FFmpeg input与output 函数流程

重要结构体

AVFormatContext

AVCodecContext
AVCodec

AVPacket
AVFrame

0.公共部分

av_register_all();
avfilter_register_all();
avformat_network_init();
avdevice_register_all();
av_log_set_level(AV_LOG_ERROR);

1.input部分

avformat_alloc_context
av_find_input_format
avformat_open_input

avformat_find_stream_info

avcodec_find_decoder
avcodec_open2

av_read_frame

avcodec_decode_video2



avformat_close_input

2.output部分

avformat_alloc_output_context2

avio_open2

avcodec_find_encoder
avcodec_alloc_context3
avcodec_open2

avformat_new_stream
avcodec_copy_context ---> avcodec_find_decoder + avcodec_alloc_context3
　　　　　　　　　　　　　　 + avcodec_parameters_to_context + avcodec_parameters_from_context

avformat_write_header

avcodec_encode_video2

av_interleaved_write_frame



av_write_trailer
avcodec_close
avformat_close_input

3.裁剪视频代码

#include <stdlib.h>
#include <libavutil/timestamp.h>
#include <libavformat/avformat.h>

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
",
           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 cut_video(double from_seconds, double end_seconds, const char* in_filename, const char* out_filename) {
    AVOutputFormat *ofmt = NULL;
    AVFormatContext *ifmt_ctx = NULL, *ofmt_ctx = NULL;
    AVPacket pkt;
    int ret, i;

    av_register_all();

    if ((ret = avformat_open_input(&ifmt_ctx, in_filename, 0, 0)) < 0) {
        fprintf(stderr, "Could not open input file '%s'", in_filename);
        goto end;
    }

    if ((ret = avformat_find_stream_info(ifmt_ctx, 0)) < 0) {
        fprintf(stderr, "Failed to retrieve input stream information");
        goto end;
    }

    av_dump_format(ifmt_ctx, 0, in_filename, 0);

    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        fprintf(stderr, "Could not create output context
");
        ret = AVERROR_UNKNOWN;
        goto end;
    }

    ofmt = ofmt_ctx->oformat;

    for (i = 0; i < ifmt_ctx->nb_streams; i++) {
/*        AVStream *in_stream = ifmt_ctx->streams[i];
        AVStream *out_stream = avformat_new_stream(ofmt_ctx, in_stream->codec->codec);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream
");
            ret = AVERROR_UNKNOWN;
            goto end;
        }

        ret = avcodec_copy_context(out_stream->codec, in_stream->codec);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy context from input to output stream codec context
");
            goto end;
        }
        out_stream->codec->codec_tag = 0;
        if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
            out_stream->codec->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;
*/
        AVStream *in_stream = ifmt_ctx->streams[i];
        AVCodec * codec = avcodec_find_decoder(in_stream->codecpar->codec_id);
        AVStream *out_stream = avformat_new_stream(ofmt_ctx, codec);
        if (!out_stream) {
            fprintf(stderr, "Failed allocating output stream
");
            ret = AVERROR_UNKNOWN;
            goto end;
        }
        
        AVCodecContext * codec_ctx = avcodec_alloc_context3(codec);
        ret = avcodec_parameters_to_context(codec_ctx, in_stream->codecpar);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy in_stream codecpar to codec context
");
            goto end;
        }

        codec_ctx->codec_tag = 0;
        if (ofmt_ctx->oformat->flags & AVFMT_GLOBALHEADER)
            codec_ctx->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

        ret = avcodec_parameters_from_context(out_stream->codecpar, codec_ctx);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy codec context to out_stream codecpar context
");
            goto end;
        }
    }
    av_dump_format(ofmt_ctx, 0, out_filename, 1);


    if (!(ofmt->flags & AVFMT_NOFILE)) {
        ret = avio_open(&ofmt_ctx->pb, out_filename, AVIO_FLAG_WRITE);
        if (ret < 0) {
            fprintf(stderr, "Could not open output file '%s'", out_filename);
            goto end;
        }
    }

