一 命令行

ffmpeg -i 001.mp4 -codec copy -bsf: h264_mp4toannexb -f h264 001.h264

-i 2018.mp4: 是输入的MP4文件

-codec copy: 从mp4中拷贝

-bsf: h264_mp4toannexb: 从mp4拷贝到annexB封装

-f h264: 采用h264格式

tmp.264: 输出的文件

然后
ffplay 001.h264
就可以了 为啥可以直接播放呢? 不要说明一些视频信息吗?

因为H264的 sps 和 pps 中已经封装了各项信息了 (具体怎么获取sps pps 分析就先不写了 先弄实现 )
这个博主写的也不错可以看看

二 目前我们需要知道的(H264)

为啥要说这些? 因为不知道看不懂代码
一 H264原始码流是由一个个NALU单元组成 每个NALU单元基本就是一帧
二 怎么区分每个NALU单元呢?
	用Startcode 
三 怎么分辨关键帧呢 ?
	NALU 单元的第五字节(前四个说明视频帧长度)可以分辨不同的类型  比如关键帧是5 sps7 pps 8 
四 关键帧startcode有什么区别吗
	如果是关键帧 startcode为 0x000001 + NALU  header(1字节 x5 x大于0)    4字节
	如果是关键帧 startcode 为 0x00000001+NALU  header (1字节 x+不是5)  5字节
五 为啥又要说关键帧又要说sps pps
	sps是一组帧编码序列等数据的参数记录
	pps是每个帧编码数据的参数记录
	一般一个小视频文件(分辨率 时间机 等什么信息不改变)中一组sps 和 pps  就足够了 但是直播中为了保证网络的健壮性(sps pps 真的很小 不影响带宽)所以在每个关键帧前面都会加入一组sps 和pps
六 为啥从MP4容器中解封装出H264文件这样麻烦呢?难道不应该是直接打开容器可以了吗?
因为MP4格式里吧sps/pps 没有和h264视频数据放在一起。没有sps pps 数据播放不了。所以要抽出数据加上sps pps
七 后续补充

二 C 代码实现

先看看main函数实现(解释在代码里)
int main(int argc, char *argv[])
{
    int err_code;
    char errors[1024];

    char *src_filename = NULL;
    char *dst_filename = NULL;

    FILE *dst_fd = NULL;

    int video_stream_index = -1;
	// 前面讲过了 获取媒体信息的 
    AVFormatContext *fmt_ctx = NULL;
    AVPacket pkt;

    av_log_set_level(AV_LOG_DEBUG);
	//看看控制台有没有输入3个  1 文件名 2输入路径 3输出路径
    if(argc < 3){
        av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
        return -1;
    }

    src_filename = argv[1];
    dst_filename = argv[2];

    if(src_filename == NULL || dst_filename == NULL){
        av_log(NULL, AV_LOG_ERROR, "src or dts file is null, plz check them!\n");
        return -1;
    }

    dst_fd = fopen(dst_filename, "wb");
    if (!dst_fd) {
        av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", dst_filename);
        return -1;
    }

    /*打开媒体文件  初始化AvFormatContext*/
    //前面也讲过了
    if((err_code = avformat_open_input(&fmt_ctx, src_filename, NULL, NULL)) < 0){
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
               src_filename,
               err_code,
               errors);
        return -1;
    }

    /*输出输入媒体的信息  前面也讲过了 */
    av_dump_format(fmt_ctx, 0, src_filename, 0);

    /*
    初始化 packet
    和后面的av_packet_unref(&pkt); 
    成对存在 一个创建一个释放
    */
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    /*找一个 最好的流  看到第二个参数就懂了只要视频流*/
    video_stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if(video_stream_index < 0){
        av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
               av_get_media_type_string(AVMEDIA_TYPE_VIDEO),
               src_filename);
        return AVERROR(EINVAL);
    }

    /*开始读取了 返回值是pkt  pkt中存放的是一个或者几个整数帧的信息*/
    while(av_read_frame(fmt_ctx, &pkt) >=0 ){
        if(pkt.stream_index == video_stream_index){
			// 加入sps  pps  单独讲
            h264_mp4toannexb(fmt_ctx, &pkt, dst_fd);

        }

        //release pkt->data
        av_packet_unref(&pkt);
    }

    /*close input media file*/
    avformat_close_input(&fmt_ctx);
    if(dst_fd) {
        fclose(dst_fd);
    }

    return 0;
}

h264_mp4toannexb()
int h264_mp4toannexb(AVFormatContext *fmt_ctx, AVPacket *in, FILE *dst_fd)
{

