Batch Processing Videos (Matlab) - Issue - matlab

Question

I've wrote some code that imports a video and takes some DCT coefficients form a specific region of the image frame by frame, it then output a binary file (containing the coefficients) using a separate function. This works fine when doing it individually but I've tried to implement a batch version of the code as I'm working with over 200 videos files.
However their is a problem, when it goes through the loop it output the wrong file each time. If the number of frames in the next iteration has less frames it writes to the previous frames. I was told that i need to initialise one of my for loops before it runs. However I'm sure how to do this, i think the problem lies in the 2nd for loop but I'm not sure.
Any suggestion would be greatly appreciated!
files = dir('Videos/*.mov');
for m = 1:numel(files);
readerobj = mmreader(files(m).name);
vidFrames = read(readerobj);
numFrames = get(readerobj, 'numberOfFrames');
% Create a MATLAB movie struct from the video frames.
for k = 1 : numFrames
mov(k).cdata = vidFrames(:,:,:,k);
mov(k).colormap = [];
end
firstFrame = mov(1).cdata;
rect = [172,225,271,143;];
numFrames = length(mov);
dctCoeff = zeros((10*10),numFrames);
for i = 1 : numFrames
frameImage = imcrop(mov(i).cdata, rect);
frameImage = rgb2gray(frameImage);
dctImage = dct2(frameImage);
dctImage = dctImage(1:10,1:10);
dctCoeff(:,i) = reshape(dctImage,1,(10*10));
end
sRate = (1/29.9701)*1e7;
[status, error] = htk_write_mfc(files(m).name, size(dctCoeff,2),sRate,4*size(dctCoeff,1),9,dctCoeff);
status
error
end

Answer 1

Just remove the
numFrames = length(mov);
and it should work fine!
I was told that i need to initialise one of my for loops before it runs
Jup! You're not initializing mov.
I would also recommend you to put the code for one file into a function which you call from your first loop. That is better code!

Answer 2

Try setting mov to [] for each movie that you load. It's going to keep frames from the previous mov and extend that array to match your longest video. Before your loop for k = 1 : ... try writing a line that says mov = [];.

Answer 3

I am not entirely clear on why you are creating the movie struct. You can use the read() function to read specific frames in the video file. You can try the code below:
files = dir('Videos/*.mov');
numDctCoeffs = 100;
for m = 1:numel(files)
readerObj = mmreader(files(m).name);
numFrames = readerObj.NumberOfFrames;
rect = [172,225,271,143];
sRate = (1/29.9701)*1e7;
dctCoeff = zeros(numDctCoeffs, numFrames);
for cnt = 1:numFrames
frameImage = imcrop(read(readerObj, cnt), rect);
frameImage = rgb2gray(frameImage);
dctImage = dct2(frameImage);
dctImage = dctImage(1:10,1:10);
dctCoeff(:, cnt) = dctImage(:);
[status, error] = htk_write_mfc(files(m).name, size(dctCoeff,2),sRate,4*size(dctCoeff,1),9,dctCoeff);
end
end
Also mmreader has been replaced with VideoReader in newer versions of MATLAB. The syntax is the same just a name change.
Hope this helps.
Dinesh