Why does video slow down towards the end?

Hello,

I've written a code to display circles on my video every 100th frame (9000 frames in total, 25fps). At the first frame, I draw 2 circles on the image and store the vertices of these circles. Later on, I display these circles again at each 100th frame. I've read somewhere that plotting could slow down the execution of this code, so could this be the reason why my video is playing way slower at the end compared to the beginning?

vid = VideoReader(video_name);
curr_axes = axes;
hfig = figure(1);
set(hfig,'WindowButtonDownFcn',@count_the_clicks) %Use callback to click
framecount = 0;

%Play video
while vid.hasFrame()
temp = vid.readFrame();
framecount = framecount + 1;
image(temp,'Parent',curr_axes);

%Draw 2 circles on the first videoframe, so that you can plot these circles
%again at each 100th frame (see below).
if framecount == 1
    hfig6 = copyobj(hfig,0);
    set(hfig6,'visible','off')
    figure(hfig);
    disp('Carefully select 2 objects you want to label by drawing ellipses')
    p2 = imellipse;
    vert_p2 = getVertices(p2);
    wait(p2);
    p3 = imellipse;
    vert_p3 = getVertices(p3);
    wait(p3);
    vert_p_all = horzcat(vert_p2',vert_p3')';
end
curr_axes.Visible = 'off';
pause(0.55/vid.FrameRate)

%At each 100th frame, plot the drawn circles
if  mod(framecount,100)==0
    hold on
    scatter(vert_p_all(:,1),vert_p_all(:,2),16,'oy','filled')
    p1 = imrect; %Double-click to proceed to the next 100 frames of the video
    wait(p1);
end
end

Answer 1

[Aman Vyas]

Hi,

You can try either of the following or both of the following as both would result in preventing the lag or delay in the video as it progresses.

1) You are trying to store images in the axes, since function image() is used.Try inserting a "cla('reset')" just before and see if that speeds it up. Otherwise you may be loading all of the images in there instead of replacing them.

2) Also, you have used "hold on" and haven't used "hold off" because of which all of the images are loaded to the same axes everytime and slows down as video progresses. Try using "hold off" function in the loop.

For more info on hold off function you can visit the links:

https://www.mathworks.com/help/matlab/ref/hold.html?s_tid=srchtitle

Hope it helps !

[Miguel Fernando]

Not using hold off slowed the video processing so much that Matlab couldn't finish in one night a very short video with a few frames. Thank you very much for your help!

[DGM]

It would probably be faster yet to keep the handles for the image and scatter objects and simply update them each frame instead of destroying them and recreating them each frame.

For instance, update the CData property of the image() object, and the XData,YData of the scatter() object.

The general theme is simple:

• The only objects in the figure should be those necessary. Don't accumulate unused objects.
• Change only what's necessary each frame.
• Property edits are usually faster than object creation. (I don't know of a good exception)

video_name = 'rhinos.avi'; % the file name
playbackspeed = 1; % the normalized playback speed
pauseeveryN = 50; % pause every N frames

vid = VideoReader(video_name);
curr_axes = axes;
hfig = figure(1);
framecount = 0;

% Play video
for f = 1:vid.NumFrames % there's no good reason to use a while loop
    thisframe = vid.readFrame(); % use a meaningful name
    
    % Draw 2 circles on the first videoframe
    if f == 1
        % create the image object once
        hi = image(thisframe,'Parent',curr_axes);
        
        msg = ['Carefully select 2 objects you want to label by drawing ellipses. ' ...
            'Double-click on each ellipse to finish drawing it.'];
        disp(msg)
        
        p2 = imellipse(curr_axes); %#ok<*IMELLPS>
        wait(p2); % wait until the user is done first
        vert_p2 = getVertices(p2); % save _after_ the changes are made, not before!
        
        p3 = imellipse(curr_axes);
        wait(p3);
        vert_p3 = getVertices(p3);
        
        % just concatenate them the right way around to begin with
        vert_p_all = [vert_p2; vert_p3];
        
        % we don't need these for anything
        % ... unless the whole pausing thing was a workaround for losing these
        delete([p2 p3])
    else
        % update the image object
        hi.CData = thisframe;
    end
    pause(1/(playbackspeed*vid.FrameRate))
    
    % Every Nth frame, plot the drawn circles
    % i have no idea why this is done.  it seems like this is unnecessary
    % and was probably only being done because the imellipse objects were buried.
    if  mod(f,pauseeveryN)==0
        if f == pauseeveryN
            % create the scatter during the first pause
            hold on
            hs = scatter(vert_p_all(:,1),vert_p_all(:,2),16,'oy','filled');
        else
            % and simply just change its visibility every other time
            hs.Visible = 'on';
        end
        
        % just pause and wait for user to hit enter 
        % instead of drawing unnecessary random objects as a pausing mechanism
        disp('Press Enter to Continue')
        pause
        
        % hide it on all other frames
        hs.Visible = 'off';
    end
end