    AVPacket *out = NULL;
    AVPacket spspps_pkt;

    int len;
    uint8_t unit_type;
    int32_t nal_size;
    uint32_t cumul_size    = 0;
    const uint8_t *buf;
    const uint8_t *buf_end;
    int            buf_size;
    int ret = 0, i;
//申请空间
    out = av_packet_alloc();

    buf      = in->data;
    buf_size = in->size;
    buf_end  = in->data + in->size; //当成 *p +n理解 

    do {
        ret= AVERROR(EINVAL);
        //因为每个视频帧的前 4 个字节是视频帧的长度
        //如果buf中的数据都不能满足4字节,所以后面就没有必要再进行处理了
        if (buf + 4 > buf_end)
            goto fail;

        //将前四字节转换成整型,也就是取出视频帧长度
        for (nal_size = 0, i = 0; i<4; i++)
            nal_size = (nal_size << 8) | buf[i];

        buf += 4; //跳过4字节(也就是视频帧长度),从而指向真正的视频帧数据 
        unit_type = *buf & 0x1f; //视频帧的第一个字节里有NAL TYPE

        //如果视频帧长度大于从 AVPacket 中读到的数据大小,说明这个数据包肯定是出错了
        if (nal_size > buf_end - buf || nal_size < 0)
            goto fail;

        /* 前面说了 如果第五个字节等于5 那么 就是关键帧IDR帧的意思 关键帧前面要加入sps pps 信息 */
        if (unit_type == 5) {

            //在每个I帧之前都加 SPS/PPS  
            //FFmpeg 中 sps pps 在extradata 中
            //细说
            h264_extradata_to_annexb( fmt_ctx->streams[in->stream_index]->codecpar->extradata,
                                      fmt_ctx->streams[in->stream_index]->codecpar->extradata_size,
                                      &spspps_pkt,
                                      AV_INPUT_BUFFER_PADDING_SIZE);
         	// 加入startcode 因为有无sps pps 是不同的
            if ((ret=alloc_and_copy(out,
                               spspps_pkt.data, spspps_pkt.size,
                               buf, nal_size)) < 0)
                goto fail;
        } else {
            if ((ret=alloc_and_copy(out, NULL, 0, buf, nal_size)) < 0)
                goto fail;
        }

		// 数据写入
        len = fwrite( out->data, 1, out->size, dst_fd);
        if(len != out->size){
            av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
                    len,
                    out->size);
        }
        fflush(dst_fd);
// 遍历pkt 前面说了 一个pkt里面不止一帧
next_nal:
        buf        += nal_size;
        cumul_size += nal_size + 4;//s->length_size;
    } while (cumul_size < buf_size);

fail:
    av_packet_free(&out);

    return ret;
}
int h264_extradata_to_annexb()

`int h264_extradata_to_annexb(const uint8_t *codec_extradata, const int codec_extradata_size, AVPacket *out_extradata, int padding)
{
uint16_t unit_size = 0;
uint64_t total_size = 0;
uint8_t *out = NULL;
uint8_t unit_nb = 0;
uint8_t sps_done = 0;
uint8_t sps_seen = 0;
uint8_t pps_seen = 0;
uint8_t sps_offset = 0;
uint8_t pps_offset = 0;

/**
 * AVCC
 * bits
 *  8   version ( always 0x01 )
 *  8   avc profile ( sps[0][1] )
 *  8   avc compatibility ( sps[0][2] )
 *  8   avc level ( sps[0][3] )
 *  6   reserved ( all bits on )
 *  2   NALULengthSizeMinusOne    // 这个值是(前缀长度-1),值如果是3,那前缀就是4,因为4-1=3
 *  3   reserved ( all bits on )
 *  5   number of SPS NALUs (usually 1)
 *
 *  repeated once per SPS:
 *  16     SPS size
 *
 *  variable   SPS NALU data
 *  8   number of PPS NALUs (usually 1)
 *  repeated once per PPS
 *  16    PPS size
 *  variable PPS NALU data
 */

const uint8_t *extradata = codec_extradata + 4; //extradata存放数据的格式如上,前4个字节没用,所以将其舍弃
static const uint8_t nalu_header[4] = { 0, 0, 0, 1 }; //每个H264裸数据都是以 0001 4个字节为开头的

extradata++;//跳过一个字节,这个也没用 

sps_offset = pps_offset = -1;

