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ffmpeg——avformat_write_header写文件头
2016-10-28 09:48:00         来源:一步的专栏  
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一:介绍

ffmpeg 的 Mux 主要分为 三步操作:

avformat_write_header : 写文件头

av_write_frame/av_interleaved_write_frame: 写packet

av_write_trailer : 写文件尾

本文主要分析 avformat_write_header

 

二:分析

\

avformat_write_header:

 

int avformat_write_header(AVFormatContext *s, AVDictionary **options)
{
    int ret = 0;

	//初始化Mux,一些参数检查和设置
    if ((ret = init_muxer(s, options)) < 0)
        return ret;

	// Write_header
    if (!s->oformat->check_bitstream) {
        ret = write_header_internal(s);
        if (ret < 0)
            goto fail;
    }

	//初始化pts
    if ((ret = init_pts(s)) < 0)
        goto fail;

    if (s->avoid_negative_ts < 0) {
        av_assert2(s->avoid_negative_ts == AVFMT_AVOID_NEG_TS_AUTO);
        if (s->oformat->flags & (AVFMT_TS_NEGATIVE | AVFMT_NOTIMESTAMPS)) {
            s->avoid_negative_ts = 0;
        } else
            s->avoid_negative_ts = AVFMT_AVOID_NEG_TS_MAKE_NON_NEGATIVE;
    }

    return 0;

fail:
    if (s->oformat->deinit)
        s->oformat->deinit(s);
    return ret;
}

来具体看看 avformat_write_header 都具体做了什么:

 

init_muxer (初始化Mux,参数检查与设置):

 

static int init_muxer(AVFormatContext *s, AVDictionary **options)
{
    int ret = 0, i;
    AVStream *st;
    AVDictionary *tmp = NULL;
    AVCodecParameters *par = NULL;
    AVOutputFormat *of = s->oformat;
    const AVCodecDescriptor *desc;
    AVDictionaryEntry *e;

	//通过AVClass接口设置AVFormatContext 的Options参数(一般options为NULL,所以可以忽略)
    if (options)
        av_dict_copy(&tmp, *options, 0);

    if ((ret = av_opt_set_dict(s, &tmp)) < 0)
        goto fail;
	//通过AVClass接口设置AVFormatContext->priv_data的Options参数。
	//(还记得么? priv_data保存的是AVOutputFormat 的预存AVClass接口。例如MOV封装预存的是个MOVMuxContext )
    if (s->priv_data && s->oformat->priv_class && *(const AVClass**)s->priv_data==s->oformat->priv_class &&
        (ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)
        goto fail;

	//新版ffmpeg3.1.4 废除了codec(忽略此部分)
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
    if (s->nb_streams && s->streams[0]->codec->flags & AV_CODEC_FLAG_BITEXACT) {
        if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
#if FF_API_LAVF_BITEXACT
            av_log(s, AV_LOG_WARNING,
                   "Setting the AVFormatContext to bitexact mode, because "
                   "the AVCodecContext is in that mode. This behavior will "
                   "change in the future. To keep the current behavior, set "
                   "AVFormatContext.flags |= AVFMT_FLAG_BITEXACT.\n");
            s->flags |= AVFMT_FLAG_BITEXACT;
#else
            av_log(s, AV_LOG_WARNING,
                   "The AVFormatContext is not in set to bitexact mode, only "
                   "the AVCodecContext. If this is not intended, set "
                   "AVFormatContext.flags |= AVFMT_FLAG_BITEXACT.\n");
#endif
        }
    }
FF_ENABLE_DEPRECATION_WARNINGS
#endif

    // some sanity checks
    if (s->nb_streams == 0 && !(of->flags & AVFMT_NOSTREAMS)) {
        av_log(s, AV_LOG_ERROR, "No streams to mux were specified\n");
        ret = AVERROR(EINVAL);
        goto fail;
    }

	//循环处理每个流通道Stream,完成对codecpar ,time_base 的参数检查
    for (i = 0; i < s->nb_streams; i++) {
        st  = s->streams[i];
        par = st->codecpar;

		//ffmpeg3.1.4 codec 已废除
#if FF_API_LAVF_CODEC_TB && FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        if (!st->time_base.num && st->codec->time_base.num) {
            av_log(s, AV_LOG_WARNING, "Using AVStream.codec.time_base as a "
                   "timebase hint to the muxer is deprecated. Set "
                   "AVStream.time_base instead.\n");
            avpriv_set_pts_info(st, 64, st->codec->time_base.num, st->codec->time_base.den);
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif

