[FFMPEG开发]FFMPEG中的pts与音视频同步的关系以及编解码过程中的注意事项

最近几个月在win上做音视频开发，基本上都是围绕着ffmpeg库进行的工作。其中一个问题就是音画不同步，由于本人是新手一枚，也不太清楚是什么原因，翻看了雷神的多篇博客也没有详细的解释。看着前辈留下的代码，只好一脸懵逼去看ffmpeg的源码，然后分析bug以及怎么修复。最后看到了一个东西pts，关于pts这篇文章介绍的十分仔细ffmpeg的pts概念。但是对于初学的我来说还是不太清楚怎么去跟开发对应。所以我就整理了下，在做音视频模块中什么地方会比较频繁遇到pts。废话就不说了，直接进入开发者模式假设我们需要对一个视频进行处理，然后产生一个输出。

input_video->decode->encode->output_video。通常都需要进行一次这样的处理。在ffmpeg中这几个过程密切相关的有容器有AVFormatContext、AVStream、AVCodecContext。这是跟音视频同步相关的主角。
另外接触ffmpeg建议从这几个简单的官方demo入手，毕竟简单做起是最好ffmpeg官方的demo这里就通过transcoding.c进行一个介绍吧(代码比较长，如果已经比较有基础，直接跳到图文那部分)

#include 
#include 
#include 
#include 
#include 
#include 

static AVFormatContext *ifmt_ctx;
static AVFormatContext *ofmt_ctx;
typedef struct FilteringContext {
    AVFilterContext *buffersink_ctx;
    AVFilterContext *buffersrc_ctx;
    AVFilterGraph *filter_graph;
} FilteringContext;
static FilteringContext *filter_ctx;

typedef struct StreamContext {
    AVCodecContext *dec_ctx;
    AVCodecContext *enc_ctx;
} StreamContext;
static StreamContext *stream_ctx;
//输入流打开，给ifmt_ctx赋值，并且打开解码器
static int open_input_file(const char *filename)
{
    int ret;
    unsigned int i;

    ifmt_ctx = NULL;
    if ((ret = avformat_open_input(&ifmt_ctx, filename, NULL, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot open input file\n");
        return ret;
    }

    if ((ret = avformat_find_stream_info(ifmt_ctx, NULL)) < 0) {
        av_log(NULL, AV_LOG_ERROR, "Cannot find stream information\n");
        return ret;
    }

    stream_ctx = av_mallocz_array(ifmt_ctx->nb_streams, sizeof(*stream_ctx));
    if (!stream_ctx)
        return AVERROR(ENOMEM);

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        AVStream *stream = ifmt_ctx->streams[i];
        AVCodec *dec = avcodec_find_decoder(stream->codecpar->codec_id);
        AVCodecContext *codec_ctx;
        if (!dec) {
            av_log(NULL, AV_LOG_ERROR, "Failed to find decoder for stream #%u\n", i);
            return AVERROR_DECODER_NOT_FOUND;
        }
        codec_ctx = avcodec_alloc_context3(dec);
        if (!codec_ctx) {
            av_log(NULL, AV_LOG_ERROR, "Failed to allocate the decoder context for stream #%u\n", i);
            return AVERROR(ENOMEM);
        }
        ret = avcodec_parameters_to_context(codec_ctx, stream->codecpar);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Failed to copy decoder parameters to input decoder context "
                   "for stream #%u\n", i);
            return ret;
        }
        /* Reencode video & audio and remux subtitles etc. */
        if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO
                || codec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            if (codec_ctx->codec_type == AVMEDIA_TYPE_VIDEO)
                codec_ctx->framerate = av_guess_frame_rate(ifmt_ctx, stream, NULL);
            /* Open decoder */
            ret = avcodec_open2(codec_ctx, dec, NULL);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to open decoder for stream #%u\n", i);
                return ret;
            }
        }
        stream_ctx[i].dec_ctx = codec_ctx;
    }

    av_dump_format(ifmt_ctx, 0, filename, 0);
    return 0;
}
//打开输出管理器，并通过解码器给编码器进行设置
static int open_output_file(const char *filename)
{
    AVStream *out_stream;
    AVStream *in_stream;
    AVCodecContext *dec_ctx, *enc_ctx;
    AVCodec *encoder;
    int ret;
    unsigned int i;

    ofmt_ctx = NULL;
    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, filename);
    if (!ofmt_ctx) {
        av_log(NULL, AV_LOG_ERROR, "Could not create output context\n");
        return AVERROR_UNKNOWN;
    }


