参考文章
ffmpeg下载地址：ffmpeg

using System;
using System.Diagnostics;
using System.IO;
using System.Threading;
using UnityEngine;

public class Gifs2Mp4s { 
    private static Gifs2Mp4s instance;
    public static Gifs2Mp4s Instance { 
        get
        { 
            if (instance == null) { 
                instance = new Gifs2Mp4s();
            }
            return instance;
        }
    }
    private string gifsPath;
    private readonly string gifsDir = "/_ToolDataConfig/ToolTeachGuideConfig/Pics/";

    /// <summary>
    /// ffmpeg软件的位置
    /// </summary>
    //readonly string ffmpegPath = Application.streamingAssetsPath + "/ffmpeg/ffmpeg.exe";
    readonly string ffmpegPath = Application.dataPath.Replace("Assets" , "") +"ffmpeg/ffmpeg.exe";
    ProcessStartInfo info;
    readonly Thread t;
    /// <summary>
    /// gif文件转换成MP4文件
    /// 全部步骤完成之后杀掉所有的ffmpeg进程（建议手动调用KillFFMPEGProcess）
    /// </summary>
    /// <param name="ac">转换成功之后的回调</param>
    /// /// <param name="dirParh">gif文件夹路径</param>
    public void Gifs2MP4s(string dirParh = "" , Action ac = null) { 
        string [] itemsPath;
        if (string.IsNullOrEmpty(dirParh)) { 
            itemsPath = Directory.GetFiles(Application.streamingAssetsPath + gifsDir , "*.gif");
        }
        else { 
            itemsPath = Directory.GetFiles(dirParh , "*.gif");
        }
        for (int i = 0 ; i < itemsPath.Length ; i++) { 
            string itemPath = itemsPath [i];
            UnityEngine.Debug.Log($"gifPath:{ itemPath}");
            GetMP4(itemPath);
        }
        ac?.Invoke();
    }
    private void GetMP4(string path) { 
        if (string.IsNullOrEmpty(path)) return;
        gifsPath = Path.GetDirectoryName(path);
        string mp4Path = Path.Combine(gifsPath , Path.GetFileNameWithoutExtension(path) + ".mp4");
        UnityEngine.Debug.Log($"mp4Path:{ mp4Path}");
        if (File.Exists(mp4Path)) { 
            File.Delete(mp4Path);
        }
        info = new ProcessStartInfo(ffmpegPath);
        info.WindowStyle = ProcessWindowStyle.Hidden;
        info.CreateNoWindow = true;
        info.Arguments = $"-f gif -i { path} -pix_fmt yuv420p { mp4Path}";
        try { 
            Process.Start(info);
            Thread.Sleep(20);
        }
        catch (System.Exception e) { 
            UnityEngine.Debug.Log($"转换失败：{ e.ToString()}");
        }
    }
    /// <summary>
    /// 生成之后手动调用，杀掉后台所有的ffmpeg进程
    /// </summary>
    public void KillFFMPEGProcess() { 
        Process [] processes = Process.GetProcessesByName("ffmpeg");
        UnityEngine.Debug.Log("进程数：" + processes.Length);
        foreach (var item in processes) { 
            item.Kill();
        }
    }
}