		//ffmpeg3.1.4 codec 已废除
#if FF_API_LAVF_AVCTX
FF_DISABLE_DEPRECATION_WARNINGS
        if (st->codecpar->codec_type == AVMEDIA_TYPE_UNKNOWN &&
            st->codec->codec_type    != AVMEDIA_TYPE_UNKNOWN) {
            av_log(s, AV_LOG_WARNING, "Using AVStream.codec to pass codec "
                   "parameters to muxers is deprecated, use AVStream.codecpar "
                   "instead.\n");
            ret = avcodec_parameters_from_context(st->codecpar, st->codec);
            if (ret < 0)
                goto fail;
        }
FF_ENABLE_DEPRECATION_WARNINGS
#endif

        /* update internal context from codecpar, old bsf api needs this
         * FIXME: remove when autobsf uses new bsf API */
         //内部使用,无需关注
        ret = avcodec_parameters_to_context(st->internal->avctx, st->codecpar);
        if (ret < 0)
            goto fail;

		//检查AVStream->time_base 是否被设置,如果没有:
		// 音频设置为: 1/sample_rate    ;    视频设置为:1/90000
        if (!st->time_base.num) {
            /* fall back on the default timebase values */
            if (par->codec_type == AVMEDIA_TYPE_AUDIO && par->sample_rate)
                avpriv_set_pts_info(st, 64, 1, par->sample_rate);
            else
                avpriv_set_pts_info(st, 33, 1, 90000);
        }

		//对音视频参数检查
        switch (par->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            if (par->sample_rate <= 0) {
                av_log(s, AV_LOG_ERROR, "sample rate not set\n");
                ret = AVERROR(EINVAL);
                goto fail;
            }
            if (!par->block_align)
                par->block_align = par->channels *
                                   av_get_bits_per_sample(par->codec_id) >> 3;
            break;
        case AVMEDIA_TYPE_VIDEO:
            if ((par->width <= 0 || par->height <= 0) &&
                !(of->flags & AVFMT_NODIMENSIONS)) {
                av_log(s, AV_LOG_ERROR, "dimensions not set\n");
                ret = AVERROR(EINVAL);
                goto fail;
            }
            if (av_cmp_q(st->sample_aspect_ratio, par->sample_aspect_ratio)
                && fabs(av_q2d(st->sample_aspect_ratio) - av_q2d(par->sample_aspect_ratio)) > 0.004*av_q2d(st->sample_aspect_ratio)
            ) {
                if (st->sample_aspect_ratio.num != 0 &&
                    st->sample_aspect_ratio.den != 0 &&
                    par->sample_aspect_ratio.num != 0 &&
                    par->sample_aspect_ratio.den != 0) {
                    av_log(s, AV_LOG_ERROR, "Aspect ratio mismatch between muxer "
                           "(%d/%d) and encoder layer (%d/%d)\n",
                           st->sample_aspect_ratio.num, st->sample_aspect_ratio.den,
                           par->sample_aspect_ratio.num,
                           par->sample_aspect_ratio.den);
                    ret = AVERROR(EINVAL);
                    goto fail;
                }
            }
            break;
        }

		//检查输出封装(AVOutputFormat)是否支持设定的流编码格式。(在AVOutputFormat支持的编码列表中寻找匹配)
        desc = avcodec_descriptor_get(par->codec_id);
        if (desc && desc->props & AV_CODEC_PROP_REORDER)
            st->internal->reorder = 1;

        if (of->codec_tag) {
            if (   par->codec_tag
                && par->codec_id == AV_CODEC_ID_RAWVIDEO
                && (   av_codec_get_tag(of->codec_tag, par->codec_id) == 0
                    || av_codec_get_tag(of->codec_tag, par->codec_id) == MKTAG('r', 'a', 'w', ' '))
                && !validate_codec_tag(s, st)) {
                // the current rawvideo encoding system ends up setting
                // the wrong codec_tag for avi/mov, we override it here
                par->codec_tag = 0;
            }
            if (par->codec_tag) {
                if (!validate_codec_tag(s, st)) {
                    char tagbuf[32], tagbuf2[32];
                    av_get_codec_tag_string(tagbuf, sizeof(tagbuf), par->codec_tag);
                    av_get_codec_tag_string(tagbuf2, sizeof(tagbuf2), av_codec_get_tag(s->oformat->codec_tag, par->codec_id));
                    av_log(s, AV_LOG_ERROR,
                           "Tag %s/0x%08x incompatible with output codec id '%d' (%s)\n",
                           tagbuf, par->codec_tag, par->codec_id, tagbuf2);
                    ret = AVERROR_INVALIDDATA;
                    goto fail;
                }
            } else
                par->codec_tag = av_codec_get_tag(of->codec_tag, par->codec_id);
        }