    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        out_stream = avformat_new_stream(ofmt_ctx, NULL);
        if (!out_stream) {
            av_log(NULL, AV_LOG_ERROR, "Failed allocating output stream\n");
            return AVERROR_UNKNOWN;
        }

        in_stream = ifmt_ctx->streams[i];
        dec_ctx = stream_ctx[i].dec_ctx;

        if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO
                || dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
            /* in this example, we choose transcoding to same codec */
            encoder = avcodec_find_encoder(dec_ctx->codec_id);
            if (!encoder) {
                av_log(NULL, AV_LOG_FATAL, "Necessary encoder not found\n");
                return AVERROR_INVALIDDATA;
            }
            enc_ctx = avcodec_alloc_context3(encoder);
            if (!enc_ctx) {
                av_log(NULL, AV_LOG_FATAL, "Failed to allocate the encoder context\n");
                return AVERROR(ENOMEM);
            }

            /* In this example, we transcode to same properties (picture size,
             * sample rate etc.). These properties can be changed for output
             * streams easily using filters */
            if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
                enc_ctx->height = dec_ctx->height;
                enc_ctx->width = dec_ctx->width;
                enc_ctx->sample_aspect_ratio = dec_ctx->sample_aspect_ratio;
                /* take first format from list of supported formats */
                if (encoder->pix_fmts)
                    enc_ctx->pix_fmt = encoder->pix_fmts[0];
                else
                    enc_ctx->pix_fmt = dec_ctx->pix_fmt;
                /* video time_base can be set to whatever is handy and supported by encoder */
                enc_ctx->time_base = av_inv_q(dec_ctx->framerate);
            } else {
                enc_ctx->sample_rate = dec_ctx->sample_rate;
                enc_ctx->channel_layout = dec_ctx->channel_layout;
                enc_ctx->channels = av_get_channel_layout_nb_channels(enc_ctx->channel_layout);
                /* take first format from list of supported formats */
                enc_ctx->sample_fmt = encoder->sample_fmts[0];
                enc_ctx->time_base = (AVRational){1, enc_ctx->sample_rate};
            }

            if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
                enc_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

            /* Third parameter can be used to pass settings to encoder */
            ret = avcodec_open2(enc_ctx, encoder, NULL);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Cannot open video encoder for stream #%u\n", i);
                return ret;
            }
            ret = avcodec_parameters_from_context(out_stream->codecpar, enc_ctx);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Failed to copy encoder parameters to output stream #%u\n", i);
                return ret;
            }

            out_stream->time_base = enc_ctx->time_base;
            stream_ctx[i].enc_ctx = enc_ctx;
        } else if (dec_ctx->codec_type == AVMEDIA_TYPE_UNKNOWN) {
            av_log(NULL, AV_LOG_FATAL, "Elementary stream #%d is of unknown type, cannot proceed\n", i);
            return AVERROR_INVALIDDATA;
        } else {
            /* if this stream must be remuxed */
            ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);
            if (ret < 0) {
                av_log(NULL, AV_LOG_ERROR, "Copying parameters for stream #%u failed\n", i);
                return ret;
            }
            out_stream->time_base = in_stream->time_base;
        }

    }
    av_dump_format(ofmt_ctx, 0, filename, 1);

    if (!(ofmt_ctx->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Could not open output file '%s'", filename);
            return ret;
        }
    }

