Advanced.cs

#region GUI

var ffmpeg = new FFmpeg();
ffmpeg.UI.Dump();

#endregion

class FFmpeg
{
    TimeSpan duration;

    public FFmpeg()
    {
        inputFilePath.TextInput += (_, _) =>
        {
            outputFileName.Text = Path.GetFileName(inputFilePath.Text);
        };

        btn.Click += async (_, _) =>
        {
            var args = new List<string>();
            args.Add("-i");
            args.Add(Path.GetFileName(inputFilePath.Text));
            args = args.Concat(Arguments).ToList();
            args.Add(outputFileName.Text);

            await Cli
                .Wrap("ffmpeg")
                .WithWorkingDirectory(Path.GetDirectoryName(inputFilePath.Text))
                .WithArguments(args, true)
                .WithStandardErrorPipe(PipeTarget.ToDelegate(HandleOutput))
                .ExecuteAsync();

            Util.Progress = 100;
        };
    }

    #region Controls

    private FilePicker inputFilePath = new();
    private TextBox outputFileName = new();
    private Button btn = new("   Run   ");

    private TextBox bitRate = new();
    private TextBox resolution = new();
    private TextBox frameRate = new();
    private TextBox crf = new TextBox();
    private SelectBox vcodec = new(new[] { "libx264", "libx265", "libxvid", "vp9", }, 0);

    public Control UI => new StackPanel(
            false,
            new StackPanel
            (
                false,
                new Span("Input"), inputFilePath,
                new Span("Output"), new Div(outputFileName, btn),
                new FieldSet
                (
                    "Options",
                    new StackPanel
                    (
                        false,
                        new Span("Bit Rate (e.g. 2M, 500k, etc.)"), bitRate,
                        new Span("Resolution (e.g. 1920x1080)"), resolution,
                        new Span("Frame Rate (e.g. 30, 60, etc.)"), frameRate,
                        new Span("Constant Rate Factor (0~51, default: 23)"), crf,
                        new Span("Video Codec"), vcodec
                    )
                )
            )
        );

    #endregion

    public List<string> Arguments
    {
        get
        {
            var args = new List<string>();

            void AddIfNotEmpty(string text, string arg)
            {
                if (string.IsNullOrWhiteSpace(text))
                    return;
                args.Add(arg);
                args.Add(text);
            }

            AddIfNotEmpty(bitRate.Text, "-b");
            AddIfNotEmpty(frameRate.Text, "-r");
            AddIfNotEmpty(resolution.Text, "-s");
            AddIfNotEmpty(crf.Text, "-crf");

            args.Add("-vcodec");
            args.Add(vcodec.SelectedOption.ToString()!);

            return args;
        }
    }

    private void HandleOutput(string line)
    {
        var durationMatch = Regex.Match(line, @"Duration: (\d+:\d+:\d+\.\d+)");
        if (durationMatch.Success)
        {
            duration = TimeSpan.Parse(durationMatch.Groups[1].Value);
            Util.Progress = 0;
            return;
        }
        var timeMatch = Regex.Match(line, @"time=(\d+:\d+:\d+\.\d+)");
        if (timeMatch.Success)
        {
            var current = TimeSpan.Parse(timeMatch.Groups[1].Value);
            var percent = current.TotalSeconds / duration.TotalSeconds;
            Util.Progress = (int)(percent * 100);
            return;
        }
    }
}

LINQPad FFmpeg.cs

#region GUI

var inputFilePath = new FilePicker().Dump("Input");
var outputFileName = new TextBox().Dump("Output");
var bitRate = new TextBox().Dump("Bit Rate");
var resolution = new TextBox().Dump("Resolution");
var frameRate = new TextBox().Dump("Frame Rate");
var button = new Button("Run").Dump();

Util.ProgressBar bar = null;

#endregion

TimeSpan duration = default;

button.Click += async (_, _) =>
{
    var args = new List<string>();
    args.Add("-i");
    args.Add(Path.GetFileName(inputFilePath.Text));
    if (!string.IsNullOrWhiteSpace(bitRate.Text))
    {
        args.Add("-b");
        args.Add(bitRate.Text);
    }
    if (!string.IsNullOrWhiteSpace(frameRate.Text))
    {
        args.Add("-r");
        args.Add(frameRate.Text);
    }
    if (!string.IsNullOrWhiteSpace(resolution.Text))
    {
        args.Add("-s");
        args.Add(resolution.Text);
    }
    args.Add(outputFileName.Text);

    await Cli
        .Wrap("ffmpeg")
        .WithWorkingDirectory(Path.GetDirectoryName(inputFilePath.Text))
        .WithArguments(args, true)
        .WithStandardErrorPipe(PipeTarget.ToDelegate(HandleOutput))
        .ExecuteAsync();
};

void HandleOutput(string line)
{
    var durationMatch = Regex.Match(line, @"Duration: (\d+:\d+:\d+\.\d+)");
    if (durationMatch.Success)
    {
        duration = TimeSpan.Parse(durationMatch.Groups[1].Value);
        bar = new Util.ProgressBar { HideWhenCompleted = true }.Dump();
        return;
    }
    var timeMatch = Regex.Match(line, @"time=(\d+:\d+:\d+\.\d+)");
    if (timeMatch.Success)
    {
        var current = TimeSpan.Parse(timeMatch.Groups[1].Value);
        var percent = current.TotalSeconds / duration.TotalSeconds;
        bar.Fraction = percent;
        bar.Caption = $"{percent * 100:F0}%";
        return;
    }
    if (line.Contains("final ratefactor"))
        bar.Fraction = 1.0;
}