Build videos using every Nth image from original video

I have a high speed video of a spinning object. The camera is triggered by the object position and it takes 20 pictures for each trigger. So I want to extract every 20th frame to a separate video as that will give me separate videos of the different positions around the object. I expect the input video to have around 4,000 frames.

What I want is to create one video with frames 1, 21, 41... another with frames 2, 22, 42... etc., up to frames 8, 28, 48...

Here is my current code. Is there a better way to do this?

% variables for extracting fixed position videos
    numImagesPerTrigger = 20;
    numLocsToOutput = 8;
    
%% Code to create a movie with every Nth image
    % Get properties of the video 
    vidObjIn = VideoReader(InputFileName);
    numFrames = vidObjIn.NumberOfFrames;
    FrameRate = vidObjIn.FrameRate;

    % Must reopen the video after getting its properties.
    vidObjIn = VideoReader(InputFileName);

    % Create separate videos for each part
    for Offset = 0 : numLocsToOutput - 1    
        % Create the output file
        OutputFileName = char(string(SelectedFilePath) + ...
            string(InputFileBaseName)+ '-(' + Offset + ')' + ...
            string(InputFileext));
        vidObjOut = VideoWriter(OutputFileName, 'MPEG-4');
        vidObjOut.FrameRate = 1;
        open(vidObjOut);

        % Select all frames at this location
        for Frame = Offset : numImagesPerTrigger : numFrames - 1
           vidObjIn.CurrentTime = (Frame) / FrameRate; 
           vidFrame = readFrame(vidObjIn);
           writeVideo(vidObjOut, vidFrame);
        end

        % Clean up output file
        close(vidObjOut)

    end

It seems to me that the line:

    vidObjIn.CurrentTime = (Frame) / FrameRate; 

is cumbersome. I am concerned that it is possibly prone to error if there are a large number of frames. I would prefer to select the actual frame number directly.

Answer 1

[Salman Ahmed]

Hi Jim,

Since you would like to select a Frame from the VideoReader object (vidObjIn) directly, you could use the read function as shown below. Hope this resolves your concern.

vidFrame = read(vidObjIn, Frame);

[Jim McIntyre]

Thank you. This is better.