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

Question

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

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