1.获取数据

ffmpeg读取mp4中的H264数据,并不能直接得到NALU,There is also no saving in the file0x00000001的分隔符.下面这张图为packet.data中的数据

从图中可以发现,packet中的数据起始处没有分隔符(0x00000001), 也不是0x65、0x67、0x68、0x41等字节,所以可以肯定这不是标准的nalu.

其实,前4个字0x000032ce表示的是nalu的长度,从第5个字节开始才是nalu的数据.所以直接将前4个字节替换为0x00000001即可得到标准的nalu数据.

2.获取pps及sps

pps及sps不能从packet获得,而是保存在AVCodecContext的extradata数据域中.如下：

如何从extradata中解析出sps及pps呢？ffmpegA stream filter is provided in "h264_mp4toannexb"完成这项工作,关键代码如下

//h264_mp4toannexb_bsf.c
static int h264_mp4toannexb_filter(AVBitStreamFilterContext *bsfc,
                                   AVCodecContext *avctx, const char *args,
                                   uint8_t  **poutbuf, int *poutbuf_size,
                                   const uint8_t *buf, int      buf_size,
                                   int keyframe) {
    H264BSFContext *ctx = bsfc->priv_data;
    uint8_t unit_type;
    int32_t nal_size;
    uint32_t cumul_size = 0;
    const uint8_t *buf_end = buf + buf_size;
 
 
    /* nothing to filter */
    if (!avctx->extradata || avctx->extradata_size < 6) {
        *poutbuf = (uint8_t*) buf;
        *poutbuf_size = buf_size;
        return 0;
    }
    
    //
    //从extradata中分析出SPS、PPS
    //
    /* retrieve sps and pps NAL units from extradata */
    if (!ctx->extradata_parsed) {
        uint16_t unit_size;
        uint64_t total_size = 0;
        uint8_t *out = NULL, unit_nb, sps_done = 0, sps_seen = 0, pps_seen = 0;
        const uint8_t *extradata = avctx->extradata+4;  //跳过前4个字节
        static const uint8_t nalu_header[4] = {0, 0, 0, 1};
 
 
        /* retrieve length coded size */
        ctx->length_size = (*extradata++ & 0x3) + 1;    //用于指示表示编码数据长度所需字节数
        if (ctx->length_size == 3)
            return AVERROR(EINVAL);
 
 
        /* retrieve sps and pps unit(s) */
        unit_nb = *extradata++ & 0x1f; /* number of sps unit(s) */
        if (!unit_nb) {
            goto pps;
        } else {
            sps_seen = 1;
        }
 
 
        while (unit_nb--) {
            void *tmp;
 
 
            unit_size = AV_RB16(extradata);
            total_size += unit_size+4;
            if (total_size > INT_MAX - FF_INPUT_BUFFER_PADDING_SIZE ||
                extradata+2+unit_size > avctx->extradata+avctx->extradata_size) {
                av_free(out);
                return AVERROR(EINVAL);
            }
            tmp = av_realloc(out, total_size + FF_INPUT_BUFFER_PADDING_SIZE);
            if (!tmp) {
                av_free(out);
                return AVERROR(ENOMEM);
            }
            out = tmp;
            memcpy(out+total_size-unit_size-4, nalu_header, 4);
            memcpy(out+total_size-unit_size,   extradata+2, unit_size);
            extradata += 2+unit_size;
pps:
            if (!unit_nb && !sps_done++) {
                unit_nb = *extradata++; /* number of pps unit(s) */
                if (unit_nb)
                    pps_seen = 1;
            }
        }
 
 
        if(out)
            memset(out + total_size, 0, FF_INPUT_BUFFER_PADDING_SIZE);
 
 
        if (!sps_seen)
            av_log(avctx, AV_LOG_WARNING, "Warning: SPS NALU missing or invalid. The resulting stream may not play.\n");
        if (!pps_seen)
            av_log(avctx, AV_LOG_WARNING, "Warning: PPS NALU missing or invalid. The resulting stream may not play.\n");
 
 
        av_free(avctx->extradata);
        avctx->extradata      = out;
        avctx->extradata_size = total_size;
        ctx->first_idr        = 1;
        ctx->extradata_parsed = 1;
    }
 
 
    *poutbuf_size = 0;
    *poutbuf = NULL;
    do {
        if (buf + ctx->length_size > buf_end)
            goto fail;  //buf为NULL时,The following code will no longer execute
 
 
        //
        //The number of bytes used to hold the data length,is analyzing theextradata计算出来的
        //
        if (ctx->length_size == 1) {
            nal_size = buf[0];
        } else if (ctx->length_size == 2) {
            nal_size = AV_RB16(buf);
        } else
            nal_size = AV_RB32(buf);
 
 
        buf += ctx->length_size;
        unit_type = *buf & 0x1f;
 
 
        if (buf + nal_size > buf_end || nal_size < 0)
            goto fail;
 
 
        /* prepend only to the first type 5 NAL unit of an IDR picture */
        if (ctx->first_idr && unit_type == 5) {
            //
            //copy IDR 帧时,需要将sps及pps一同拷贝
            //
            if (alloc_and_copy(poutbuf, poutbuf_size,
                               avctx->extradata, avctx->extradata_size,
                               buf, nal_size) < 0)
                goto fail;
            ctx->first_idr = 0;
        } else {
            //
            //非IDR帧,没有sps及pps
            if (alloc_and_copy(poutbuf, poutbuf_size,
                               NULL, 0,
                               buf, nal_size) < 0)
                goto fail;
            if (!ctx->first_idr && unit_type == 1)
                ctx->first_idr = 1;
        }
 
 
        buf += nal_size;
        cumul_size += nal_size + ctx->length_size;
    } while (cumul_size < buf_size);
 
 
    return 1;
 
 
fail:
    av_freep(poutbuf);
    *poutbuf_size = 0;
    return AVERROR(EINVAL);
}

一般情况下,extradata中包含一个sps、一个pps 的nalu, It's easy to see from the code aboveextradata的数据格式.分析后的sps及ppsstill storedextradata域中,and added a starter.从代码中还可以看出,The above function will dosps、pps及packet中的数据,都copy到poutbuf指示的内存中,如果不需要copy到指定内存,直接给bufThe parameter can be passed a null value.

本文福利, C++音视频学习资料包、技术视频,内容包括（音视频开发,面试题,FFmpeg ,webRTC ,rtmp ,hls ,rtsp ,ffplay ,srs）↓↓↓↓↓↓见下面↓↓文章底部点击领取↓↓

3.使用ffmpegThe stream filter getssps及pps

流过滤器"h264_mp4toannexb", 在av_register_all()function will be registered.用法示例如下：

int ParseH264ExtraDataInMp4(int stream_id)
{
    uint8_t *dummy = NULL;
    int dummy_size;
    AVBitStreamFilterContext* bsfc =  av_bitstream_filter_init("h264_mp4toannexb");
 
 
    if(bsfc == NULL)
    {
        return -1;
    }
 
 
    av_bitstream_filter_filter(
            bsfc, format_ctx_->streams[stream_id]->codec, NULL, &dummy, &dummy_size, NULL, 0, 0);
 

 
    av_bitstream_filter_close(bsfc);
 
 
    return 0;
}

 本文福利, C++音视频学习资料包、技术视频,内容包括（音视频开发,面试题,FFmpeg ,webRTC ,rtmp ,hls ,rtsp ,ffplay ,srs）↓↓↓↓↓↓见下面↓↓文章底部点击领取↓↓