最近想把ffmpeg官方例子过一遍，达到初步了解ffmpeg的目的，本文只是给自己一个记录，也是在网上没有找到一样的文章，发出来供大家指点。

直接使用官方demo，把mp4转换成avi格式会报下面的错误(avi转MP4,MP4转其他都没问题)

 下面的代码，可以实现其他格式转avi，但是还是存在问题，转换完后的视频再想转回去就会丢失视频画面，只有音频了，刚开始以为是要放到av_rescale_q_rnd函数前面，测试是一样的结果，报错如下图，输出的dts和duration变了（去掉两个宏定义，就没这个问题，应该是漏了步骤）

 源码如下：

#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

#define USE_H264  1
#define USE_AAC   1

static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt, const char *tag)
{
    AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

    printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",
           tag,
           av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base),
           av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base),
           av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base),
           pkt->stream_index);
}

int main(int argc, char **argv)
{
    AVOutputFormat *ofmt = NULL;
    AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;
    AVPacket pkt;
    const char *in_filename, *out_filename;
    int ret, i;
    int stream_index = 0;
    int *stream_mapping = NULL;
    int stream_mapping_size = 0;

#if USE_H264
    AVCodecParameters *h264Codecpar = NULL;
    int h264VideoIndex = -1;
#endif
#if USE_AAC
   AVCodecParameters *aacCodecpar = NULL;
   int aacVideoIndex = -1;
#endif

    if (argc < 3) {
        printf("usage: %s input output\n"
               "API example program to remux a media file with libavformat and libavcodec.\n"
               "The output format is guessed according to the file extension.\n"
               "\n", argv[0]);
        return 1;
    }

    in_filename  = argv[1];
    out_filename = argv[2];

    if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {
        fprintf(stderr, "Could not open input file '%s'", in_filename);
        goto end;
    }

    if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {
        fprintf(stderr, "Failed to retrieve input stream information");
        goto end;
    }

    av_dump_format(ifmt_ctx, 0, in_filename, 0);

    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        fprintf(stderr, "Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    stream_mapping_size = ifmt_ctx->nb_streams;
    stream_mapping = av_mallocz_array(stream_mapping_size, sizeof(*stream_mapping));
    if (!stream_mapping) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    ofmt = ofmt_ctx->oformat;

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        AVStream *out_stream;
        AVStream *in_stream = ifmt_ctx->streams[i];
        AVCodecParameters *in_codecpar = in_stream->codecpar;
        
#if USE_H264        
        if(in_codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
        {
            h264Codecpar = in_codecpar;
            h264VideoIndex = i;
        }
#endif

#if USE_AAC
        if(in_codecpar->codec_type == AVMEDIA_TYPE_AUDIO)
        {
            aacCodecpar = in_codecpar;
            aacVideoIndex = i;
        }
#endif

        if (in_codecpar->codec_type != AVMEDIA_TYPE_AUDIO &&
            in_codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&
            in_codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE) {
            stream_mapping[i] = -1;
            continue;
        }

        stream_mapping[i] = stream_index++;

        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy codec parameters\n");
            goto end;
        }
        out_stream->codecpar->codec_tag = 0;
    }
    av_dump_format(ofmt_ctx, 0, out_filename, 1);

    if (!(ofmt->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            fprintf(stderr, "Could not open output file '%s'", out_filename);
            goto end;
        }
    }

#if USE_H264
    const AVBitStreamFilter *h264Filter  =  av_bsf_get_by_name("h264_mp4toannexb");
    AVBSFContext *h264BsfCtx;
    ret = av_bsf_alloc(h264Filter,&h264BsfCtx);
    if(ret < 0)
        return -1;
    avcodec_parameters_copy(h264BsfCtx->par_in,h264Codecpar);
    ret = av_bsf_init(h264BsfCtx);
#endif
#if USE_AAC
    const AVBitStreamFilter *aacFilter  =  av_bsf_get_by_name("aac_adtstoasc");
    AVBSFContext *aacBsfCtx;
    ret = av_bsf_alloc(aacFilter,&aacBsfCtx);
    if(ret < 0)
        return -1;
    avcodec_parameters_copy(aacBsfCtx->par_in,aacCodecpar);
    ret = av_bsf_init(aacBsfCtx);
#endif    

    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file\n");
        goto end;
    }

    while (1) {
        AVStream *in_stream, *out_stream;

        ret = av_read_frame(ifmt_ctx, &pkt);
        if (ret < 0)
            break;

        in_stream  = ifmt_ctx->streams[pkt.stream_index];
        if (pkt.stream_index >= stream_mapping_size ||
            stream_mapping[pkt.stream_index] < 0) {
            av_packet_unref(&pkt);
            continue;
        }

        pkt.stream_index = stream_mapping[pkt.stream_index];
        out_stream = ofmt_ctx->streams[pkt.stream_index];
        log_packet(ifmt_ctx, &pkt, "in");

        /* copy packet */
        pkt.pts = av_rescale_q_rnd(pkt.pts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        pkt.dts = av_rescale_q_rnd(pkt.dts, in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_INF|AV_ROUND_PASS_MINMAX);
        pkt.duration = av_rescale_q(pkt.duration, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        log_packet(ofmt_ctx, &pkt, "out");

#if USE_H264
        if(pkt.stream_index == h264VideoIndex)
        {
            ret = av_bsf_send_packet(h264BsfCtx, &pkt);
            if(ret < 0) {
                printf("bsg_send_packet is error!\n");
                continue;
            }
            ret = av_bsf_receive_packet(h264BsfCtx, &pkt);
            if(ret < 0) {
                printf("bsg_receive_packet is error!\n");
                continue;
            }            
        }
#endif

#if USE_AAC
        if(pkt.stream_index == aacVideoIndex)
        {
            ret = av_bsf_send_packet(aacBsfCtx, &pkt);
            if(ret < 0) {
                printf("bsg_send_packet is error!\n");
                continue;
            }
            ret = av_bsf_receive_packet(aacBsfCtx, &pkt);
            if(ret < 0) {
                printf("bsg_receive_packet is error!\n");
                continue;
            }            
        }
#endif


        ret = av_interleaved_write_frame(ofmt_ctx, &pkt);
        if (ret < 0) {
            fprintf(stderr, "Error muxing packet\n");
            break;
        }
        av_packet_unref(&pkt);
    }

    av_write_trailer(ofmt_ctx);
end:
#if USE_H264
    av_bsf_free(&h264BsfCtx);
#endif
#if USE_AAC
    av_bsf_free(&aacBsfCtx);
#endif

    avformat_close_input(&ifmt_ctx);

    /* close output */
    if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    av_freep(&stream_mapping);

    if (ret < 0 && ret != AVERROR_EOF) {
        fprintf(stderr, "Error occurred: %s\n", av_err2str(ret));
        return 1;
    }

    return 0;
}

参考链接：音视频系列4：新手如何入门ffmpeg（以FLV解码H.264为例） - 知乎