/* retrieve sps and pps unit(s) */
unit_nb = *extradata++ & 0x1f; /* 取 SPS 个数,理论上可以有多个, 但我没有见到过多 SPS 的情况*/
if (!unit_nb) {
    goto pps;
}else {
    sps_offset = 0;
    sps_seen = 1;
}

while(unit_nb--) {
    int err;

    unit_size   = AV_RB16(extradata);
    total_size += unit_size + 4; //加上4字节的h264 header, 即 0001
    if (total_size > INT_MAX - padding) {
        av_log(NULL, AV_LOG_ERROR,
               "Too big extradata size, corrupted stream or invalid MP4/AVCC bitstream\n");
        av_free(out);
        return AVERROR(EINVAL);
    }

    //2:表示上面 unit_size 的所占字结数
    //这句的意思是 extradata 所指的地址,加两个字节,再加 unit 的大小所指向的地址
    //是否超过了能访问的有效地址空间
    if (extradata + 2 + unit_size > codec_extradata + codec_extradata_size) {
        av_log(NULL, AV_LOG_ERROR, "Packet header is not contained in global extradata, "
               "corrupted stream or invalid MP4/AVCC bitstream\n");
        av_free(out);
        return AVERROR(EINVAL);
    }

    //分配存放 SPS 的空间
    if ((err = av_reallocp(&out, total_size + padding)) < 0)
        return err;

    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:
    //当 SPS 处理完后,开始处理 PPS
    if (!unit_nb && !sps_done++) {
        unit_nb = *extradata++; /* number of pps unit(s) */
        if (unit_nb) {
            pps_offset = total_size;
            pps_seen = 1;
        }
    }
}

//余下的空间清0
if (out){
    memset(out + total_size, 0, padding);
}

if (!sps_seen)
    av_log(NULL, AV_LOG_WARNING,
           "Warning: SPS NALU missing or invalid. "
           "The resulting stream may not play.\n");

if (!pps_seen)
    av_log(NULL, AV_LOG_WARNING,
           "Warning: PPS NALU missing or invalid. "
           "The resulting stream may not play.\n");

out_extradata->data      = out;
out_extradata->size      = total_size;

return 0;

}
`

alloc_and_copy
static int alloc_and_copy(AVPacket *out,
                          const uint8_t *sps_pps, uint32_t sps_pps_size,
                          const uint8_t *in, uint32_t in_size)
{
    uint32_t offset         = out->size;
    uint8_t nal_header_size = 4;
    int err;
        //扩充out 容量
    err = av_grow_packet(out, sps_pps_size + in_size + nal_header_size);
    if (err < 0)
        return err;

    if (sps_pps)
        // 写入sps pps  第一个参数是找写入位置
        memcpy(out->data + offset, sps_pps, sps_pps_size);
    // 写入原始数据
    memcpy(out->data + sps_pps_size + nal_header_size + offset, in, in_size);
    if (!offset) {
        // 000001
        AV_WB32(out->data + sps_pps_size, 1);
    } else {
        // 00000001
        (out->data + offset + sps_pps_size)[0] =
        (out->data + offset + sps_pps_size)[1] = 0;
        (out->data + offset + sps_pps_size)[2] = 1;
    }

    return 0;
}

全部代码

#include <stdio.h>
#include <libavutil/log.h>
#include <libavformat/avio.h>
#include <libavformat/avformat.h>

#ifndef AV_WB32
#   define AV_WB32(p, val) do {                 \
        uint32_t d = (val);                     \
        ((uint8_t*)(p))[3] = (d);               \
        ((uint8_t*)(p))[2] = (d)>>8;            \
        ((uint8_t*)(p))[1] = (d)>>16;           \
        ((uint8_t*)(p))[0] = (d)>>24;           \
    } while(0)
#endif

#ifndef AV_RB16
#   define AV_RB16(x)                           \
    ((((const uint8_t*)(x))[0] << 8) |          \
      ((const uint8_t*)(x))[1])
#endif
// 添加startcode
static int alloc_and_copy(AVPacket *out,
                          const uint8_t *sps_pps, uint32_t sps_pps_size,
                          const uint8_t *in, uint32_t in_size)
{
    uint32_t offset         = out->size;
    uint8_t nal_header_size = 4;
    int err;
        //扩充out 容量
    err = av_grow_packet(out, sps_pps_size + in_size + nal_header_size);
    if (err < 0)
        return err;

    if (sps_pps)
        // 写入sps pps  第一个参数是找写入位置
        memcpy(out->data + offset, sps_pps, sps_pps_size);
    // 写入原始数据
    memcpy(out->data + sps_pps_size + nal_header_size + offset, in, in_size);
    if (!offset) {
        // 000001
        AV_WB32(out->data + sps_pps_size, 1);
    } else {
        // 00000001
        (out->data + offset + sps_pps_size)[0] =
        (out->data + offset + sps_pps_size)[1] = 0;
        (out->data + offset + sps_pps_size)[2] = 1;
    }

    return 0;
}

int h264_extradata_to_annexb(const uint8_t *codec_extradata, const int codec_extradata_size, AVPacket *out_extradata, int padding)
{
    uint16_t unit_size  = 0;
    uint64_t total_size = 0;
    uint8_t *out        = NULL;
    uint8_t unit_nb     = 0;
    uint8_t sps_done    = 0;
    uint8_t sps_seen    = 0;
    uint8_t pps_seen    = 0;
    uint8_t sps_offset  = 0;
    uint8_t pps_offset  = 0;

