ffmpeg日记4001-原理介绍-视频切割原理

原理

打开输入---->打开输出---->根据输入来创建流---->拷贝流设置---->循环读帧---->判断时间点是否到达切割点，并做设置---->设置pts和dts---->写入---->善后

重点是pts和dts如何设置。参考《ffmpeg学习日记25-pts，dts概念的理解》

示例代码

CMakeLists.txt

cmake_minimum_required(VERSION 3.14)

project(clipVideo )

#set(CMAKE_CXX_STANDARD 11)
#set(CMAKE_CXX_STANDARD_REQUIRED YES)

set(CMAKE_AUTOMOC ON) # Meta-Object Compiler
set(CMAKE_AUTORCC ON) # Resource Compiler
set(CMAKE_AUTOUIC ON) # User Interface Compiler

set(CMAKE_BUILD_TYPE Debug)

#find_package(PkgConfig REQUIRED)
#pkg_check_modules(AVLIB REQUIRED IMPORTED_TARGET libavcodec libavformat libavutil libswresample libswscale)

if(CMAKE_SYSTEM_NAME MATCHES "Linux")
        set(CURRENT_SYSTEM Linux)
elseif(CMAKE_SYSTEM_NAME MATCHES "Windows")
        set(CURRENT_SYSTEM Windows)
endif()

set(FFMPEG_LIB_DIR ${PROJECT_SOURCE_DIR}/../../lib)
set(FFMPEG_INCLUDE_DIR ${PROJECT_SOURCE_DIR}/../../include)

include_directories(
    ${PROJECT_SOURCE_DIR}
#    ${PROJECT_SOURCE_DIR}/../../include
    ${FFMPEG_INCLUDE_DIR}
    )

link_libraries(
#    ${PROJECT_SOURCE_DIR}/../../lib
    ${FFMPEG_LIB_DIR}
    )

#对于find_package找不到的外部依赖库，可以用add_library添加
# SHARED表示添加的是动态库
# IMPORTED表示是引入已经存在的动态库

add_library( avcodec STATIC IMPORTED)
add_library( avfilter STATIC IMPORTED )
add_library( swresample STATIC IMPORTED )
add_library( swscale STATIC IMPORTED )
add_library( avformat STATIC IMPORTED )
add_library( avutil STATIC IMPORTED )


#指定所添加依赖库的导入路径
set_target_properties( avcodec PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/avcodec.lib )
set_target_properties( avfilter PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/avfilter.lib )
set_target_properties( swresample PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/swresample.lib )
set_target_properties( swscale PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/swscale.lib )
set_target_properties( avformat PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/avformat.lib )
set_target_properties( avutil PROPERTIES IMPORTED_LOCATION ${FFMPEG_LIB_DIR}/avutil.lib )


add_executable(${PROJECT_NAME} main.cpp)

target_link_libraries(${PROJECT_NAME}
    avcodec avfilter avformat avutil
#    postproc swresample swscale
    )

main.cpp

#include <iostream>
#include <cinttypes>

#ifdef __cplusplus
extern "C" {
#endif

#include <stdio.h>
#include "libavutil/avutil.h"

#include <libavcodec/avcodec.h>

#include <libavformat/avformat.h>
#include <libavformat/avio.h>

#include <libavutil/imgutils.h>
#include <libavutil/samplefmt.h>

#include <libavutil/timestamp.h>

#include <libavutil/time.h>
#include <libavutil/pixdesc.h>
#include <libavutil/audio_fifo.h>
#include <libavutil/avassert.h>
#include <libavutil/avstring.h>
#include <libavutil/frame.h>
#include <libavutil/opt.h>

#include <libswresample/swresample.h>

#ifdef __cplusplus
}
#endif

#ifdef _WIN32
#include <direct.h>
#elif __APPLE__ || __linux__
#include<unistd.h>
#endif

using namespace std;

int main()
{
    cout << "Hello World!" << endl;
    printf("ffmpeg version:%s\n",av_version_info());

    std::cout << "current path:" << getcwd(NULL,0) << std::endl;

    int ret = 0;
    float fromSeconds = 3.0,endSeconds = 8.0;

    AVFormatContext *ifmt_ctx = NULL,*ofmt1_ctx = NULL;
    std::string ifileName = "../../../15s.mp4",ofileName1 = "1.mp4";
    AVPacket pkt;

    if ( (ret = avformat_open_input(&ifmt_ctx,ifileName.c_str(),NULL,NULL)) < 0){
        std::cout << "can not open the in put file format context!" << std::endl;
        return 0;
    }

    if( (ret = avformat_find_stream_info(ifmt_ctx,NULL)) < 0){
        std::cout << "can not find the input stream info" << std::endl;
        return 0;
    }

    avformat_alloc_output_context2(&ofmt1_ctx,NULL,NULL,ofileName1.c_str());
    if(!ofmt1_ctx){
        std::cout << "could not creat output1 context" << std::endl;
        return 0;
    }


