Add subtitles

There are two ways to add subtitles into video. The first one is to burn it into video. The next one is to add new stream with subtitles, as in .mkv format.

Burning subtitles

IVideoStream allows to burn subtitle into video:

IMediaInfo inputFile = await FFmpeg.GetMediaInfo(Resources.MkvWithAudio);
string outputPath = Path.ChangeExtension(Path.GetTempFileName(), ".mp4");

IVideoStream videoStream = inputFile.VideoStreams.First().AddSubtitles(Resources.SubtitleSrt);

IConversionResult conversionResult = await FFmpeg.Conversions.New()
    .AddStream(videoStream)
    .SetOutput(outputPath)
    .Start();

Add subtitles

Subtitles are streams too so could be added to conversion like other streams:

IMediaInfo mediaInfo = await FFmpeg.GetMediaInfo(inputPath);
IMediaInfo subtitleInfo = await FFmpeg.GetMediaInfo(subtitlePath);

ISubtitleStream subtitleStream = subtitleInfo.SubtitleStreams.First()
.SetLanguage(language);

return New()
    .AddStream(mediaInfo.Streams)
    .AddStream(subtitleStream)
    .SetOutput(outputPath);

or easier using FFmpeg.Conversions.FromSnippet.AddSubtitle 

string output = Path.ChangeExtension(Path.GetTempFileName(), ".mkv");

IConversion conversion = await FFmpeg.Conversions.FromSnippet.AddSubtitle(Resources.MkvWithAudio, output, Resources.SubtitleSrt);
IConversionResult result = await conversion.Start();

Methods

/// <summary>
///     Set subtitle language
/// </summary>
/// <param name="language">Language</param>
/// <returns>ISubtitleStream</returns>
ISubtitleStream SetLanguage(string language);

/// <summary>
///     Set subtitle codec
/// </summary>
/// <param name="codec">Subtitle codec</param>
/// <returns>ISubtitleStream</returns>
ISubtitleStream SetCodec(SubtitleCodec codec);

/// <summary>
///     Set Subtitle codec
/// </summary>
/// <param name="codec">Subtitle codec</param>
/// <returns>IVideoStream</returns>
ISubtitleStream SetCodec(string codec);

/// <summary>
///     "-re" parameter. Read input at native frame rate. Mainly used to simulate a grab device, or live input stream (e.g. when reading from a file). Should not be used with actual grab devices or live input streams (where it can cause packet loss). By default ffmpeg attempts to read the input(s) as fast as possible. This option will slow down the reading of the input(s) to the native frame rate of the input(s). It is useful for real-time output (e.g. live streaming).
/// </summary>
/// <param name="readInputAtNativeFrameRate">Read input at native frame rate. False set parameter to default value.</param>
/// <returns>IConversion object</returns>
ISubtitleStream UseNativeInputRead(bool readInputAtNativeFrameRate);

/// <summary>
///     "-stream_loop" parameter. Set number of times input stream shall be looped. 
/// </summary>
/// <param name="loopCount">Loop 0 means no loop, loop -1 means infinite loop.</param>
/// <returns>IConversion object</returns>
ISubtitleStream SetStreamLoop(int loopCount);