        if (par->codec_type != AVMEDIA_TYPE_ATTACHMENT)
            s->internal->nb_interleaved_streams++;
    }

	//此处实际上在前面已经做过
    if (!s->priv_data && of->priv_data_size > 0) {
        s->priv_data = av_mallocz(of->priv_data_size);
        if (!s->priv_data) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        if (of->priv_class) {
            *(const AVClass **)s->priv_data = of->priv_class;
            av_opt_set_defaults(s->priv_data);
            if ((ret = av_opt_set_dict2(s->priv_data, &tmp, AV_OPT_SEARCH_CHILDREN)) < 0)
                goto fail;
        }
    }

	//metadata 的设置(根据需要设置)
    /* set muxer identification string */
    if (!(s->flags & AVFMT_FLAG_BITEXACT)) {
        av_dict_set(&s->metadata, "encoder", LIBAVFORMAT_IDENT, 0);
    } else {
        av_dict_set(&s->metadata, "encoder", NULL, 0);
    }

    for (e = NULL; e = av_dict_get(s->metadata, "encoder-", e, AV_DICT_IGNORE_SUFFIX); ) {
        av_dict_set(&s->metadata, e->key, NULL, 0);
    }

    if (options) {
         av_dict_free(options);
         *options = tmp;
    }

    if (s->oformat->init && (ret = s->oformat->init(s)) < 0) {
        if (s->oformat->deinit)
            s->oformat->deinit(s);
        goto fail;
    }

    return 0;

fail:
    av_dict_free(&tmp);
    return ret;
}

 

write_header_internal (写文件头):

 

static int write_header_internal(AVFormatContext *s)
{
    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_HEADER);
    if (s->oformat->write_header) {
        int ret = s->oformat->write_header(s);
        if (ret >= 0 && s->pb && s->pb->error < 0)
            ret = s->pb->error;
        s->internal->write_header_ret = ret;
        if (ret < 0)
            return ret;
        if (s->flush_packets && s->pb && s->pb->error >= 0 && s->flags & AVFMT_FLAG_FLUSH_PACKETS)
            avio_flush(s->pb);
    }
    s->internal->header_written = 1;
    if (!(s->oformat->flags & AVFMT_NOFILE) && s->pb)
        avio_write_marker(s->pb, AV_NOPTS_VALUE, AVIO_DATA_MARKER_UNKNOWN);
    return 0;
}
可见 writer_header_internal 直接调用匹配的 AVOutputFormat 的 write_header方法。因此,具体的 write_header 方法由各种封装自己定义并实现。(例如MOV封装:avformat_write_header——>write_header_internal——>AVOutputFormat(MOV)——>mov_write_header)。

 

priv_pts (设置AVStream->priv_pts):

 

static int init_pts(AVFormatContext *s)
{
    int i;
    AVStream *st;

    /* init PTS generation */
    for (i = 0; i < s->nb_streams; i++) {
        int64_t den = AV_NOPTS_VALUE;
        st = s->streams[i];

        switch (st->codecpar->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            den = (int64_t)st->time_base.num * st->codecpar->sample_rate;
            break;
        case AVMEDIA_TYPE_VIDEO:
            den = (int64_t)st->time_base.num * st->time_base.den;
            break;
        default:
            break;    
        }

        if (!st->priv_pts)
            st->priv_pts = av_mallocz(sizeof(*st->priv_pts));
        if (!st->priv_pts)
            return AVERROR(ENOMEM);

        if (den != AV_NOPTS_VALUE) {
            if (den <= 0)
                return AVERROR_INVALIDDATA;

            frac_init(st->priv_pts, 0, 0, den);
        }
    }

    return 0;
}
此处设置的是priv_pts(将后续使用)。
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