How can I merge video files without crashing my memory?

I am trying to create a program that randomly chooses two seconds from a variable number of relatively short movie clips (short videos created for the purpose of putting together a '2 seconds a day' type movie). Everything is working fine, but I crash my systems memory after 8 videos or so, and I'm trying to put together around 100. I'm just learning how to program and my main problem revolves around how to free up memory. Below is my code.

if true
  % code
files = dir( 'C:/Users/psychuser/Desktop/Pictures and Misc/Phone/*.mov' );   %finds all movies in a certain folder
n = 1;
for i= 1 : size( files , 1 );
  originalfilename = sprintf( 'C:/Users/psychuser/Desktop/Pictures and Misc/Phone/%s' , files(i,1).name );%creates variable file name
  obj = VideoReader( originalfilename );  %turns video into variable 'obj'
  video = obj.read;                   %read video and properties into matlab file
  x = round( obj.FrameRate );                  %from that properties file turn framerate of video into variable
  y = obj.NumberOfFrames;             %from that properties file turn total number of frames into variable
  z = ( y - ( x * 2 ) );                      %z subtracts the frame rate from the total number of frames so that the new movie doesn't exceed the total number of frames
  if ( x * 2 ) <= y                          %if total number of seconds is greater than 2 seconds choose random two seconds
      j = ( n : (n + ( x * 2 )));             %j is a counter that knows where the last video ended and how big the current video is
      startframe = randi ( z );              %choose random start point between first frame and last possible frame
      choicerng = ( startframe : ( startframe + ( x * 2 ) ) );
      video1 ( : , : , : , j ) = video( : , : , : , choicerng ); %merge new video to old 'combined' video
  else                                        % else choose the whole video
      j = ( n : ( ( y - 1 ) + n ) );          %Y is rounding up for these videos creating a problem with a less than two second clip, to resolve this i subtracted 1 frame, needs a better fix
      choicerng = ( 1 : y );                  % choose whole video
      video1 ( : , : , : , j ) = video( : , : , : , choicerng );  %merge new video to old 'combined' video
  end
  n = max(j) + 1;                             % last frame in the combined video
  clear x y z originalfilename obj video choicerng startframe     %clear variables
end    %everything below here writes out the video to a new file called twoseconds
newfilename = 'C:/Users/psychuser/Desktop/Pictures and Misc/ChristiPhone/TwoSecond/twoseconds.avi';
writeobj = VideoWriter( newfilename );
open( writeobj );
writeVideo( writeobj , video1 );
close( writeobj );
clear all
end

Answer 1

[Image Analyst]

Try not reading the whole video into memory. Use read() to read just the frames you want in.

[Chris Foster]

Thank you for your help. This specific advice (e.g., read() ) didn't resolve the issue because MATLAB currently says this command will be not included in future releases. That said, the advice in general was helpful in making me think about the problem in a different way.

[Image Analyst]

Wow, you're right. It's new but it didn't last long before being deprecated. It looks like they say use readFrame instead of read. That should be a simple fix (just renaming the method) and should resolve the issue, I would think.

It's always a dilemma when to change my demos. If I change read() to readFrame() then people with old versions of MATLAB won't be able to run my demo.

In some ways readFrame() looks worse than read() because with read() you could specify what frame number to extract whereas it doesn't look like readFrame has tha option.

[Walter Roberson]

Yes, you have to set the object properties to indicate which frame you want to read next. A clumsy system.

Answer 2

[Walter Roberson]

See http://www.mathworks.com/help/matlab/import_export/read-video-files.html for how to use the readFrame method of VideoReader; it has an example of setting the current time and reading until a target time. You would not need to calculate the number of frames.

Also instead of storing the frames in memory and writing everything out at the same time, you can use VideoWriter to write individual frames.

You could use a procedure such as

open your output with videowriter
for 1 to number of videos to subset
  pick a random video
  open a reader onto the video
  ask how long it is in seconds
  if it is less than 2 seconds set lower time bound to 0 and upper time bound to actual length
  otherwise pick a random floating point number between 0 and length minus 2 seconds, set lower time bound to that value and upper time bound to 2 seconds later
  set reader start time to lower time bound
  loop until the current reader time exceeds upper time bound, reading the next frame from the reader and writing it immediately to the writer
  close the reader
end of for loop
close the writer

[Chris Foster]

I did end up using seconds instead of frame rate and similar to "image analyst"'s advice I did read and write one frame at a time.

Answer 3

[Chris Foster]

Thanks to image analyst and Walter Roberson for the help. Below is what I ended up with. It works great but all the videos have to have the same frame size. I'll try to fix that and update this post if I figure it out.

if true
  % code
end
files = dir( 'C:/Users/psychuser/Desktop/Pictures and Misc/ChristiPhone/*.mov' );   %finds all movies in a certain folder
n = 1;                              %frame counter
y = 2;                              %!!!!!!!number of seconds you want to choose!!!!!!!!!
newfilename = 'C:/Users/psychuser/Desktop/Pictures and Misc/ChristiPhone/TwoSecond/twoseconds';
writeobj = VideoWriter( newfilename , 'MPEG-4' );
open( writeobj );
for i= 1 : size( files , 1 );
    originalfilename = sprintf( 'C:/Users/psychuser/Desktop/Pictures and Misc/ChristiPhone/%s' , files(i,1).name );%gets name of file
    obj = VideoReader( originalfilename );          %turns video properties into variable 'obj'
    x = obj.Duration;                               %from that properties file get the duration of video
    if x > y                                        %if total number of seconds is greater than desired length of clip 
        z = fix( x - y );                           %z is the last possible second to start reading the video such that the desired length of clip is never longer than actual video
        if z == 0                                   %because we are removing the decimals, z could be 0, which randi won't be able to use
            starttime = 0;
        else
        starttime = randi ( z );                    %choose random start point between first frame and last possible frame
        end
        endtime = ( starttime + y );                %random starttime plus desired clip length
        obj.CurrentTime = starttime;                %set the start time of the to be copied clip
        while obj.CurrentTime <= endtime            %read a frame and write a frame to new video until desired length is attained
            vidHeight = obj.Height;
            vidWidth = obj.Width;
            video1 = struct('cdata',zeros(vidHeight,vidWidth,3,'uint8'),'colormap',[]); %fill in all the properties
            video1(n).cdata = readFrame(obj);
            writeVideo( writeobj , video1(n).cdata );
            n = n + 1;
            clear video1
        end
    else                                        % if to-be-copied video is less than two seconds then choose the whole video
        while  hasFrame(obj)
            vidHeight = obj.Height;
            vidWidth = obj.Width;
            video1 = struct('cdata',zeros(vidHeight,vidWidth,3,'uint8'),'colormap',[]);
            video1(n).cdata = readFrame(obj);
            writeVideo( writeobj , video1(n).cdata );
            n = n + 1;
            clear video1
        end
    end
    clear x z originalfilename obj video choicerng starttime endtime vidHeight vidWidth     %clear variables
end    
close( writeobj );                              %finalize video
clear all

[Walter Roberson]

I recommend you learn to use fullfile() instead of sprintf()'ing your filenames.

Your line

starttime = randi ( z );

only starts on integer second boundaries. To be more flexible, you could use

starttime = rand() * z;

which will start between 0 and z.

Your reading and writing of frames is not efficient. Change to

            video1data = readFrame(obj);
            writeVideo( writeobj , video1data );

with no need for "n" at all. You can compress it all down to

            writeVideo(writeobj, readFrame(obj));