VideoReader read function indicies must be positive or logical values

Hello,

I'm trying to read specific frames of a video in, add text and then write it out as a new video. I keep getting the error message below after it has read the first frame:

"Error using ProcessVideo (line 11)

Array indices must be positive integers or logical values."

Edited to add that have tried further it looks like if I change the iFrame loop to increment by 1 (i.e. iFrame=1:1:NFrames) then it works. So it looks like read can't hanlde non-sequential calls which is in contradiction with the MATLAB example linked below. While I can read in all frames and discard most of them this makes it very slow so is not desireable.

My script is:

clear all
close all
files=dir('\\192.168.1.184\d\CCTV\*.mp4'); %Path containing files
vout=VideoWriter([files(1).folder, '\VideoOut.mp4'],'MPEG-4'); %Output video
open(vout);
for ifile=1:numel(files) %Loop over all files
    fprintf(['File ', num2str(ifile), ' of ', num2str(numel(files)), '\n']); %Display progress
    vin=VideoReader([files(ifile).folder, '\', files(ifile).name]); %Open video
    NFrames=floor(vin.Duration*vin.FrameRate); %Calculate number of frames (as vin.NumFrames is not populated for some reason)
    for iFrame=1:15:NFrames %Loop over desired frames
        Frame=read(vin,iFrame); %Read in desired frame
        Frame=AddTextToImage(Frame,files(ifile).name,[1700 125],[1 1 1],'Arial',125); %Annotate with file name
        writeVideo(vout,Frame); %Write frame to output video
    end
    clear vin %Shouldn't be necesarry - added during debug
end
close(vout); %Finish writing video

So line 11 is where I attempt to read the frame. The syntax I am using appears to match the example in the read videos using frame index section of https://www.mathworks.com/help/matlab/ref/videoreader.html .

I've checked and nothing is shadowing read, vin or iFrame:

>> which read
read is a built-in method  % asyncioimpl.InputStream method
>> which vin
vin is a variable.
>> which iFrame
iFrame is a variable.

So I'm a bit confused why this error is occuring as iFrame is a positive integer.

If anyone has any ideas it would be most appreciated.

I'm running MATLAB R2020b update 3.

AddTextToImage is from https://www.mathworks.com/matlabcentral/fileexchange/40959-add-text-to-image.

[Nitin Kapgate]

Can you share the MAT file which includes the variables like vin, NFrames, iFrame for 1 iteration of the for loop so that I can reproduce the issue at my end?

[Benjamin Bloomfield]

I tried to upload a mat file containing vin, iFrame, NFrames however it is 10.5MB which the file exchange will not let me upload (I don't know if I can email it or some other way of uploading it)? I don't know why it is so large as if I call whos all of the variables are 8 bytes.

One interesting observation I found this morning is if I try reading an mp4 that MATLAB wrote then the above script works but if I try reading a different file (from a Swann NVR) it doesn't work. With the file that MATLAB wrote vin.NumFrames is populated with a sensible value but for the other file vin.NumFrames has no value. So there must be some compatability issue with the file (even though it plays fine in VLC media player).

I found a workaround which is to set vin.CurrentTime and then call readFrame so this is no longer urgent but it would be nice to understand as I spent quite a while trying to understand why the code in the help doesn't work before I found a way that does.

Answer 1

[Yaxuan Li]

I got the error like

"Array indices must be positive integers or logical values."

while using

numFrames = obj.NumberFrames;