Lagging with updating a Figure

I am having an issue when it comes to a figure I have in a GUI. I have it so that an filled object updates its position and moves around the plot based on the input from an accelerometer that is hooked up to my computer. I have it moving around in the way I want however after not very much time the figure begins to lag more and more. I havent yet found a way to fix this issue. If you have an ideas or ways to fix this your help would be much appreciated.

[Stephen23]

How are you plotting this? What code are you using? There a many ways to plot data using MATLAB, and there are also many ways to access and change that plotted data, so how do you expect us to know what you are doing if you do not give us any information at all?

There are lots of talented people here who volunteer their time to help others, but they can't read minds and they can't read your computer screen. Please help them by reading this, and following its advice:

http://de.mathworks.com/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer

In particular you can upload code using the paperclip button, and please explain exactly how you are using/calling your code, and what you expect it to do.

[Kevin]

handles.E = 1;
handles.N = 1;

while strcmp(get(handles.Start,'String'),'STOP');
        [gx,gy] = readAcc(handles.accelerometer, handles.calco);
        rawmag=sqrt(gx^2+gy^2);
        if strcmp(get(handles.DifficultyFilter,'String'),'EASY');
            GX = handles.alpha*gx;
            GY = handles.alpha*gy;
            filtmag=(1-handles.alpha)*handles.magfiltdata(end)+handles.alpha*rawmag;
        else
            GX = handles.alphaHARD*gx;
            GY = handles.alphaHARD*gy;
            filtmag=(1-handles.alphaHARD)*handles.magfiltdata(end)+handles.alphaHARD*rawmag;
        end
        axes(handles.Backdrop);
        r = .5;
        t = linspace(0,2*pi);
        handles.E = handles.E+GX/2;
        handles.N = handles.N-GY/2;
        fill(handles.E + r*cos(t),handles.N + r*sin(t),'y');
        drawnow
        guidata(hObject, handles);
end

here readAcc is a function that reads and records the accelerometer data. both handles.alpha and handles.alphaHARD are scalars in order to filter the data somewhat.

Answer 1

[Stephen23]

Although you don't give any information about the figure and axes properties (e.g. use of hold, the DrawMode or NextPlot properties), based on the information that you gave it may be due to the repeated calling of fill:

fill(handles.E + r*cos(t),handles.N + r*sin(t),'y');

which creates a new patch object on every call. This might mean that the axes are slowly filling up with patch objects, or that MATLAB is somehow trying to keep track of every patch object you have created (i.e. a history). Rather than create a new patch object on every iteration, consider instead creating the patch object before the first iteration, and then using set to update its properties. You will need to use the patch handle for this:

h = fill(...);

and read the patch properties documentation and decide which properties you wish to update on each iteration.

In my experience updating patch/line properties does not result in lagging pictures with the iteration progress, and is a conceptually neater way of programming than creating a new patch object on every iteration.

[Kevin]

I am unfamiliar with the patch concept in order to accomplish what I am trying to do. Is there anyway that you may be able to provide me with some for of syntax example in order to give me a better idea of how to go about this?

[Stephen23]

The most commonly used graphics "objects" are lines and patches. Different plotting command create these: fill creates a patch object, as the documentation states. You can access (with get and set ) their properties to change how they look. Try this in your command window:

>> h = fill([1,0.5,0,0],[0.1,0.9,1,0],'y');
>> get(h)

this displays all of the patch properties. Some of the properties are relevant to your task, in particular you should look at the ones listed under Data heading.

You can change these values simply by using the set command:

set(h,'XData',[...],'YData',[...]);

where you supply the new values in those vectors.

[Kevin]

When I go about trying to get it to work this way. I specified the fill outside of the while loop and then inside the while loop I have the set functions as

set(h,'XData',handles.E,'YData',handles.N);

When I did this my circle disappeared off of the screen and the lag starts to kick in a few seconds later.

[Kevin]

I ended up getting it to work another way where I have the fill command in the while loop like this

r = .5;
t = linspace(0,2*pi);
handles.E = handles.E+GX/2;
handles.N = handles.N-GY/2;
handles.W = fill(handles.E + r*cos(t),handles.N + r*sin(t),'y');
hold on

............ and at the end of my while loop

          pause(.01);
          delete('handles.Workplease');
          hold off

This eliminated the lag for me so I figured I would let you know. Thank you for your assistance with this. I will try and mess around with the set function in order to understand it better

Answer 2

[Image Analyst]

I think Stephen is probably right. You're using up more and more memory and it slows down. I know that, in older versions of MATLAB, when I would repeatedly call imshow() it would store all those images, just covering up the one before, using up more and more memory until it ground to a halt. I could call cla or cla('reset') before I put stuff into the axes and that seemed to fix it with no slowdown anymore. Try clearing out old clutter from your axes by calling cla and see if that helps.