    /* init muxer, write output file header */
    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error occurred when opening output file\n");
        return ret;
    }

    return 0;
}
//初始化过滤器，其实不要也行，因为过滤器涉及操作不多
static int init_filter(FilteringContext* fctx, AVCodecContext *dec_ctx,
        AVCodecContext *enc_ctx, const char *filter_spec)
{
    char args[512];
    int ret = 0;
    const AVFilter *buffersrc = NULL;
    const AVFilter *buffersink = NULL;
    AVFilterContext *buffersrc_ctx = NULL;
    AVFilterContext *buffersink_ctx = NULL;
    AVFilterInOut *outputs = avfilter_inout_alloc();
    AVFilterInOut *inputs  = avfilter_inout_alloc();
    AVFilterGraph *filter_graph = avfilter_graph_alloc();

    if (!outputs || !inputs || !filter_graph) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    if (dec_ctx->codec_type == AVMEDIA_TYPE_VIDEO) {
        buffersrc = avfilter_get_by_name("buffer");
        buffersink = avfilter_get_by_name("buffersink");
        if (!buffersrc || !buffersink) {
            av_log(NULL, AV_LOG_ERROR, "filtering source or sink element not found\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        snprintf(args, sizeof(args),
                "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
                dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
                dec_ctx->time_base.num, dec_ctx->time_base.den,
                dec_ctx->sample_aspect_ratio.num,
                dec_ctx->sample_aspect_ratio.den);

        ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                args, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create buffer source\n");
            goto end;
        }

        ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
                NULL, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create buffer sink\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "pix_fmts",
                (uint8_t*)&enc_ctx->pix_fmt, sizeof(enc_ctx->pix_fmt),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output pixel format\n");
            goto end;
        }
    } else if (dec_ctx->codec_type == AVMEDIA_TYPE_AUDIO) {
        buffersrc = avfilter_get_by_name("abuffer");
        buffersink = avfilter_get_by_name("abuffersink");
        if (!buffersrc || !buffersink) {
            av_log(NULL, AV_LOG_ERROR, "filtering source or sink element not found\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        if (!dec_ctx->channel_layout)
            dec_ctx->channel_layout =
                av_get_default_channel_layout(dec_ctx->channels);
        snprintf(args, sizeof(args),
                "time_base=%d/%d:sample_rate=%d:sample_fmt=%s:channel_layout=0x%"PRIx64,
                dec_ctx->time_base.num, dec_ctx->time_base.den, dec_ctx->sample_rate,
                av_get_sample_fmt_name(dec_ctx->sample_fmt),
                dec_ctx->channel_layout);
        ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                args, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create audio buffer source\n");
            goto end;
        }

        ret = avfilter_graph_create_filter(&buffersink_ctx, buffersink, "out",
                NULL, NULL, filter_graph);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot create audio buffer sink\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "sample_fmts",
                (uint8_t*)&enc_ctx->sample_fmt, sizeof(enc_ctx->sample_fmt),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output sample format\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "channel_layouts",
                (uint8_t*)&enc_ctx->channel_layout,
                sizeof(enc_ctx->channel_layout), AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output channel layout\n");
            goto end;
        }

        ret = av_opt_set_bin(buffersink_ctx, "sample_rates",
                (uint8_t*)&enc_ctx->sample_rate, sizeof(enc_ctx->sample_rate),
                AV_OPT_SEARCH_CHILDREN);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Cannot set output sample rate\n");
            goto end;
        }
    } else {
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    /* Endpoints for the filter graph. */
    outputs->name       = av_strdup("in");
    outputs->filter_ctx = buffersrc_ctx;
    outputs->pad_idx    = 0;
    outputs->next       = NULL;

    inputs->name       = av_strdup("out");
    inputs->filter_ctx = buffersink_ctx;
    inputs->pad_idx    = 0;
    inputs->next       = NULL;

    if (!outputs->name || !inputs->name) {
        ret = AVERROR(ENOMEM);
        goto end;
    }

    if ((ret = avfilter_graph_parse_ptr(filter_graph, filter_spec,
                    &inputs, &outputs, NULL)) < 0)
        goto end;

    if ((ret = avfilter_graph_config(filter_graph, NULL)) < 0)
        goto end;

    /* Fill FilteringContext */
    fctx->buffersrc_ctx = buffersrc_ctx;
    fctx->buffersink_ctx = buffersink_ctx;
    fctx->filter_graph = filter_graph;

end:
    avfilter_inout_free(&inputs);
    avfilter_inout_free(&outputs);

    return ret;
}

static int init_filters(void)
{
    const char *filter_spec;
    unsigned int i;
    int ret;
    filter_ctx = av_malloc_array(ifmt_ctx->nb_streams, sizeof(*filter_ctx));
    if (!filter_ctx)
        return AVERROR(ENOMEM);

