如何使用FFMpeg (C#)转码数据流

using System;
using FFmpeg.AutoGen;

namespace FFMpegExample
{
    class Program
    {
        static unsafe void Main(string[] args)
        {
            // 初始化FFmpeg库
            ffmpeg.av_register_all();
            ffmpeg.avcodec_register_all();

            // 打开输入流
            AVFormatContext* inputFormatContext = null;
            if (ffmpeg.avformat_open_input(&inputFormatContext, "input.mp4", null, null) != 0)
            {
                Console.WriteLine("无法打开输入流");
                return;
            }

            // 查找音视频流信息
            if (ffmpeg.avformat_find_stream_info(inputFormatContext, null) < 0)
            {
                Console.WriteLine("无法获取流信息");
                return;
            }

            // 查找音频流
            int audioStreamIndex = -1;
            for (int i = 0; i < inputFormatContext->nb_streams; i++)
            {
                if (inputFormatContext->streams[i]->codec->codec_type == AVMediaType.AVMEDIA_TYPE_AUDIO)
                {
                    audioStreamIndex = i;
                    break;
                }
            }

            if (audioStreamIndex == -1)
            {
                Console.WriteLine("未找到音频流");
                return;
            }

            // 打开音频解码器
            AVCodecContext* audioCodecContext = inputFormatContext->streams[audioStreamIndex]->codec;
            AVCodec* audioCodec = ffmpeg.avcodec_find_decoder(audioCodecContext->codec_id);
            if (audioCodec == null)
            {
                Console.WriteLine("无法找到音频解码器");
                return;
            }

            if (ffmpeg.avcodec_open2(audioCodecContext, audioCodec, null) < 0)
            {
                Console.WriteLine("无法打开音频解码器");
                return;
            }

            // 创建输出流
            AVFormatContext* outputFormatContext = null;
            if (ffmpeg.avformat_alloc_output_context2(&outputFormatContext, null, null, "output.mp3") < 0)
            {
                Console.WriteLine("无法创建输出流");
                return;
            }

            // 添加音频流到输出流
            AVStream* audioOutputStream = ffmpeg.avformat_new_stream(outputFormatContext, null);
            if (audioOutputStream == null)
            {
                Console.WriteLine("无法创建音频流");
                return;
            }

            // 复制音频流参数
            if (ffmpeg.avcodec_parameters_copy(audioOutputStream->codecpar, audioCodecContext) < 0)
            {
                Console.WriteLine("无法复制音频流参数");
                return;
            }

            // 打开输出流
            if ((outputFormatContext->oformat->flags & ffmpeg.AVFMT_NOFILE) == 0)
            {
                if (ffmpeg.avio_open(&outputFormatContext->pb, "output.mp3", ffmpeg.AVIO_FLAG_WRITE) < 0)
                {
                    Console.WriteLine("无法打开输出流");
                    return;
                }
            }

            // 写入文件头
            if (ffmpeg.avformat_write_header(outputFormatContext, null) < 0)
            {
                Console.WriteLine("无法写入文件头");
                return;
            }

            // 转码数据流
            AVPacket packet;
            ffmpeg.av_init_packet(&packet);
            while (ffmpeg.av_read_frame(inputFormatContext, &packet) >= 0)
            {
                if (packet.stream_index == audioStreamIndex)
                {
                    // 解码音频帧
                    AVFrame* frame = ffmpeg.av_frame_alloc();
                    int frameFinished;
                    ffmpeg.avcodec_decode_audio4(audioCodecContext, frame, &frameFinished, &packet);

                    if (frameFinished != 0)
                    {
                        // 编码音频帧
                        AVPacket outputPacket;
                        ffmpeg.av_init_packet(&outputPacket);
                        outputPacket.data = null;
                        outputPacket.size = 0;

                        ffmpeg.avcodec_encode_audio2(audioCodecContext, &outputPacket, frame, &frameFinished);
                        if (frameFinished != 0)
                        {
                            outputPacket.stream_index = audioOutputStream->index;

                            // 写入输出流
                            ffmpeg.av_interleaved_write_frame(outputFormatContext, &outputPacket);
                            ffmpeg.av_packet_unref(&outputPacket);
                        }
                    }

                    ffmpeg.av_frame_free(&frame);
                }

                ffmpeg.av_packet_unref(&packet);
            }

            // 写入文件尾
            ffmpeg.av_write_trailer(outputFormatContext);

            // 释放资源
            ffmpeg.avcodec_close(audioCodecContext);
            ffmpeg.avformat_close_input(&inputFormatContext);
            ffmpeg.avformat_free_context(outputFormatContext);
        }
    }
}