    /**
     * AVCC
     * bits
     *  8   version ( always 0x01 )
     *  8   avc profile ( sps[0][1] )
     *  8   avc compatibility ( sps[0][2] )
     *  8   avc level ( sps[0][3] )
     *  6   reserved ( all bits on )
     *  2   NALULengthSizeMinusOne    // 这个值是(前缀长度-1),值如果是3,那前缀就是4,因为4-1=3
     *  3   reserved ( all bits on )
     *  5   number of SPS NALUs (usually 1)
     *
     *  repeated once per SPS:
     *  16     SPS size
     *
     *  variable   SPS NALU data
     *  8   number of PPS NALUs (usually 1)
     *  repeated once per PPS
     *  16    PPS size
     *  variable PPS NALU data
     */

    const uint8_t *extradata = codec_extradata + 4; //extradata存放数据的格式如上,前4个字节没用,所以将其舍弃
    static const uint8_t nalu_header[4] = { 0, 0, 0, 1 }; //每个H264裸数据都是以 0001 4个字节为开头的

    extradata++;//跳过一个字节,这个也没用 

    sps_offset = pps_offset = -1;

    /* retrieve sps and pps unit(s) */
    unit_nb = *extradata++ & 0x1f; /* 取 SPS 个数,理论上可以有多个, 但我没有见到过多 SPS 的情况*/
    if (!unit_nb) {
        goto pps;
    }else {
        sps_offset = 0;
        sps_seen = 1;
    }

    while(unit_nb--) {
        int err;

        unit_size   = AV_RB16(extradata);
        total_size += unit_size + 4; //加上4字节的h264 header, 即 0001
        if (total_size > INT_MAX - padding) {
            av_log(NULL, AV_LOG_ERROR,
                   "Too big extradata size, corrupted stream or invalid MP4/AVCC bitstream\n");
            av_free(out);
            return AVERROR(EINVAL);
        }

        //2:表示上面 unit_size 的所占字结数
        //这句的意思是 extradata 所指的地址,加两个字节,再加 unit 的大小所指向的地址
        //是否超过了能访问的有效地址空间
        if (extradata + 2 + unit_size > codec_extradata + codec_extradata_size) {
            av_log(NULL, AV_LOG_ERROR, "Packet header is not contained in global extradata, "
                   "corrupted stream or invalid MP4/AVCC bitstream\n");
            av_free(out);
            return AVERROR(EINVAL);
        }

        //分配存放 SPS 的空间
        if ((err = av_reallocp(&out, total_size + padding)) < 0)
            return err;

        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:
        //当 SPS 处理完后,开始处理 PPS
        if (!unit_nb && !sps_done++) {
            unit_nb = *extradata++; /* number of pps unit(s) */
            if (unit_nb) {
                pps_offset = total_size;
                pps_seen = 1;
            }
        }
    }

    //余下的空间清0
    if (out){
        memset(out + total_size, 0, padding);
    }

    if (!sps_seen)
        av_log(NULL, AV_LOG_WARNING,
               "Warning: SPS NALU missing or invalid. "
               "The resulting stream may not play.\n");

    if (!pps_seen)
        av_log(NULL, AV_LOG_WARNING,
               "Warning: PPS NALU missing or invalid. "
               "The resulting stream may not play.\n");

    out_extradata->data      = out;
    out_extradata->size      = total_size;

    return 0;
}

int h264_mp4toannexb(AVFormatContext *fmt_ctx, AVPacket *in, FILE *dst_fd)
{

    AVPacket *out = NULL;
    AVPacket spspps_pkt;

    int len;
    uint8_t unit_type;
    int32_t nal_size;
    uint32_t cumul_size    = 0;
    const uint8_t *buf;
    const uint8_t *buf_end;
    int            buf_size;
    int ret = 0, i;

    out = av_packet_alloc();

    buf      = in->data;
    buf_size = in->size;
    buf_end  = in->data + in->size;

    do {
        ret= AVERROR(EINVAL);
        //因为每个视频帧的前 4 个字节是视频帧的长度
        //如果buf中的数据都不能满足4字节,所以后面就没有必要再进行处理了
        if (buf + 4 > buf_end)
            goto fail;

