using LGPL compiled ffmpeg shared for FFmpeg n5.0 from [here](https:/…

…/github.com/BtbN/FFmpeg-Builds/releases)

* specifically - ffmpeg-n5.0-latest-win64-lgpl-shared-5.0.zip

targets openH264  rather than the GPL x264 encoder

modifies FFmpeg.AutoGen.Example to output an MP4 rather than .h264 file by way of:
* adding AVFormatContext and AVStream
* using ffmpeg.avio_open to create / write the file instead of System.IO.xxx
* writing a header and trailer
* using ffmpeg.av_interleaved_write_frame()

includes a somewhat mangled but updated and somewhat working version of the Transcoding gist shared by Ruslan-B

.gitignore

@@ -106,4 +106,5 @@ UpgradeLog*.XML
 *.stackdump
 
 # Local 
-/FFmpeg.AutoGen.Example/frame.*.jpg
+/FFmpeg.AutoGen.Example/frame.*.jpg
+/video-test*

FFmpeg.AutoGen.Example.Transcoding/FFmpeg.AutoGen.Example.Transcoding.csproj

@@ -0,0 +1,15 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <ImplicitUsings>disable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <AllowUnsafeBlocks>True</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\FFmpeg.AutoGen\FFmpeg.AutoGen.csproj" />
+  </ItemGroup>
+
+</Project>

FFmpeg.AutoGen.Example.Transcoding/FFmpegBinariesHelper.cs

@@ -0,0 +1,36 @@
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+
+namespace FFmpeg.AutoGen.Example
+{
+    public class FFmpegBinariesHelper
+    {
+        internal static void RegisterFFmpegBinaries()
+        {
+            if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+            {
+                var current = Environment.CurrentDirectory;
+                var probe = Path.Combine("FFmpeg", "bin", Environment.Is64BitProcess ? "x64" : "x86");
+
+                while (current != null)
+                {
+                    var ffmpegBinaryPath = Path.Combine(current, probe);
+
+                    if (Directory.Exists(ffmpegBinaryPath))
+                    {
+                        Console.WriteLine($"FFmpeg binaries found in: {ffmpegBinaryPath}");
+                        ffmpeg.RootPath = ffmpegBinaryPath;
+                        return;
+                    }
+
+                    current = Directory.GetParent(current)?.FullName;
+                }
+            }
+            else if (RuntimeInformation.IsOSPlatform(OSPlatform.Linux))
+                ffmpeg.RootPath = "/lib/x86_64-linux-gnu/";
+            else
+                throw new NotSupportedException(); // fell free add support for platform of your choose
+        }
+    }
+}

FFmpeg.AutoGen.Example.Transcoding/FFmpegHelper.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace FFmpeg.AutoGen.Example
+{
+    internal static class FFmpegHelper
+    {
+        public static unsafe string av_strerror(int error)
+        {
+            var bufferSize = 1024;
+            var buffer = stackalloc byte[bufferSize];
+            ffmpeg.av_strerror(error, buffer, (ulong) bufferSize);
+            var message = Marshal.PtrToStringAnsi((IntPtr) buffer);
+            return message;
+        }
+
+        public static int ThrowExceptionIfError(this int error)
+        {
+            if (error < 0) throw new ApplicationException(av_strerror(error));
+            return error;
+        }
+    }
+}

FFmpeg.AutoGen.Example.Transcoding/Program.cs

@@ -0,0 +1,12 @@
+namespace FFmpeg.AutoGen.Example
+{
+    public class Program
+    {
+        private static void Main(string[] args)
+        {
+            FFmpegBinariesHelper.RegisterFFmpegBinaries();
+            var transcoding = new Transcoding();
+            transcoding.main(args.Length, args);
+        }
+    }
+}