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        filter_ctx[i].buffersrc_ctx  = NULL;
        filter_ctx[i].buffersink_ctx = NULL;
        filter_ctx[i].filter_graph   = NULL;
        if (!(ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO
                || ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO))
            continue;


        if (ifmt_ctx->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO)
            filter_spec = "null"; /* passthrough (dummy) filter for video */
        else
            filter_spec = "anull"; /* passthrough (dummy) filter for audio */
        ret = init_filter(&filter_ctx[i], stream_ctx[i].dec_ctx,
                stream_ctx[i].enc_ctx, filter_spec);
        if (ret)
            return ret;
    }
    return 0;
}
//编码并输出
static int encode_write_frame(AVFrame *filt_frame, unsigned int stream_index, int *got_frame) {
    int ret;
    int got_frame_local;
    AVPacket enc_pkt;
    int (*enc_func)(AVCodecContext *, AVPacket *, const AVFrame *, int *) =
        (ifmt_ctx->streams[stream_index]->codecpar->codec_type ==
         AVMEDIA_TYPE_VIDEO) ? avcodec_encode_video2 : avcodec_encode_audio2;

    if (!got_frame)
        got_frame = &got_frame_local;

    av_log(NULL, AV_LOG_INFO, "Encoding frame\n");
    /* encode filtered frame */
    enc_pkt.data = NULL;
    enc_pkt.size = 0;
    av_init_packet(&enc_pkt);
    ret = enc_func(stream_ctx[stream_index].enc_ctx, &enc_pkt,
            filt_frame, got_frame);
    av_frame_free(&filt_frame);
    if (ret < 0)
        return ret;
    if (!(*got_frame))
        return 0;

    /* prepare packet for muxing */
    enc_pkt.stream_index = stream_index;
    av_packet_rescale_ts(&enc_pkt,
                         stream_ctx[stream_index].enc_ctx->time_base,
                         ofmt_ctx->streams[stream_index]->time_base);

    av_log(NULL, AV_LOG_DEBUG, "Muxing frame\n");
    /* mux encoded frame */
    ret = av_interleaved_write_frame(ofmt_ctx, &enc_pkt);
    return ret;
}
//发送到过滤器进行再编码
static int filter_encode_write_frame(AVFrame *frame, unsigned int stream_index)
{
    int ret;
    AVFrame *filt_frame;

    av_log(NULL, AV_LOG_INFO, "Pushing decoded frame to filters\n");
    /* push the decoded frame into the filtergraph */
    ret = av_buffersrc_add_frame_flags(filter_ctx[stream_index].buffersrc_ctx,
            frame, 0);
    if (ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Error while feeding the filtergraph\n");
        return ret;
    }

    /* pull filtered frames from the filtergraph */
    while (1) {
        filt_frame = av_frame_alloc();
        if (!filt_frame) {
            ret = AVERROR(ENOMEM);
            break;
        }
        av_log(NULL, AV_LOG_INFO, "Pulling filtered frame from filters\n");
        ret = av_buffersink_get_frame(filter_ctx[stream_index].buffersink_ctx,
                filt_frame);
        if (ret < 0) {
            /* if no more frames for output - returns AVERROR(EAGAIN)
             * if flushed and no more frames for output - returns AVERROR_EOF
             * rewrite retcode to 0 to show it as normal procedure completion
             */
            if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
                ret = 0;
            av_frame_free(&filt_frame);
            break;
        }

        filt_frame->pict_type = AV_PICTURE_TYPE_NONE;
        ret = encode_write_frame(filt_frame, stream_index, NULL);
        if (ret < 0)
            break;
    }

    return ret;
}
//刷出编码器剩余数据
static int flush_encoder(unsigned int stream_index)
{
    int ret;
    int got_frame;

    if (!(stream_ctx[stream_index].enc_ctx->codec->capabilities &
                AV_CODEC_CAP_DELAY))
        return 0;

