What is drawnow doing?

Hello everyone.

I'm working on a code where I need to alternate the display of two complex plots in a very short time. The time for creating each plot is quite high (4-5 seconds) and I want to alternate them with a lag of 0.1 seconds. Creating a new plot every 0.1 seconds is therefore completely useless.

My idea was to create the two plots in two different axes and alternate the visibility of them. The problem is that drawnow keeps taking a very long time just to show one axes instead of the other.

Is there any way I can speed up this process?

Example of code:

figure
x1 = rand(100000,1);
y1 = rand(100000,1);
x2 = rand(100000,1);
y2 = rand(100000,1);
t = tic;
delay = 0.5;
flag = 0;
while 1
    if toc(t) > delay
        if flag
            plot(x1,y1,'.')
            flag = 0;
        else
            plot(x2,y2,'.')
            flag = 1;
        end
        drawnow
    end
end

Answer 1

[Walter Roberson]

figure
x1 = rand(100000,1);
y1 = rand(100000,1);
x2 = rand(100000,1);
y2 = rand(100000,1);
p1 = plot(x1,y1,'.', 'visible', 'off');
p2 = plot(x2,y2,'.', 'visible', 'off');

t = tic;
delay = 0.5;
flag = 0;
while 1
    if toc(t) > delay
        if flag
            set(p2, 'visible', 'off');
            set(p1, 'visible', 'on');
            flag = 0;
        else
            set(p1, 'visible', 'off');
            set(p2, 'visible', 'on');
            flag = 1;
        end
        drawnow
    end
end

[Thorsten]

Second set should be set(p1, ... not p2. And there should be a

 hold on

[Alessandro Masullo]

Thank you for the answer, Walter, but this is the answer that I suggested too (not in the code, though), and it is still too slow.

If you run the following code, you can see that the actual delay between the two plots is twice the lag (in my case). If the plot is even heavier (rand(100000,4)), the time for drawing the axis reaches up to two seconds!

figure,hold on
x1 = rand(100000,2);
y1 = rand(100000,2);
x2 = rand(100000,2);
y2 = rand(100000,2);
p1 = plot(x1,y1,'.', 'visible', 'off');
p2 = plot(x2,y2,'.', 'visible', 'off');
t = tic;
delay = 0.1;
flag = 0;
while 1
    t2 = tic;
    if toc(t) > delay
        if flag
            set(p1, 'visible', 'off');
            set(p2, 'visible', 'on');
            flag = 0;
        else
            set(p1, 'visible', 'on');
            set(p2, 'visible', 'off');
            flag = 1;
        end
        drawnow
        disp(toc(t2))
    end
end

Another solution that I had in mind was using getframe and alternate the two buffered getframes instead of the real plots. Though I wouldn't like to use this solution because I would need to take care of figure resizing and this kind of stuff...

[Mike Garrity]

You're on the right track here. The basic idea is that we want to park the two plots on the graphics card, and then just tell the card which one to draw each frame. The code you've got almost does that.

Here's a version I tweaked a tiny bit:

figure
hold on
drawnow
rng default
x1 = rand(100000,2);
y1 = rand(100000,2);
x2 = rand(100000,2);
y2 = rand(100000,2);
p1 = plot(x1,y1,'.', 'visible', 'off');
p2 = plot(x2,y2,'.', 'visible', 'off');
nframes = 500;
ft = zeros(1,nframes);
t = tic;
delay = 0.1;
flag = 0;
for i=1:nframes
  t2 = tic;
  if flag
    set(p1,'Visible','off');
    set(p2,'Visible','on');
    flag = 0;
  else
    set(p1,'Visible','on');
    set(p2,'Visible','off');
    flag = 1;
  end
  drawnow
  ft(i) = toc(t2);
end
sprintf('%g FPS', 1/median(ft))

When I run this on my machine (an nVidia K600) I get 38 frames per second.

But we can do better. The problem with this is that we're changing a property on the line object. When we do this, MATLAB decides something's changed and it resends the data to the card. We don't want to do that.

