YData = Yball;

end

It also looks you are trying to use H as both an axes and a quiver object. The scatter command can take an axes object as a first input, but an axes object does not have an XData or YData property.

I cannot run your code without more of the variables defined, but I attempted to update it based on what I think you are trying to do:

H = quiver(NaN, NaN, NaN, NaN) % Replace with however you are creating your quiver plot.

AX = H.Parent; % Get the axes that is the parent of the quiver plot.

hold(AX,'on')

S = scatter(NaN, NaN, 12, 'm', 'filled'); % Create a scatter plot.

hold(Ax,'off')

for c = 0:game

% make a step

x(3:end) = x(3:end) + speed * sin(direction);

y(3:end) = y(3:end) + speed * cos(direction);

% packet transmission

Xball = rand();

Yball = rand();

% update u v components

u = sin(direction);

v = cos(direction);

% update graph

pause(0.5); % Note: Pause will also call |drawnow|

set(H, 'XData', x(3:end), 'YData', y(3:end), 'udata', u,'vdata', v, 'MarkerFaceColor', 'b');

set(S, 'XData', Xball, 'YData', Yball);

drawnow;

end

Benjamin Kraus
on 22 Mar 2018

New version of my code based on your updated code:

x = rand(1,10);

y = rand(1,10);

direction = 3*rand(1,10);

u = sin(direction);

v = cos(direction);

H = quiver(x, y, u, v, 'filled', 'Marker', 'o', 'LineWidth', 1.8, 'AutoScaleFactor', .1);

AX = H.Parent; % Get the axes that is the parent of the quiver plot.

hold(AX,'on')

S = scatter(NaN, NaN, 12, 'm', 'filled'); % Create a scatter plot.

hold(AX,'off')

for c = 0:100

% make a step

x = x + 5 * sin(direction);

y = y + 5 * cos(direction);

Xball = 10*rand();

Yball = 10*rand();

u = sin(direction);

v = cos(direction);

% update graph

pause(0.5); % Note: Pause will also call |drawnow|

set(H, 'XData', x(3:end), 'YData', y(3:end), 'udata', u(3:end),'vdata', v(3:end), 'MarkerFaceColor', 'b');

set(S, 'XData', Xball, 'YData', Yball);

drawnow;

end

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## 2 Comments

## KSSV (view profile)

Direct link to this comment:https://ch.mathworks.com/matlabcentral/answers/389959-plot-a-moving-dot-inside-a-quiver-plot-that-is-constantly-being-updated#comment_548272

## Raldi (view profile)

Direct link to this comment:https://ch.mathworks.com/matlabcentral/answers/389959-plot-a-moving-dot-inside-a-quiver-plot-that-is-constantly-being-updated#comment_548343

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