Question

I’m having a difficult time understanding the paradigm of Matlab classes vs compared to c++. I wrote code the other day, and I thought it should work. It did not… until I added

<handle 

after the classdef.

So I have two classes, landmarks and robot, both are called from within the simulation class. This is the main loop of obj.simulation.animate() and it works, until I try to plot two things at once.

DATA.path is a record of all the places a robot has been on the map, and it’s updated every time the position is updated.

When I try to plot it, by uncommenting the two marked lines below, I get this error:

??? Error using ==> set
Invalid handle object.

Error in ==> simulation>simulation.animate at 45
set(l.lm,’XData’,obj.landmarks.apparentPositions(:,1),’YData’,obj.landmarks.apparentPositions(:,2));

%INITIALIZE GLOBALS
global DATA XX
XX = [obj.robot.x ; obj.robot.y];
DATA.i=1;
DATA.path = XX;

%Setup Plots
fig=figure;
xlabel('meters'), ylabel('meters')
set(fig, 'name', 'Phil''s AWESOME 80''s Robot Simulator')
xymax = obj.landmarks.mapSize*3;
xymin = -(obj.landmarks.mapSize*3);
l.lm=scatter([0],[0],'b+');
%"UNCOMMENT ME"l.pth= plot(0,0,'k.','markersize',2,'erasemode','background'); % vehicle path
axis([xymin xymax xymin xymax]);


%Simulation Loop
for n = 1:720,
    %Calculate and Set Heading/Location
    XX = [obj.robot.x;obj.robot.y];
    store_data(XX);
    if n == 120,
        DATA.path
    end
    %Update Position
    headingChange = navigate(n); 
    obj.robot.updatePosition(headingChange); 
    obj.landmarks.updatePerspective(obj.robot.heading, obj.robot.x, obj.robot.y);

    %Animate
    %"UNCOMMENT ME" set(l.pth, 'xdata', DATA.path(1,1:DATA.i), 'ydata', DATA.path(2,1:DATA.i));
    set(l.lm,'XData',obj.landmarks.apparentPositions(:,1),'YData',obj.landmarks.apparentPositions(:,2));
    rectangle('Position',[-2,-2,4,4]);
    drawnow

This is the classdef for landmarks

classdef landmarks <handle
properties
    fixedPositions;  %# positions in a fixed coordinate system. [ x, y ]
    mapSize;  %Map Size.  Value is side of square
    x;
    y;
    heading;
    headingChange;
end
properties (Dependent)
    apparentPositions
end
methods
    function obj = landmarks(mapSize, numberOfTrees)
        obj.mapSize = mapSize;
        obj.fixedPositions = obj.mapSize * rand([numberOfTrees, 2]) .* sign(rand([numberOfTrees, 2]) - 0.5);
    end
    function apparent = get.apparentPositions(obj)
        currentPosition = [obj.x ; obj.y];
        apparent = bsxfun(@minus,(obj.fixedPositions)',currentPosition)';
        apparent = ([cosd(obj.heading)  -sind(obj.heading) ; sind(obj.heading)  cosd(obj.heading)] * (apparent)')';
    end
    function updatePerspective(obj,tempHeading,tempX,tempY)
        obj.heading = tempHeading;
        obj.x = tempX;
        obj.y = tempY;
    end
end
end

To me, this is how I understand things. I created a figure l.lm that has about 100 xy points. I can rotate this figure by using

set(l.lm,'XData',obj.landmarks.apparentPositions(:,1),'YData',obj.landmarks.apparentPositions(:,2));

When I do that, things work. When I try to plot a second group of XY points, stored in DATA.path, it craps out and I can’t figure out why.

I need to plot the robots path, stored in DATA.path, AND the landmarks positions. Ideas on how to do that?

Jonas:
I’m not saying you’re wrong, because I don’t know the answer, but I have code from another application that plots this way without calling axes(‘NextPlot’,’add’);

if dtsum==0 & ~isempty(z) % plots related to observations
    set(h.xf, 'xdata', XX(4:2:end), 'ydata', XX(5:2:end))
    plines= make_laser_lines (z,XX(1:3));
    set(h.obs, 'xdata', plines(1,:), 'ydata', plines(2,:))
    pfcov= make_feature_covariance_ellipses(XX,PX);
    set(h.fcov, 'xdata', pfcov(1,:), 'ydata', pfcov(2,:)) 
end
drawnow

The above works on the other code, but not mine. I’ll try implementing your suggestion and let you know.

Answer 1

When you call plot multiple times on the same figure, the previous plot is by default erased, and the handle to the previous plot points to nothing. Thus the error.

To fix this, you need to set the NextPlot property of the axes to add. You can do this by calling hold on (that’s what you’d do if you were plotting from command line), or you can write

fig=figure;
%# create a set of axes where additional plots will be added on top of each other 
%# without erasing
axes('NextPlot','add');

If you want, you can store the axes handle as well, and use plot(ah,x,y,...) to make sure that you plot into the right set of axes and not somewhere strange if you happen to click on a different figure window between the time the figure is opened and the plot command is issued.