This is the archived version of Roland Weigelt's weblog that ran from 2003 to 2023 at weblogs.asp.net

Ways to Create a Thumbnail for Video and Image Files in C#

In my spare time, I work on a software for the courtside advertising system of a local sports arena. To create the playlists for the advertising loops on the LED modules, users can drag and drop preview thumbnails of video and image files. Which brings us to the topic of this blog post, i.e. how to create these thumbnails.

Ways to create thumbnails

Win API

When I started the project in C#/WPF many years ago, I used some Win API code from a StackOverflow answer by Daniel Peñalba.

StorageFile.GetThumbnailAsync

After a failed attempt at a UWP rewrite, my software now runs on .NET Core 3.1, with a WPF management client, a ASP.NET/SignalR server and a HTML5/Typescript playback client (using a Chrome browser in kiosk mode, although I may change that when Microsoft Edge WebView2 comes out of preview).

Working with UWP introduced me to StorageFile.GetThumbnailAsync, which I kept using because the StorageFile API also gives me easy access to metadata (e.g. width, height and duration of a video).

The StorageFile API is not exactly known for high performance, but in my case this is not an issue, because the number of media files added in one go is typically in single digits.

FFmpeg

As mentioned above, StorageFile.GetThumbnailAsync works for me. So, why am I considering another method for the next version of my software?

For an answer I have to digress a bit: My software initially used individual MediaElement controls for the eleven LED modules. While I did manage to sync up the video playback, making it robust and reliable for many hours of looped playback required many workarounds in the code. To make matters worse, the reliability depended on the video format.

The videos to be played come from a variety of sources; some are produced by advertising agencies or video production companies, others are the work of semi-professionals. Some match the requested specs, many do not. In the end, I made it a habit to transcode each video myself.

Fortunately, this did not cause a noticeable loss of quality. Which encouraged me to try a different approach when I rewrote the playback to be performed in a browser. Now my software uses FFmpeg to combine the individual videos into one MP4 file. FFmpeg is “A complete, cross-platform solution to record, convert and stream audio and video” and this quote from the website is not an understatement.

In addition to offering a wide variety of features to manipulate inputs and outputs, FFmpeg also has been able to handle virtually any media file it has come across so far. Until now I could not fully benefit from this robustness because StorageFile.GetThumbnailAsync does not work with the same range of file formats. For example, on my computer, MOV files cause an exception (note the emphasis, YMMV).

FFmpeg to the rescue: Creating a thumbnail from a video or image file is something that FFmpeg can do easily – with two caveats:

  1. It is slower than GetThumbnailAsync (at least when calling the executable as a separate process, like I do)
  2. It does not work for general purpose files, only for videos or images.

My plan is to continue to use StorageFile.GetThumbnailAsync as the default method and to use FFmpeg as a fallback in case of an exception.

Demo code

I have posted a Visual Studio demo project on https://github.com/RWeigelt/ThumbnailCreationDemo. The code shows the use of StorageFile.GetThumbnailAsync and FFmpeg (for Win API, see the StackOverflow answer). Please note that the code is not intended to be a ready-to-use library; instead, you should run the demo, maybe set a few breakpoints, follow the code and use the parts that are of interest to you.

A few prerequisites:

  • Your Visual Studio 2019 (I used 16.7.1) should be able to compile .NET Core 3.1 command line applications.
  • You must have FFmpeg on your computer. You can get it from https://ffmpeg.org.

  • Before you run the project, change the following line in “Program.cs” so the path matches the location of the FFmpeg executable on your computer:

    const string _FfmpegExeFilePath = @"C:\Program Files\FFmpeg\bin\ffmpeg.exe";

The project comes with two example files, “Image.png” and “Video.wmv”, with 768 x 80 pixels each (the size of a LED advertising module). During a build, the files will be copied to the output directory.

Running the project inside Visual Studio creates two thumbnail files for each media file (“…thumb1.png” and “…thumb2.png”). At the end you will be offered to open an Explorer window for the output directory, so you do not need dig around in the file system.

While the code for using StorageFile.GetThumbnailAsync is straightforward, the usage of FFmpeg needs some explanation.

The call of ffmpeg.exe is hidden behind a Task using a TaskCompletionSource. For the purpose of the demo code, the Task returns the duration in milliseconds (instead of e.g. the exit code of the FFmpeg process). This is something you may want to change for production code.

The command line parameters:

  • For video files,
    • -ss 5 means “seek to the position five seconds into the media file”.
    • -an prevents audio streams from being handled in any way
  • -i specifies the input file
  • -vframes 1 gets one frame from the input file
  • -s specifies the output size
  • -y to confirm overwriting an existing file
  • The last parameter is the output file; the extension of the thumbnail file tells FFmpeg that we want the file in PNG format.

Note that the order of the parameters makes a difference: Parameters in front of the input file (-i) are treated as options that affect the input, parameter behind affect the output.

For instance, the -ss 5 in front of the -i simply seeks into the file (as one would expect). If the -ss 5 is placed behind the -i, then FFmpeg will actually decode five seconds of the input file frame by frame, which takes more time. The difference is noticeable even for this simple demo, try it out. For more information see https://trac.ffmpeg.org/wiki/Seeking.

What about metadata?

As I wrote, on my computer, calling StorageFile.GetThumbnailAsync causes an exception for a MOV file (yes, this could surely be fixed with the right codec pack, but for the moment I keep things the way they are to test error handling).

But I can retrieve metadata using StorageFile.Properties – that is because the metadata is not determined at the moment I ask for it, it simply is read from the file without actually looking at its content. This is fortunate because for my purposes, reading the metadata via the StorageFile API is much easier than using e.g. ffprobe’s output.

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