How to use composite objects using spmd

I am trying to implement video-on-demand using the spmd parallel computing function. I have a saved video encoded at 3 different qualities and segmented into 3second segments, and I am trying to save video frames to the video buffer (modelled as a structure) while simultaneously playing from the buffer at a constant rate of 24 frames per second. The quality of each segment must be selected based on the buffer level. However, I am getting the error "Error detected on worker 1. Subscripted assignment between dissimilar structures" from running the following code:

mov = struct; % video buffer
bufsize = length(mov); %buffer level
spmd
    switch labindex
        case 1
            segnum = 0; %segment number
            i=1;
            c=1;
            br = 20; %assumed channel bandwidth in Mbps
            
            %Check last segment number and get segment names from file
            res = ["low" "mid" "sample"]; %available resolutions
            resnum = 1;
            filename = sprintf('%sdash.m3u8',res(resnum));
            s = importdata(filename);
            lastseg = s{end-1,1};
            [lastsegnum,n,err] = sscanf(lastseg,(res(resnum)+'dash%d.ts'));
                        
            %Get frame size for highest quality
            vidobj = VideoReader("sampledash1.ts");
            v = read(vidobj,1);
            framesize = size(v);
            
            %Get next segment with appropriate quality
            while segnum<=lastsegnum
                if bufsize<72 %24fps*3sec = 72 frames in one segment
                    resnum = 1;
                elseif bufsize>=72 && br<216
                    resnum = 2;
                elseif bufsize>=216
                    resnum = 3;
                end
                segment = sprintf('%sdash%d.ts',res(resnum),segnum);
                vidobj = VideoReader(segment);
                f = dir(segment);
                fsz = f.bytes; %size of segment
                x = rand;
                delay = ((fsz*8)/(br*1e6)) + x; %network delay
                pause(delay);
                while hasFrame(vidobj)
                    mov(c) = im2frame(readFrame(vidobj));
                    mov(c).cdata  = imresize(mov(c).cdata,[framesize(1) framesize(2)]);
                    labSend(mov(c).cdata,labindex+1,11);
                    bufsize = bufsize+1;
                    c = c+1;
                end   
                labSend(segnum,labindex+1,12);
                segnum=segnum+1;
            end

        case 2
            depvid = vision.DeployableVideoPlayer('Size','Full-screen');
            i=1;
            while ~isempty(mov) %start playing the video only after the first two segments have been downloaded
                frame = labReceive(labindex-1,11);
                segnum = labReceive(labindex-1,12);
                vid(i).cdata = frame;
                i=i+1;
                if segnum>1
                    step(depvid,vid(1).cdata)
                    pause(1/24)
                    vid(1) = [];
                    i = i-1;
                end
            end
    end
end

I am a bit confused about why I am getting this error. I think it may be due to the Composite objects created by the function, but I'm not sure how.

Answer 1

[Raymond Norris]

It's not related to your parallel code, the issue is the initialization of mov. This can be reproduced by the following

function nagi

mov = struct();
%mov.cdata = [];
%mov.colormap = [];

M.cdata = ones(240,320,3,'uint8');
M.colormap = [];
mov(1) = M;

So it's getting tripped up here

mov(c) = im2frame(readFrame(vidobj));

If you initialize mov as I've commented it out, you'll be fine.

Side note: I'm assuming at some point, labindex 1 won't have anything else to send labindex 2, correct? If so, you're going to run into the issue that labindex 2 is continuing to wait to receive from labindex 1. Keep in mind, for labindex 2, mov is never empty. For all intents and purposes, it only sees

mov = struct;

(Or however you end up initializing it). Therefore, at somepoint, MATLAB will throw

A communication mismatch error was encountered:
The other lab became idle during labReceive.

"The other lab" in this case is labindex 1.

[Hala Nagi]

Oh, that worked to get rid of the error. Thank you!

I am still unable to see the video playing, though. Any figures that I try to open from inside the spmd function don't open and the code keeps running endlessly. Is there something I'm missing?

[Raymond Norris]

You mentioned in a comment

%start playing the video ...

but I don't see where that was actually happening, is that the call to step? Keep in mind that technically the graphics can display (and you could even print the figures to a file), but you can't see the graphics in a spmd or parfor block. Case in point, try

parfor idx = 1:2, plot(rand(10)); end
spmd, plot(rand(10)); end

The reason is because the workers are "headless" -- they don't show their graphics. What you can use is a parallel.pool.DataQueue. Here's a simple toy example

Q = parallel.pool.DataQueue;
afterEach(Q,@updatePlot);

spmd
    for idx = 1:50
        if labindex==2
            % Worker 2 is the workhorse, responsible for sending data back
            % to the client, which in turn plots the result
            
            % "Calculate" some data
            r = round(rand*100)+5;
            
            % Send it to the client
            send(Q,r)
        end
    end
end

function updatePlot(r)

% Plot the results
surfc(1:r,1:r,rand(r))
axis tight, shg

end

[Hala Nagi]

That worked! Thank you so so much!!