    while (1) {
        av_log(NULL, AV_LOG_INFO, "Flushing stream #%u encoder\n", stream_index);
        ret = encode_write_frame(NULL, stream_index, &got_frame);
        if (ret < 0)
            break;
        if (!got_frame)
            return 0;
    }
    return ret;
}

int main(int argc, char **argv)
{
    int ret;
    AVPacket packet = { .data = NULL, .size = 0 };
    AVFrame *frame = NULL;
    enum AVMediaType type;
    unsigned int stream_index;
    unsigned int i;
    int got_frame;
    int (*dec_func)(AVCodecContext *, AVFrame *, int *, const AVPacket *);

    if (argc != 3) {
        av_log(NULL, AV_LOG_ERROR, "Usage: %s  
\n", argv[0]);
        return 1;
    }

    if ((ret = open_input_file(argv[1])) < 0)
        goto end;
    if ((ret = open_output_file(argv[2])) < 0)
        goto end;
    if ((ret = init_filters()) < 0)
        goto end;

    /* read all packets */
    while (1) {
        if ((ret = av_read_frame(ifmt_ctx, &packet)) < 0)
            break;
        stream_index = packet.stream_index;
        type = ifmt_ctx->streams[packet.stream_index]->codecpar->codec_type;
        av_log(NULL, AV_LOG_DEBUG, "Demuxer gave frame of stream_index %u\n",
                stream_index);

        if (filter_ctx[stream_index].filter_graph) {
            av_log(NULL, AV_LOG_DEBUG, "Going to reencode&filter the frame\n");
            frame = av_frame_alloc();
            if (!frame) {
                ret = AVERROR(ENOMEM);
                break;
            }
            av_packet_rescale_ts(&packet,
                                 ifmt_ctx->streams[stream_index]->time_base,
                                 stream_ctx[stream_index].dec_ctx->time_base);
            dec_func = (type == AVMEDIA_TYPE_VIDEO) ? avcodec_decode_video2 :
                avcodec_decode_audio4;
            ret = dec_func(stream_ctx[stream_index].dec_ctx, frame,
                    &got_frame, &packet);
            if (ret < 0) {
                av_frame_free(&frame);
                av_log(NULL, AV_LOG_ERROR, "Decoding failed\n");
                break;
            }

            if (got_frame) {
                frame->pts = frame->best_effort_timestamp;
                ret = filter_encode_write_frame(frame, stream_index);
                av_frame_free(&frame);
                if (ret < 0)
                    goto end;
            } else {
                av_frame_free(&frame);
            }
        } else {
            /* remux this frame without reencoding */
            av_packet_rescale_ts(&packet,
                                 ifmt_ctx->streams[stream_index]->time_base,
                                 ofmt_ctx->streams[stream_index]->time_base);

            ret = av_interleaved_write_frame(ofmt_ctx, &packet);
            if (ret < 0)
                goto end;
        }
        av_packet_unref(&packet);
    }

    /* flush filters and encoders */
    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        /* flush filter */
        if (!filter_ctx[i].filter_graph)
            continue;
        ret = filter_encode_write_frame(NULL, i);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Flushing filter failed\n");
            goto end;
        }

        /* flush encoder */
        ret = flush_encoder(i);
        if (ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Flushing encoder failed\n");
            goto end;
        }
    }

    av_write_trailer(ofmt_ctx);
end:
    av_packet_unref(&packet);
    av_frame_free(&frame);
    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
        avcodec_free_context(&stream_ctx[i].dec_ctx);
        if (ofmt_ctx && ofmt_ctx->nb_streams > i && ofmt_ctx->streams[i] && stream_ctx[i].enc_ctx)
            avcodec_free_context(&stream_ctx[i].enc_ctx);
        if (filter_ctx && filter_ctx[i].filter_graph)
            avfilter_graph_free(&filter_ctx[i].filter_graph);
    }
    av_free(filter_ctx);
    av_free(stream_ctx);
    avformat_close_input(&ifmt_ctx);
    if (ofmt_ctx && !(ofmt_ctx->oformat->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    if (ret < 0)
        av_log(NULL, AV_LOG_ERROR, "Error occurred: %s\n", av_err2str(ret));

