Create a video from a 4D array

Hello, I have a problem in this code.

function [] = writeBufferToFinalVideo(buffer)
video = VideoWriter('example','MPEG-4'); 
video.FrameRate = 25; 
open(video)

for i = 1:size(buffer,4)    
    img = buffer(i); 
    img(img>1) = 1;                           
    img(img<0) = 0; 
    writeVideo(video,img); 
end
close(video);
end

My buffer is a 4-dimensional array that has frames inside. It is composed of: (height, width, numChannels, framesPerSymbol). The latter is the number of frames that are in a symbol, that is, the size of my buffer.

In my opinion, I think the code is correct, but I always get an empty video without the frames in my buffer.

[Ameer Hamza]

What is the class if buffer?

class(buffer)

[Alber]

I made the buffer with an array buffer = zeros(height, width, numChannels, framesPerSymbol) and with other function I put the frames of the original video input the buffer.

Answer 1

[Ameer Hamza]

If buffer is a 4D-array, then you will need to index it like this

img = buffer(:,:,:,i);

[Alber]

Still not working, I'm running it here in case it's of any use to you, maybe the loops are wrong

while hasFrame(videoObject)
    frame = double(readFrame(videoObject));
    % We update the framesInBuffer counter
    framesInBuffer = framesInBuffer + 1;
    frameBuffer = double(shiftBuffer(frameBuffer,frame,framesInBuffer));
    
    if framesInBuffer > framesPerSymbol
        framesInBuffer = framesPerSymbol;
        bypassEncoding = false;
    else
        bypassEncoding = true;
    end
    if ~bypassEncoding
        if canWeEncode(frameBuffer,alpha,threshold)     % True condition
            encodedBuffer = steganographicEncoding(frameBuffer,width,height,codeRows,codeCols);
            writeBufferToFinalVideo(encodedBuffer);
        else                                            % False condition
            writeFrameToFinalVideo(squeeze(frameBuffer(:,:,:,1)));
        end
    end
end

[Ameer Hamza]

Can you show the output of

size(encodedBuffer)

Also can you see what is the size of buffer(:,:,:,i) inside the function by using a breakpoint.

[Alber]

[Alber]

if I put a break after the line writeBufferToFinalVideo(encodedBuffer), I can generate a video but, only white frames

[Ameer Hamza]

Ok. It is a bit clear now. Can you show the output of

class(encodedBuffer)

and also show the output of

max(encodedBuffer, [], 'all');

[Alber]

the output of

class(encodedBuffer)

is 'single' and the output of

max(encodedBuffer, [], 'all')

is 236

[Ameer Hamza]

Alberto, try your code after changing the line to

frameBuffer = double(shiftBuffer(frameBuffer,frame,framesInBuffer))/255;

[Alber]

Yes!!! This is the solution, Thank you!!

[Ameer Hamza]

I am glad to be of help.

Reason: if the datatype of the matrix is double, MATLAB considers the RGB color values between 0 and 1. Whereas, for your matrix, the values were given in an 8-bit format from 0 to 255. MATLAB takes all value over one as white (that is why you saw white video frames) Dividing by 255, normalizes the value between 0 and 1. I guess, the following will also work for your original code

writeBufferToFinalVideo(uint8(encodedBuffer));

Answer 2

[Image Analyst]

Make sure buffer is uint8.

buffer = zeros(height, width, numChannels, framesPerSymbol, 'uint8')