Number Of Frames in Video File with Matlab 2015b

Hi,

In Matlab 2015b, NumberOfFrames property of VideoReader is deprecated. So, what is the best way to learn number of frames in a video?

[Cristina Vasco]

When I use NumberOfFrames in a script, there's some hover-over advice to "use 'CurrentTime' instead." This is not particularly helpful and seems wrong. Could someone update this help?

Answer 1

[Dinesh Iyer]

The NumberOfFrames property is on the path to deprecation but querying this property still works. This deprecation indicates that this might be removed in some future release. This was first announced in 14b but has not been removed for the past three releases.

If this property is infact removed, then there are two ways to determine this.

First way is to compute it using the formula:

vidObj = VideoReader('myfile.mp4');
numFrames = ceil(vidObj.FrameRate*vidObj.Duration);

This formula above gives only an "estimate" of the number of frames. For fixed frame-rate files, the value will be within 1 of the actual number of frames due to rounding issues. Many video files are variable frame-rate and so for those files the actual number of frames can be more or less than the estimated value.

To get the exact value, you have no choice but to scan through the entire file using the code below:

    vidObj = VideoReader('myfile.mp4');
    numFrames = 0;
    while hasFrame(vidObj)
        readFrame(vidObj);
        numFrames = numFrames + 1;
    end

If your application requires knowledge of exact number of frames use the second method. If you want to know this value for pre-allocating arrays use the first method.

Hope this helps.

Dinesh

[Cristina Vasco]

Thanks Dinesh, this was helpful.

[Image Analyst]

Since it's useful, and seems to require a time consuming bit of code to determine the number of frames, why have they deprecated it?

[Walter Roberson]

Efficiency. Populating it requires parsing the entire file for variable frame-rate videos.

Also I would point out that if/when the toolbox starts supporting live streaming video, then NumberOfFrames will not have any useful meaning.

[Prashanth Pradeep]

So what exactly does 'use current time' mean? Regardless of efficiency, isn't finding out the exact number of frames helpful if we want to extract each frame and process it.

[Akshay Ravindran]

I am getting a variable number of frame numbers in running the same code on two different machines

i.e. I am reading the number of frames using the above code snippet in two separate machines both with Matlab2017 installed, on the same video file. However, I am getting two different values for frame numbers on each machine. Could you please let me know why that is so?

[Billy Pilgrim]

Hi Dinesh,

Thanks for your answer above. I've been using the 'while hasFrame' approach for a while now for getting exact frame numbers.

I was wondering if you had any approaches that might expedite the process? Preallocating isn't a thing in this case because numFrames is just a single value, not an array we're adding to? Similarly, there's no way to read a subset of the frame rather than the entire frame, correct? Just wondering if you had any ideas of tricks for speeding things up...

[Walter Roberson]

You could potentially estimate the number of frames, then subtract a bit in case of estimation error, and then use the read() method instead of readFrame() to position to a particular frame number, and then readFrame from there to find the actual last frame.

However, it is not clear that read() with a frame number is efficient: it might have to read all of the frames in order to count them.

What is efficient is positioning by time. But positioning by time does not give you a frame count.