How to lower frame rate of a video without changing the duration of the video

i want to lower the frame rate of a video without actually changing the duration of the video, i realise that this means i am losing data in the process but i need this for a specifc task, i tried doing on my own but it kept resulting in a lower frame rate however it added to the duration of the vid, my code:

function [] = tests(frameRate)
vid = VideoReader('anyVideo.mp4', 'tag', 'myreader1');

newVid = VideoWriter('NewVid.avi');
newVid.FrameRate = frameRate;

open(newVid);

numFrames = vid.NumberOfFrames;
vidTime = numFrames/Vid.FrameRate;
newNum = vidTime * frameRate;

steps = numFrames / (numFrames - newNum); % Max Count
disp(steps)
ceil(steps);
count = 1;
for frame = 1 : numFrames
    
    % Extract the frame from the movie structure.
    thisFrame = read(vid, frame); 
     if count < steps
         writeVideo(newVid,thisFrame);
         count = count + 1;
     else
         count = 1;
     end
end
close(newVid);

disp(newVid.FrameCount)

Answer 1

[Geoff Hayes]

Ahmed - since you are downsampling, can't you use the sampling frequencies to determine which frames to extract from your original video file? For example,

% assume frameRate < vid.FrameRate
frameToExtract = ceil(vid.FrameRate/frameRate);
for frame = 1 : numFrames
    % Extract the frame from the movie structure.
    thisFrame = read(vid, frame); 
    if frame == 1 || mod(frame - 1, frameToExtract) == 0
        writeVideo(newVid,thisFrame);
    end
end

We extract the first frame and then every frameToExtract after. I haven't tested the above but I think that it should work (or produce a reasonable extraction). This differs from your code in that we extract every nth sample rather than consecutive samples (if I'm reading your code correctly).

[Ahmed Al-Qarqaz]

can you please provide more detail about the if-stament condition in your code , i don't uderstand the intuition

if frame == 1 || mod(frame - 1, frameToExtract) == 0

[Geoff Hayes]

Ahmed - the intent of the condition is that if we are on the first iteration of the loop (which corresponds to the first frame of the video)

frame == 1

or we are the frameToExtract iteration (and so we use mod to see if frame-1 is a multiple of frameToExtract)

mod(frame - 1, frameToExtract) == 0

then we write that frame to file. For example, if frameToExtract is 4, and we iterate over

1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,..

then the following frames shoud be written to file

1,5,9,13,17,...

See this code

numFrames = 42;
frameToExtract = 4;

for frame = 1 : numFrames
    % Extract the frame from the movie structure.
    %thisFrame = read(vid, frame); 
    if frame == 1 || mod(frame - 1, frameToExtract) == 0
        fprintf('Writing frame %d to file...\n', frame);
        %writeVideo(newVid,thisFrame);
    end
end

[Ahmed Al-Qarqaz]

oh now i understand, i tried to understand it through iterating through a list on paper, but now i got it thanks a lot !!

when i think about it that's actually a pretty smart way of doing it.

[Akshaya Srinidhi Vijayareka]

Hi !

Thank for this code.

I'm trying to reeduce the frame rate of a video. I'm trying out your code. But i'm not able to read the output video file saved in the path. Can you help me with this ?

Thank you!

[Geoff Hayes]

Akshaya - please clarify what code you are using and what the error message is.