Plot a moving dot inside a quiver plot that is constantly being updated

Asked by Raldi on 22 Mar 2018

Latest activity Commented on by Benjamin Kraus on 22 Mar 2018

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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?

2 Comments

KSSV on 22 Mar 2018

Most of the variables are not defined......we cannot run the code.

Raldi on 22 Mar 2018

Updated the code so it can now run

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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

1 Comment

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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