        //将前四字节转换成整型,也就是取出视频帧长度
        for (nal_size = 0, i = 0; i<4; i++)
            nal_size = (nal_size << 8) | buf[i];

        buf += 4; //跳过4字节(也就是视频帧长度),从而指向真正的视频帧数据 
        unit_type = *buf & 0x1f; //视频帧的第一个字节里有NAL TYPE

        //如果视频帧长度大于从 AVPacket 中读到的数据大小,说明这个数据包肯定是出错了
        if (nal_size > buf_end - buf || nal_size < 0)
            goto fail;

        /* prepend only to the first type 5 NAL unit of an IDR picture, if no sps/pps are already present */
        if (unit_type == 5) {

            //在每个I帧之前都加 SPS/PPS
            h264_extradata_to_annexb( fmt_ctx->streams[in->stream_index]->codecpar->extradata,
                                      fmt_ctx->streams[in->stream_index]->codecpar->extradata_size,
                                      &spspps_pkt,
                                      AV_INPUT_BUFFER_PADDING_SIZE);
            /**
                                    不太懂
             */
            if ((ret=alloc_and_copy(out,
                               spspps_pkt.data, spspps_pkt.size,
                               buf, nal_size)) < 0)
                goto fail;
        } else {
            if ((ret=alloc_and_copy(out, NULL, 0, buf, nal_size)) < 0)
                goto fail;
        }


        len = fwrite( out->data, 1, out->size, dst_fd);
        if(len != out->size){
            av_log(NULL, AV_LOG_DEBUG, "warning, length of writed data isn't equal pkt.size(%d, %d)\n",
                    len,
                    out->size);
        }
        fflush(dst_fd);
/**
 不太懂
 */
next_nal:
        buf        += nal_size;
        cumul_size += nal_size + 4;//s->length_size;
    } while (cumul_size < buf_size);

fail:
    av_packet_free(&out);

    return ret;
}

int main(int argc, char *argv[])
{
    int err_code;
    char errors[1024];

    char *src_filename = NULL;
    char *dst_filename = NULL;

    FILE *dst_fd = NULL;

    int video_stream_index = -1;

    AVFormatContext *fmt_ctx = NULL;
    AVPacket pkt;

    av_log_set_level(AV_LOG_DEBUG);

    if(argc < 3){
        av_log(NULL, AV_LOG_DEBUG, "the count of parameters should be more than three!\n");
        return -1;
    }

    src_filename = argv[1];
    dst_filename = argv[2];

    if(src_filename == NULL || dst_filename == NULL){
        av_log(NULL, AV_LOG_ERROR, "src or dts file is null, plz check them!\n");
        return -1;
    }

    dst_fd = fopen(dst_filename, "wb");
    if (!dst_fd) {
        av_log(NULL, AV_LOG_DEBUG, "Could not open destination file %s\n", dst_filename);
        return -1;
    }

    /*open input media file, and allocate format context*/
    if((err_code = avformat_open_input(&fmt_ctx, src_filename, NULL, NULL)) < 0){
        av_strerror(err_code, errors, 1024);
        av_log(NULL, AV_LOG_DEBUG, "Could not open source file: %s, %d(%s)\n",
               src_filename,
               err_code,
               errors);
        return -1;
    }

    /*dump input information*/
    av_dump_format(fmt_ctx, 0, src_filename, 0);

    /*initialize packet*/
    av_init_packet(&pkt);
    pkt.data = NULL;
    pkt.size = 0;

    /*find best video stream*/
    video_stream_index = av_find_best_stream(fmt_ctx, AVMEDIA_TYPE_VIDEO, -1, -1, NULL, 0);
    if(video_stream_index < 0){
        av_log(NULL, AV_LOG_DEBUG, "Could not find %s stream in input file %s\n",
               av_get_media_type_string(AVMEDIA_TYPE_VIDEO),
               src_filename);
        return AVERROR(EINVAL);
    }

    /*read frames from media file*/
    while(av_read_frame(fmt_ctx, &pkt) >=0 ){
        if(pkt.stream_index == video_stream_index){

            h264_mp4toannexb(fmt_ctx, &pkt, dst_fd);

        }

        //release pkt->data
        av_packet_unref(&pkt);
    }

    /*close input media file*/
    avformat_close_input(&fmt_ctx);
    if(dst_fd) {
        fclose(dst_fd);
    }

    return 0;
}

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