FFmpeg 视频截取

视频截取步骤

1、打开输入文件
2、创建并打开一个空文件存储 flv 格式音视频数据
3、遍历输入文件的每一路流，每个输入流对应创建一个输出流，并将输入流中的编解码参数直接拷贝到输出流中。
4、写入新的多媒体文件的头
5、快进视频流到要截取的时间
6、在循环遍历输入文件的每一帧，对每一个packet进行时间基的转换
7、将处理好的pkt写入输出文件
8、超过要结束的时间跳出循环
9、写入新的多媒体文件尾
10、释放相关资源

相关API

文件操作
相关API

源码：

#include "SeekVideo.h"
#include 
#include 
#include 

static void log_packet(const AVFormatContext *fmt_ctx, const AVPacket *pkt, const char *tag)
{
    AVRational *time_base = &fmt_ctx->streams[pkt->stream_index]->time_base;

    printf("%s: pts:%s pts_time:%s dts:%s dts_time:%s duration:%s duration_time:%s stream_index:%d\n",
           tag,
           av_ts2str(pkt->pts), av_ts2timestr(pkt->pts, time_base),
           av_ts2str(pkt->dts), av_ts2timestr(pkt->dts, time_base),
           av_ts2str(pkt->duration), av_ts2timestr(pkt->duration, time_base),
           pkt->stream_index);
}

int cutVideo(double start_secondtime, double end_secondtime, const char *in_filename, const char *out_filename) {
    AVOutputFormat *outfmt = NULL; //输出格式
    AVFormatContext *infmt_ctx = NULL, *outfmt_ctx = NULL; //输入/输出上下文
    AVCodecContext *codec_ctx = NULL;//解码器上下文
    AVPacket pkt;
    int ret, i;
    
    av_register_all();
    
    //打开输入文件为 infmt_ctx 分配内存
    ret = avformat_open_input(&infmt_ctx, in_filename, NULL, NULL);
    if (ret < 0) {
        fprintf(stderr, "Could not open input file '%s'", in_filename);
        goto end;
    }
    
    //检索输入文件的流信息
    ret = avformat_find_stream_info(infmt_ctx, 0);
    if (ret < 0) {
        fprintf(stderr, "Failed to retrieve input stream information");
        goto end;
    }
    //打印输入文件的相关信息
    av_dump_format(infmt_ctx, 0, in_filename, 0);
    
    //为输出上下文分配内存
    avformat_alloc_output_context2(&outfmt_ctx, NULL, NULL, out_filename);
    if (!outfmt_ctx) {
        fprintf(stderr, "Could not create output context\n");
        ret = AVERROR_UNKNOWN;
        goto end;
    }
    
    outfmt = outfmt_ctx->oformat;
    for (i = 0; i < infmt_ctx->nb_streams; i++) {
        AVStream *in_stream = infmt_ctx->streams[i];
        
        //创建一个输出流
        AVStream *out_stream = avformat_new_stream(outfmt_ctx, NULL);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream\n");
            ret = AVERROR_UNKNOWN;
            goto end;
        }
        
        //获取解码器
        AVCodec *pAVCodec = avcodec_find_decoder(in_stream->codecpar->codec_id);
        //为解码上下文分配内存
        codec_ctx = avcodec_alloc_context3(pAVCodec);
        //把avstream中的参数复制到codec中
        ret = avcodec_parameters_to_context(codec_ctx, in_stream->codecpar);

        if (ret < 0){
            printf("Failed to copy in_stream codecpar to codec context\n");
            goto end;
        }
        codec_ctx->codec_tag = 0;
        if (outfmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
        codec_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
        
        //把codec中的参数复制到avstream中
        ret = avcodec_parameters_from_context(out_stream->codecpar, codec_ctx);
        
    }
    //打印输出文件的相关信息
    av_dump_format(outfmt_ctx, 0, out_filename, 1);
    
    if (!(outfmt->flags & AVFMT_NOFILE)) {
        ret = avio_open2(&outfmt_ctx->pb, out_filename, AVIO_FLAG_WRITE, NULL, NULL);
        if (ret < 0) {
            fprintf(stderr, "Could not open output file '%s'", out_filename);
            goto end;
        }
    }
    
    //写入新的多媒体文件的头
    ret = avformat_write_header(outfmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file\n");
        goto end;
    }
    
    //从start_secondtime秒开始截取
    ret = av_seek_frame(infmt_ctx, -1, start_secondtime*AV_TIME_BASE, AVSEEK_FLAG_ANY);
    if (ret < 0) {
        fprintf(stderr, "Error seek\n");
        goto end;
    }
    
    int64_t *dts_start_from = malloc(sizeof(int64_t) * infmt_ctx->nb_streams);
    memset(dts_start_from, 0, sizeof(int64_t) * infmt_ctx->nb_streams);
    int64_t *pts_start_from = malloc(sizeof(int64_t) * infmt_ctx->nb_streams);
    memset(pts_start_from, 0, sizeof(int64_t) * infmt_ctx->nb_streams);
    
    while (1) {
        AVStream *in_stream, *out_stream;
        
        //循环读取每一帧数据
        ret = av_read_frame(infmt_ctx, &pkt);
        if (ret < 0) {
            break;
        }
        //获取与pkt对应的的输入流
        in_stream = infmt_ctx->streams[pkt.stream_index];
        //获取与pkt对应的的输出流
        out_stream = outfmt_ctx->streams[pkt.stream_index];
        log_packet(infmt_ctx, &pkt, "in");
        
        // Convert an AVRational to a `double`.，截取到末尾时间时就跳出
        if (av_q2d(in_stream->time_base) * pkt.pts > end_secondtime) {
            av_packet_unref(&pkt);
            break;
        }
        //将截取的开始时间的dts保存起来
        if (dts_start_from[pkt.stream_index] == 0) {
            dts_start_from[pkt.stream_index] = pkt.dts;
            printf("dts_start_from: %s\n",av_ts2str(dts_start_from[pkt.stream_index]));
        }
        //将截取的开始时间的pts保存起来
        if (pts_start_from[pkt.stream_index] == 0) {
            pts_start_from[pkt.stream_index] = pkt.pts;
            printf("pts_start_from: %s\n", av_ts2str(pts_start_from[pkt.stream_index]));
        }
        
        //对pts、dts、duration 进行时间基转换,减去
        pkt.pts = av_rescale_q_rnd(pkt.pts - pts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_INF | AV_ROUND_PASS_MINMAX);
        pkt.dts = av_rescale_q_rnd(pkt.dts - dts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_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, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        log_packet(outfmt_ctx, &pkt, "out");
        printf("\n");
        
        //将处理好的pkt写入输出文件
        ret = av_interleaved_write_frame(outfmt_ctx, &pkt);
        if (ret < 0) {
            fprintf(stderr, "Error muxing packet\n");
            break;
        }
        av_packet_unref(&pkt);
    }
    free(dts_start_from);
    free(pts_start_from);
    
    //写入新的多媒体文件尾
    av_write_trailer(outfmt_ctx);
end:
    avformat_close_input(&infmt_ctx);
    
    if (outfmt_ctx && !(outfmt->flags & AVFMT_NOFILE)) {
        avio_closep(&outfmt_ctx->pb);
    }
    avformat_free_context(outfmt_ctx);
    
    if (ret < 0 && ret != AVERROR_EOF) {
        fprintf(stderr, "Error occurred:%s\n", av_err2str(ret));
        return -1;
    }
    return 0;
}