    return ret ? 1 : 0;
}

首先是AVFormatContext这里有两个相关的ifmt_ctx，ofmt_ctx，我们姑且就当作文件组织管理器吧(个人感觉这样比较合适，因为使用到这个内容多数是直接关联文件的)然后是每个fmt_ctx都有一个或者多个AVStream，这个是音频/视频流的管理结构，最后是每个stream里都有一个AVCodecContext这个是编解码器的管理器。通常在ffmpeg中能解码的都能编码。所以这个既可以做为编码器使用，也能做为解码器使用(我指的是这个结构体，千万不要以为都是同格式的编码器就可以直接用编码器解码，说多了都是泪)总体结构如下:关于这些东西的初始化官方例子可以研究下，比较基础就不解说了。
然后就是我们进行编解码，无论是直播流还是文件流。我们都需要一个函数av_read_frame()从这个函数中我们会接触第一个pts在获取的AVPacket的pts会跳来跳去，忽大忽小，这很正常，因为这个文件并不止一个流。 这个packet中有一个stream_index，有了这个流索引，我们可以换算出这个包是在视频中什么位置播放

av_read_frame(ifmt_ctx, &packet)
int virtualpts=packet.pts;
int realpts=av_rescale_q(virtualpts,ifmt_ctx->streams[packet.stream_index]->time_base,{1,AV_TIME_BASE});

通过这样的换算，我们可以获知这个音频/视频包在什么时候进行播放的(该函数换算出来的是微秒)。这是第一个会遇到pts的地方。进行Decode进行音视频解码，我们可以获得第二个pts，AVFrame->pts这个其实是从packet获取的，所以本质还是由packet引导换算的方法也是跟packet的一样。然后我们在Encode的时候会用到第三个pts这个pts其实是跟frame->pts是一致的，也没什么太大问题。如果要做一些控制需要对frame->pts进行设置(转封装)，接着就是第四个pts这个pts会在av_write_frame里使用到，用来将封装好的packet写到输出文件中。那么要进行音视频同步，同帧率同格式只需要进行以下操作(音频同理音频转封装需要重采样，视频跨像素格式需要重调整)。这样封装出来的视频是音视频同步的，不会出现异常。这也是ffmpeg demo普遍的做法，但是当你想用来转格式的时候(完成重采样跟像素调整了)就会出问题，有时候转出来的视频视频会出现马赛克，音频会偶尔出现杂音(关于杂音去除的算法比较复杂下期再说，不过这也跟pts有关)，这很正常。
首先说说马赛克
视频编码中有ibp帧的概念，其中只有i帧是完整数据的，其余都是根据i帧进行解码的，那么假设，在跨格式的时候，AVStream->time_base不一致了会出现什么?比如1:25转1:15，假设pts每次都加1，这时候会有10帧跟其他帧的pts重复，这时候问题就出现了，重复的帧会被做什么？不会做什么，只是你写不进去，因为重复的包肯定是不能写的，恰巧重复的包中有i帧，那完了，后续的帧肯定马赛克了。这种情况应该怎么处理。
预知编码后包所在位置，合理丢帧
有时候转格式了输入流的time_base分母可能比输出流的time_base分母大，这时候可能会出现帧覆盖，解码器解码的帧不发到编码器进行编码就可以解决了，至于预知，就是对当前编码到的pts进行记录，然后解码完成后对pts进行换算，如果重复了就不发送到编码器。如图所示合理丢弃会编码重复的帧就可以解决马赛克了。然后接下来是第二个问题设置了编码器为15帧，输出的视频还是跟原视频一样是25帧
这个问题是因为stream->time_base跟videoencodectx->time_base不一致的原因，处理方法也是合理丢帧，不过我们刚刚丢帧是为了丢编码后会出问题的帧，现在丢的是不符合帧率的帧
现在会进行合理丢帧，不过动态帧率的视频可能不行，原因比较复杂，不进行解释了。
第三个问题是跟设置的码率严重不一致(部分格式可能真的没办法控制)
这个原因是部分编码器是根据pts来进行码率控制的，而我们换算pts是最后才进行换算的，如果你需要控制码率则最好在发给编码器的时候就改pts，那么流程就变成这样
这样同步异常基本上就解决了。关于音频出现杂音的问题比较复杂，我下次有空再进行分解。