    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        fprintf(stderr, "Error occurred when opening output file
");
        goto end;
    }

    // int64_t start_from = 8*AV_TIME_BASE;
    ret = av_seek_frame(ifmt_ctx, -1, from_seconds*AV_TIME_BASE, AVSEEK_FLAG_ANY);
    if (ret < 0) {
        fprintf(stderr, "Error seek
");
        goto end;
    }

    int64_t *dts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(dts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);
    int64_t *pts_start_from = malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(pts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);

    while (1) {
        AVStream *in_stream, *out_stream;

        ret = av_read_frame(ifmt_ctx, &pkt);
        if (ret < 0)
            break;

        in_stream  = ifmt_ctx->streams[pkt.stream_index];
        out_stream = ofmt_ctx->streams[pkt.stream_index];

        log_packet(ifmt_ctx, &pkt, "in");

        if (av_q2d(in_stream->time_base) * pkt.pts > end_seconds) {
            av_free_packet(&pkt);
            break;
        }

        if (dts_start_from[pkt.stream_index] == 0) {
            dts_start_from[pkt.stream_index] = pkt.dts;
            printf("dts_start_from: %s
", av_ts2str(dts_start_from[pkt.stream_index]));
        }
        if (pts_start_from[pkt.stream_index] == 0) {
            pts_start_from[pkt.stream_index] = pkt.pts;
            printf("pts_start_from: %s
", av_ts2str(pts_start_from[pkt.stream_index]));
        }

        /* copy packet */
        pkt.pts = av_rescale_q_rnd(pkt.pts - pts_start_from[pkt.stream_index], in_stream->time_base, out_stream->time_base, AV_ROUND_NEAR_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_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, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        log_packet(ofmt_ctx, &pkt, "out");
        printf("
");

        ret = av_interleaved_write_frame(ofmt_ctx, &pkt);
        if (ret < 0) {
            fprintf(stderr, "Error muxing packet
");
            //break;    // 注释后，解决 错误 pts (6000) < dts (12000) in stream 0
        }
        av_free_packet(&pkt);
    }
    free(dts_start_from);
    free(pts_start_from);

    av_write_trailer(ofmt_ctx);
end:

    avformat_close_input(&ifmt_ctx);

    /* close output */
    if (ofmt_ctx && !(ofmt->flags & AVFMT_NOFILE))
        avio_closep(&ofmt_ctx->pb);
    avformat_free_context(ofmt_ctx);

    if (ret < 0 && ret != AVERROR_EOF) {
        fprintf(stderr, "Error occurred: %s
", av_err2str(ret));
        return 1;
    }

    return 0;
}

int main(int argc, char *argv[]){
    if(argc < 5){
        fprintf(stderr, "Usage: 
                command startime, endtime, srcfile, outfile");
        return -1;
    }

    double startime = atoi(argv[1]);
    double endtime = atoi(argv[2]);
    cut_video(startime, endtime, argv[3], argv[4]);

    return 0;
}

4.问题

　　问题分析：

　　出现这种错误是由于视频pts大于dts。pts是视频播放时间，dts是送入解码器解码时间。所以一帧视频播放时间必须在解码时间点之后。

　　产生错误的原因一般是对dts，pts操作不当。比如在进行视频分割时，常用的方法是视频截取后半段视频pts与dts减去前半段pts和dts。前半段pts可能比dts大（当解码的视频帧不是I帧时）后半段刚开始视频pts和dts刚好相等（当前帧为I帧时），两个一相减就会出现dts小于pts的情况。

　　解决方法：

　　1.进行判断：在av_interleaved_write_frame之前添加 if(packet.pts < packet.dts) continue; 把异常的帧简单跳过，异常帧只是极少数简单跳过不会有什么影响。这样会是的播放裁剪后的视频起始有黑屏。

　　2.获取截取起始时间后的第一个I帧，从这个I帧开始。

　　最好使用下面的方法。