But how do we change the visibility without modifying the object? The trick is to put the line object in a group and toggle the visibility on that. That would look something like this:

figure
drawnow
rng default
x1 = rand(100000,2);
y1 = rand(100000,2);
x2 = rand(100000,2);
y2 = rand(100000,2);
g1 = hggroup;
p1 = plot(x1,y1,'.','Parent',g1);
g2 = hggroup;
p2 = plot(x2,y2,'.','Parent',g2);
nframes = 500;
ft = zeros(1,nframes);
t = tic;
delay = 0.1;
flag = 0;
for i=1:nframes
  t2 = tic;
  if flag
    g1.Visible = 'off';
    g2.Visible = 'on';
    flag = 0;
  else
    g1.Visible = 'on';
    g2.Visible = 'off';
    flag = 1;
  end
  drawnow
  ft(i) = toc(t2);
end
sprintf('%g FPS', 1/median(ft))

When I run this version, I get 90 frames per second.

Does that help?

[Alessandro Masullo]

Hello Mike! Thank you a lot for your answer, I didn't get a notification about a new comment, sorry!

I knew about the memory transfer with the graphic card, but I didn't find any documentation to avoid that. I tried your code and it is incredibly faster, as you say, but still too slow for my problem.

I don't understand why, but on my machine (16 GB of ram, intel core i7 and Quadro K600) I only get 4 FPS on the slow code and 8 FPS on with the hggroup workaround.

I'm using matlab 8.5.0.197613 (R2015a), and the same code on a previous version of matlab runs incredibly faster. I read some stories about drawnow being slower on this new version of matlab, but I didn't expect such a big difference.

What is it going on?

[Mike Garrity]

It's not that drawnow is slower. What changed is that more work is being done on the graphics card. Since drawnow is where we synch up all the different threads, it is more likely to show up as a hotspot in the profiler. This usually means that the different threads are out of balance and are spending a lot of time waiting for each other.

Are you saying that when you run the code in my previous comment above, you get 8 FPS? That's very surprising, because your machine is actually very similar to the one I'm using right now. I just ran it on this machine with R2015a, and it prints out '98.9303 FPS'. If you are getting wildly different results with a similar system, could you please run the following command and add the output in a comment here?

opengl info

Thanks.

[Alessandro Masullo]

Hi Mike, this is the result of opengl info:

opengl info
                          Version: '1.1.0'
                           Vendor: 'Microsoft Corporation'
                         Renderer: 'GDI Generic'
                   MaxTextureSize: 1024
                           Visual: 'Visual 0x17c, (RGB 24 bits (8 8 8), Z depth...'
                         Software: 'true'
        SupportsGraphicsSmoothing: 0
    SupportsDepthPeelTransparency: 0
       SupportsAlignVertexCenters: 0
                       Extensions: {3x1 cell}
               MaxFrameBufferSize: 0

[Alessandro Masullo]

I think I found the answer by myself... It was the opengl software instead of hardware!

Thank you very much for your answer! (How can I accept it if it is a comment?)

[Mike Garrity]

It's really not an answer yet. We need to figure out why you are getting softwareopengl. There are a couple of reasons this might happen. Two common reasons.

1. We have a blacklist for some drivers we've seen a lot of problems with.
2. The system that reports crashes back to us looks at the call stack. If the crash was in your graphics card driver, we'll set your default to softwareopengl to prevent it happening again.

Could you start MATLAB with the -hardwareopengl switch and then do opengl info again? That's probably the easiest way to get all of the details about your graphics card. I'll see if I can find someone who can help figure out what's going on. Hopefully it'll be as simple as updating your driver to a different version.

[Alessandro Masullo]

Someone suggested me to set opengl to software many months ago, because of a bug that I was experiencing with surf.

I don't remember the exact post, but it was something like this issue: http://www.mathworks.com/matlabcentral/answers/52943-mesh-or-surf-display-problem-on-windows-7

It happened on a previous version of matlab. So far I haven't experienced the bug on the R2015a.