How to code an inner loop to get only non-empty movie frames in Matlab?

In my plotting code, I'm using a for loop

frame(S) = struct('cdata',[],'colormap',[]);    % pre-allocate frames structure

for i = 1 : round(N/S) : N

some plotting code..

axis equal 

drawnow;

frame(i) = getframe();

end

and then using VideoWriter:

video = VideoWriter('My_movie.avi', 'Uncompressed AVI');

video.FrameRate = 60;

open(video)

writeVideo(video, frame);

hold off 

close(video);

But I'm getting the error

The 'cdata' field of FRAME must not be empty.

I know what the issue is but am not sure how to resolve it.

The values for i are 1,5,9,13...

which means that frames 2,3,4,6,7,8,10,11,12, etc would be empty.

I think I need an inner loop, just before I call the getframe() function but am not sure how to do so properly and perhaps iterate over the index.

Currently, I have tried coding this inner loop:

for j = 1:S

frame(j) = getframe();

end

However, the simulation is now running extremely slow because of this inner loop.

Solution

The way you code your loop, you are not assigning data to each frame. You pre-allocate S frames (frame numbers 1 through S), but then assign data to S frames with indices 1 through N (N>S). This makes no sense to me.

But it seems that you use i also to draw your animation, where you do want to skip "frames".

The way to do so properly is like this:

frame(S) = struct('cdata',[],'colormap',[]);    % pre-allocate frames structure
for j = 1:S
   i = j * round(N/S);
   % some plotting code..
   axis equal 
   drawnow;
   frame(j) = getframe();
end

Here, each frame in the range 1:S is assigned data, but i still skips values like it does in your code.