how can I move a Point in a plot (GUI) from one side to the other in a fixed time?

I have a point in a plot and I want to move it "smoothly" to the other side of the plot within a fixed amount of time. In the example this time is 3s, and I refresh the plot "every 20ms" (20ms is what I want, but it's always more, between: 30 and 50ms). My code is:

interval=3000/20; %3000/20=150

a=0;

for i=1:1:interval

      iTime=clock; 

      set(pointHandle,'Xdata',point(1).coordinatesNow(1)+(500/150),'Ydata',point(1).coordinatesNow(2)); %Only refresh Xaxis
      point(1).coordinatesNow(1)=point(1).coordinatesNow(1)+(500/150);

      pause(0.02); %pause of 20ms

      a=a+etime(clock,iTime); %total elapsed time

end
    disp(a); %show total time

So I'd like "a" to be 3s, but most of the times is between 7 and 10s, How could I get more accuracy?

Answer 1

[Robert Cumming]

edit: changed timing to an overall target for loop instead of interval time.

See the code below:

 figure;
 ax = axes;
 x = [0:100];
 h = plot ( ax, x, x*0 );
 ax.YLim = [0 100];
 targetTime = 3;
 nSteps = 100;
 intervalTime = targetTime./nSteps;
 loop = tic;
 for ii=1:nSteps
   h.YData(:) = ii;
   drawnow();
   timeTaken = toc(loop);
   % comment out this line to see the effet of the pause
   pause(max(0,(intervalTime.*ii)-timeTaken));
 end
 toc(loop)

When I run this in matlab I get the following:

 Elapsed time is 1.174296 seconds.   (pause commented out)
 Elapsed time is 3.006906 seconds.

[Óscar]

Thanks, but I don't know why, but if I run it in my pc it the overall time is between 20.3 and 20.8, the first one it's OK but not the second one. Now if your code change eachLoopTarget=0.03, then the overall time should be 3seconds but it's at least 3.5... (and 0.5s more is too much for my app) Thank you anyway.

[Robert Cumming]

There will be a small overhead of calculating the input to the pause function and for the loop itself.

You could test what that value is (around .5 from your figures above) and modify your eachLoopTarget variable accordingly.

edit I have changed my example code to target a total time rather than a explicit interval time.

Answer 2

[Image Analyst]

What happens to the time if you reduce the .02 to something less?

[Óscar]

then the elapsed time is reduced but still, it is not what i want without calibrating it. The main reason it's not because of the plot, its because of the "pause" function that it doesn't last 20ms, it last between 55 and 30ms... so its almost imposible to calibrate it.

[Image Analyst]

It's almost impossible to get super precise timing with a multi-tasking operating system. I don't know why your uses need it to be exactly 3.00000 seconds, but if they do, maybe elevating the priority of the MATLAB process to High, or even "Real time" might work, though using Real time is very risky since it won't pay attention to anything else in your computer (like clicking on other applications) until your app is done.

Alternatively, look in to the Real Time toolboxes - maybe there's something there that can help.

[Óscar]

Thank you, I don't really need it to be 3.000 seconds, it can be 2.9 or 3.1, but the things is that 3s it's only an example the user we'll have to select it, and he'll be able to choose whichever time he wants... now it's only for 1 point, but there are going to be up to ten points... so I'm just wondering which is the best choice to get a time as accurate as possible.

Thanks

Answer 3

[Robert Cumming]

You need to work out how long it takes to update the plot, then adjust your pause command to suit at runtime so that the code runs at the speed you want it to.

[Óscar]

but the problem its not the plot, it takes to update it between 48 -52 ms, the problem is with the "pause" function that doesn't stop the program for 20ms, it stops it for 30-55ms, and it iteration it's different, so it's almost imposible to adjust it.

[Robert Cumming]

Its not that pause is taking too long, pause is also trigering a screen refresh.

If you replace pause with drawnow you will see what I mean. I'm assuming it will still be too slow.

[Óscar]

so, you mean that pause function stops the program and refresh the plot? and that's why it takes so long?, then i guess that drawnow only refresh the screen? but using this one i would totally lost track of the elapsed time right?

[Robert Cumming]

You measure the time it takes to update the plot. If that is less than your target time for each step then issue a pause command which will pause for the remainder.

[Óscar]

but the pause, even if i command it to be 10ms, it'll be around 20-40ms....

[Image Analyst]

Maybe try a small for loop where it just spins it's wheel for a short time

tic
for k = 1:3000
end
toc
Elapsed time is 0.000008 seconds.

Adjust the ending value to trim it to shorter or longer times.

[Robert Cumming]

I dont know wwhy you think pause will take longer, this time it would have no plot refresh so it should pause for the requested time.

See my other answer for an example code (I didn't edit this one as I answered it with an old account that I no longer use....)