How to check when ffmpeg completes a task?

Question

I'm just learning how to use ffmpeg a few hours ago to generate video thumbnails.

These are some results:

I'd used the same size (width - height) to Youtube's. Each image contains max 25 thumbnails (5x5) with the size 160x90.

Everything looks good until:

public async Task GetVideoThumbnailsAsync(string videoPath, string videoId)
{
    byte thumbnailWidth = 160;
    byte thumbnailHeight = 90;

    string fps = "1/2";

    videoPath = Path.Combine(_environment.WebRootPath, videoPath);

    string videoThumbnailsPath = Path.Combine(_environment.WebRootPath, $"assets/images/video_thumbnails/{videoId}");
    string outputImagePath = Path.Combine(videoThumbnailsPath, "item_%d.jpg");

    Directory.CreateDirectory(videoThumbnailsPath);

    using (var ffmpeg = new Process())
    {
        ffmpeg.StartInfo.Arguments = $" -i {videoPath} -vf fps={fps} -s {thumbnailWidth}x{thumbnailHeight} {outputImagePath}";
        ffmpeg.StartInfo.FileName = Path.Combine(_environment.ContentRootPath, "FFmpeg/ffmpeg.exe");
        ffmpeg.Start();
    }

    await Task.Delay(3000);

    await GenerateThumbnailsAsync(videoThumbnailsPath, videoId);
}

I'm getting a trouble with the line:

await Task.Delay(3000);

When I learn the way to use ffmpeg, they didn't mention about it. After some hours failed, I notice that:

An mp4 video (1 min 31 sec - 1.93Mb) requires some delay time ~1000ms. And other, an mp4 video (1 min 49 sec - 7.25Mb) requires some delay time ~3000ms.

If I don't use Task.Delay and try to get all the files immediately, it would return 0 (there was no file in the directory).

Plus, each file which has a difference length to the others requires a difference delay time. I don't know how to calculate it.

And my question is: How to check when the task has completed?

P/s: I don't mean to relate to javascript, but in js, there is something called Promise:

var promise = new Promise(function (done) {
    var todo = function () {
        done();
    };

    todo();
});

promise.then(function () {
    console.log('DONE...');
});

I want to edit the code like that.

Thank you!

I'm not familiar with ffmpeg, but C# has [events](https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/events/). I suspect if you look at the ffmpeg documentation, there are some events that you can subscribe to that are raised when the process completes. — Lews Therin, May 25 '18 at 19:06
This answer: https://stackoverflow.com/a/19104345/393701 might be of interest to you. — SirDarius, May 25 '18 at 19:07
In c#, analog of Promise is Task, and this specific use of Promise can be achieved with TaskCompletionSource. — Evk, May 25 '18 at 19:19
@SirDarius Perfect! I have tried again with the advice and it's working good. Thank you so much! — , May 25 '18 at 19:22

score 0 · Accepted Answer · answered May 25 '18 at 19:34

If you want to do it synchronously, then you could call ffmpeg.WaitForExit(), which will cause the line to wait until the process has finished. Otherwise you'll need to start a Task to run it in the background. A C# Task is somewhat similar to a Promise in JavaScript.

This answer can also be used for details on how to wrap WaitForExit so that you can use it with an await

How to check when ffmpeg completes a task?

1 Answers1