How can I write a set number of frames to a file using vision.VideoFileWriter and MATLAB R2015a?

The code involves reading a video file and rewriting the same video image into a new file with new audio samples (the total audio file has already been trimmed to the correct length). 'start_time' and 'total_time' are defined as inputs to the function, in seconds.

The audio part of the file writes perfectly, but I can't get the video output to match the indexed values I've specified. The video output is the correct length but always starts from index 0. Can anyone help? The relevant parts of my code are shown below.

videoreader = vision.VideoFileReader('input.mp4');
videowriter = vision.VideoFileWriter('output.avi', 'FileFormat', 'AVI',...
    'AudioInputPort', true, 'FrameRate', videoreader.info.VideoFrameRate);
[y, fs] = audioread('input.wav');

a = round(start_time *  videoreader.info.VideoFrameRate, 0);
b = round((start_time + total_time) * videoreader.info.VideoFrameRate, 0);

nSamples = round(fs/videoreader.info.VideoFrameRate, 0);

for i = a:b; 
    videoframe = step(videoreader);
    step(videowriter, videoframe, y((nSamples*(i-a)+1):nSamples*(i-a+1)));
end

close(videoreader);
close(videowriter);

Answer 1

[Robyn Hunt]

After consulting with some contacts, we've discovered that this is an appropriate solution to the problem:

i = 1;
while ~isDone(videoreader)
  videoframe = step(videoreader);
  if i >= a && i<b;
    step(videowriter, videoframe, y((nSamples*(i-a)+1):nSamples*(i-a+1)));
  end
  i = i + 1;
end

[Walter Roberson]

I don't see how that could make a difference compared to my suggestion, unless for some data flushing reason one further step() was needed after reading frame #b.

Answer 2

[Walter Roberson]

I might be misunderstanding, but possibly

for i = 1:b; 
  videoframe = step(videoreader);
  if i >= a
    step(videowriter, videoframe, y((nSamples*(i-a)+1):nSamples*(i-a+1)));
  end
end

[Robyn Hunt]

I tried this and the video output is still composed of the first (b-a) frames instead of frames a:b. You seem to understand my question though!

[Walter Roberson]

That does not seem possible. videowriter for Computer Vision uses system objects that overwrite the buffers each time. The first frame is going to be gone by the time "i" becomes as large as "a" for the first step() of the videowriter object.

[Robyn Hunt]

My thought process was the same as yours but it doesn't work. I can put the entirety of my code up if it'll help so you can run it yourself but I can assure you it definitely writes from the beginning of the video clip regardless of the index. I can't explain it at all.