Why is it so slow to draw a figure in Matlab?

Question

smarthu 2022-5-28

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评论： dpb 2022-6-5

I just started learning Matlab.

I am drawing very simple function curves, like y=kx or y=sin(fx).

Then I am using a slide bar to change the values of k or f.

In softwares like Geogebra, or program I made using VB.net, the figure will change with the changing values without any delay.

But in Matlab, though the figure does change, there is an obvious delay, it seems the drawing takes a lot of time.

In Matlab, I tried to draw using Script, Live Script, App.

While changing the k or f values, I have tried to update the figure with 'plot' or 'fplot' directly, or to update with 'set' to change the 'yData' or 'Function'.

How can I speed up the drawing in Matlab?

What's causing this slow drawing?

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dpb 2022-5-28

编辑：dpb 2022-5-28

fplot is a user-convenience function, not one built for performance. It does everything all over every time it is called; I wouldn't expect it to be very fast...easy, yes; high-performance, "not so much".

To get any speed here I think you'll need to create the graph/axes and directly recompute the ydata and replace it in the line handle or use line instead of plot

You might also try <animatedline> before jumping all the way to that level.

I also suspect it's the latency of the callbacks from the app contributing to more of the delay than the actual drawing; not sure there's much that can be done there. Or, there may be so many callbacks generated by moving the slider it overwhelms the compute power -- that's what the 'limirate' parameter on drawnow will take care of by throwing away a bunch of them. Whether there's any way to increase the granularity or reduce the number of callbacks on how much of a variation it takes to trigger a callback I don't know.

I'm guessing although I've done almost nothing with the app GUI overall (and what I have done have nothing that has anything except static updates of input parameters like file name selection) so I just don't have any hands on experience that is truly applicable, but my guess here is one is seeing the inherent overhead in MATLAB.

There was another similar topic not too long ago -- an attempt to measure time a user kept the cursor inside a graphic area on the screen -- it was easy to move the mouse sufficiently fast to completely miss the event -- some machinations of the code as far as how it was done helped considerably, but it never got to be under a fair latency time. I dunno if I could find that topic again now or not...

<Answer_864425> is a demo of the above subject that shows 65 msec average latency to the response above -- the discussion there shows some "tricks" that were done from OPs original implementation that helped the response there; it was, of course reading the cursor position of a fixed graphic but it did have to compute the inside/outside decision. What would be a response time of a graphic update in that case I wonder...how much time would the graphics take as opposed to the latency?

smarthu 2022-5-29

Yes, the line method draws much faster.

Now the figure changes as fast as I move the slider!

Thanks!

dpb 2022-5-30

Ah, so! I was certain it would help, not so sure it would be a total solution...

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Answer 1

Prateek Rai 2022-6-3

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Hi,

To my understanding, you are attempting to draw function curves with a parameter whose value depends on a slide bar.

Here is the possible workaround:

You can create Live Editor in MATLAB and divide the whole code into 2 sections:

Part 1: Where setup of the entire graph is placed
Part 2: Where actual changing of parameter and plotting of graph is placed

It will look like:

Section 1

%% Sine Wave plot
%% Time specifications:
   Fs = 8000;                   % samples per second
   dt = 1/Fs;                   % seconds per sample
   StopTime = 0.25;             % seconds
   t = (0:dt:StopTime-dt)';     % seconds 
%% F and x specifications:
   Fc = 60;
   x = 2*pi*Fc*t;

Section 2

k =

k

-7

-1010

-7

%% this represents Numeric Slider in Live Task

k = -7

y = sin(k*x);

% Plot the signal versus time:

figure;

plot(t,y);

xlabel('time (in seconds)');

title('Signal versus Time');

zoom xon;

By doing so,

If you change the 'k' value using slider, only graph and plot part will get affected and thus will update the graph fast.

On a similar note, if you want to just make a small part of code dynamic then try to keep it in a separate section in live editor to get faster execution.

You can refer to the following MathWorks Documentation page to learn more about MATLAB Live Editor.