The code you're showing is obviously meant to be in a loop which you haven't showned. That makes it harder to pinpoint the cause of the problem.
A red flag is this line:
M(:,:,count)=getframe(fig);
getframe always return a scalar structure so it's unclear why it's being put into a 3D array. At best, dimensions 1 and 2 of the existing M are singleton, in which case the above is the same as
M(1, 1, count) = getframe(fig);
If they're not singleton, then the scalar structure will be replicated as many times as necessary to fill these dimensions. A complete waste. It looks like the code was written expecting that getframe returns a 2D image instead of the structure it returns.
The error tells you that one of the cdata field of M is not the same size as the others. So look through that structure array, see which frame has a different size, then find out why. Since we don't see how you initialise M, it's possible that you're using a M from a previous run that had a different size.
Inferring from the code you show, it looks like you never preallocate M, just let it grow to 100, then clear it. That's very inefficient and error prone. Your loop should be more something like:
%naming the variable frames instead of M since the name M doesn't say anything about the variable content
frames = struct('cdata', cell(1, 100), 'colormap', cell(1, 100)); %preallocate structure with 100 elements
count = 1;
while ... %whatever the loop code is
%...
%code to read the image and display it. Unchanged
frames(count) = getframe(fig); %acquire frame
if count == 100
vw = VideoWriter(sprintf('filename%d.avi', videoId)); %taking a guess that you intend to modify the filename each time you write a video
open(vw);
writeVideo(vw, frames);
videoId = videoId + 1;
end
count = mod(count, 100) + 1;
end