    //拷贝参数到输出结构
    for(int i = 0;i < ifmt_ctx->nb_streams;i++){
        AVStream *instream = ifmt_ctx->streams[i];
        AVCodec *codec = avcodec_find_decoder(instream->codecpar->codec_id);
        AVStream *outStream = avformat_new_stream(ofmt1_ctx,codec);
        if(!outStream){
            std::cout << "failed allow output stream" << std::endl;
            return 0;
        }

        AVCodecContext *codecCtx = avcodec_alloc_context3(codec);
        ret = avcodec_parameters_to_context(codecCtx,instream->codecpar);
        if(ret < 0){
            std::cout << "failed to copy instream codecpar to codec context" << std::endl;
            return 0;
        }

        //这一步是在干什么呢
        codecCtx->codec_tag = 0;//这个标志是什么意思？？？
        if (ofmt1_ctx->oformat->flags & AVFMT_GLOBALHEADER){
            codecCtx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
        }

        ret = avcodec_parameters_from_context(outStream->codecpar,codecCtx);
        if (ret < 0){
            std::cout << "failed to copy codec context to outstram codecpar context" << std::endl;
            return 0;
        }
    }

    av_dump_format(ofmt1_ctx,0,ofileName1.c_str(),1);

    if(!(ofmt1_ctx->flags & AVFMT_NOFILE)){
        ret = avio_open(&ofmt1_ctx->pb,ofileName1.c_str(),AVIO_FLAG_WRITE);
        if (ret < 0){
            std::cout << "could not open output 1.mp4" << std::endl;
            return 0;
        }
    }


    //写文件头
    ret = avformat_write_header(ofmt1_ctx,NULL);
    if (ret < 0){
        std::cout << "error write header of 1.mp4" << std::endl;
        return 0;
    }

    ret = av_seek_frame(ifmt_ctx, -1, (int64_t)(fromSeconds * AV_TIME_BASE), AVSEEK_FLAG_ANY);
    std::cout << "ret:" << ret << std::endl;

    int64_t *dstStartFrom = (int64_t *)malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(dstStartFrom, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);
    int64_t *ptsStartFrom = (int64_t *)malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(ptsStartFrom, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);


    while(1){
        AVStream *instream,*outstream;
        ret = av_read_frame(ifmt_ctx,&pkt);
        if (ret < 0)
            break;
        std::cout << "pkt.pts:" << pkt.pts << std::endl;
        instream = ifmt_ctx->streams[pkt.stream_index];
        outstream = ofmt1_ctx->streams[pkt.stream_index];

        if (av_q2d(instream->time_base) * pkt.pts > endSeconds){
            av_packet_unref(&pkt);
            break;
        }

        if (dstStartFrom[pkt.stream_index] == 0)
        {
            dstStartFrom[pkt.stream_index] = pkt.dts;
//          /  printf("dstStartFrom: %s\n", av_strerror(dstStartFrom[pkt.stream_index]));
        }
        if (ptsStartFrom[pkt.stream_index] == 0)
        {
            ptsStartFrom[pkt.stream_index] = pkt.dts;
//            printf("ptsStartFrom: %s\n", av_strerror(ptsStartFrom[pkt.stream_index]));
        }


        pkt.pts = av_rescale_q_rnd(pkt.pts - ptsStartFrom[pkt.stream_index],instream->time_base,outstream->time_base,
                                   (enum AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX) );//
        pkt.dts = av_rescale_q_rnd(pkt.dts - dstStartFrom[pkt.stream_index],instream->time_base,outstream->time_base,
                                   (enum AVRounding)(AV_ROUND_NEAR_INF | AV_ROUND_PASS_MINMAX) );

        if (pkt.pts < 0){
            pkt.pts = 0;
        }
        if(pkt.dts < 0){
            pkt.dts = 0;
        }

        pkt.duration = (int)av_rescale_q( (int64_t)pkt.duration ,
                                          instream->time_base,
                                          outstream->time_base);

        pkt.pos = -1;

        ret = av_interleaved_write_frame(ofmt1_ctx,&pkt);
        if(ret){
            std::cout << "error muxing packet" << std::endl;
            break;
        }
        av_packet_unref(&pkt);
    }
    //写文件尾
    free(dstStartFrom);
    free(ptsStartFrom);
    av_write_trailer(ofmt1_ctx);

    //释放资源
    avformat_close_input(&ifmt_ctx);
    if (ofmt1_ctx && !(ofmt1_ctx->flags & AVFMT_NOFILE)){
        avio_closep(&ofmt1_ctx->pb);
    }
    avformat_free_context(ofmt1_ctx);
    return 0;
}

总结

1. 这里的操作是将一种视频格式切割，之后生成的视频是同样的格式，所以直接是参数的复制即可，如果要生成另一种格式的视频，应该要单独设置设置输出视频格式的参数。

参考

• ffmpeg实现视频切割

• FFmpeg ‘avcodec_copy_context’ deprecated （视频裁剪）

• av_seek_frame使用详解

• FFmpeg中的时间基(time_base), AV_TIME_BASE

• FFmpeg之时间戳详解

•