av_seek_frame

`av_seek_frame`是FFmpeg中的一个函数，用于定位媒体文件中的某一帧。

在使用FFmpeg解码媒体文件时，有时候我们需要跳转到特定的时间点或特定的帧位置进行处理。这时就可以使用`av_seek_frame`函数来实现。该函数可以根据时间戳或者帧数来定位媒体文件中的某一帧。

函数的具体用法如下：

int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags);

其中，`s`是输入格式上下文，`stream_index`是需要定位的流的索引，`timestamp`是需要定位到的时间戳，`flags`是定位的标志，可以指定为`AVSEEK_FLAG_BACKWARD`表示向后定位，`AVSEEK_FLAG_ANY`表示任意定位，`AVSEEK_FLAG_FRAME`表示定位到最接近的关键帧。

需要注意的是，使用`av_seek_frame`函数进行定位时，必须保证输入格式上下文中的流信息已经完全读取，否则可能会出现定位失败的情况。

相关问题

c/c++使用ffmpeg库推流本地文件，并使用av_seek_frame实现跳转功能

要使用FFmpeg库进行推流和跳转功能，需要按照以下步骤进行操作：

1. 初始化FFmpeg库

在使用FFmpeg库之前，需要先初始化FFmpeg库。可以使用av_register_all()函数进行初始化。

av_register_all();

2. 打开输入文件

使用avformat_open_input()函数打开输入文件，然后使用avformat_find_stream_info()函数查找文件中的流信息。

AVFormatContext *formatCtx = NULL;
avformat_open_input(&formatCtx, inputFile, NULL, NULL);
avformat_find_stream_info(formatCtx, NULL);

3. 打开输出文件

使用avformat_alloc_output_context2()函数创建输出文件的AVFormatContext，并使用avio_open()函数打开输出文件。

AVFormatContext *outFormatCtx = NULL;
avformat_alloc_output_context2(&outFormatCtx, NULL, NULL, outputFile);
AVIOContext *outAVIOContext = NULL;
avio_open(&outAVIOContext, outputFile, AVIO_FLAG_WRITE);
outFormatCtx->pb = outAVIOContext;

4. 为输出文件添加流

使用avformat_new_stream()函数为输出文件添加音频或视频流，并设置流的编码格式和参数。

AVStream *outStream = avformat_new_stream(outFormatCtx, NULL);
outStream->codecpar->codec_id = codecId;
outStream->codecpar->codec_type = codecType;
outStream->codecpar->width = width;
outStream->codecpar->height = height;
outStream->codecpar->sample_rate = sampleRate;
outStream->codecpar->channels = channels;
outStream->codecpar->format = AV_SAMPLE_FMT_FLTP;

5. 打开编码器

使用avcodec_find_encoder()函数查找流的编码器，并使用avcodec_open2()函数打开编码器。

AVCodec *encoder = avcodec_find_encoder(outStream->codecpar->codec_id);
AVCodecContext *encoderCtx = avcodec_alloc_context3(encoder);
avcodec_parameters_to_context(encoderCtx, outStream->codecpar);
avcodec_open2(encoderCtx, encoder, NULL);

6. 写入文件头

使用avformat_write_header()函数写入输出文件的文件头。

avformat_write_header(outFormatCtx, NULL);

7. 读取和写入数据

使用av_read_frame()函数读取输入文件中的数据，并使用av_write_frame()函数将数据写入输出文件。如果需要跳转到指定时间点，可以使用av_seek_frame()函数进行跳转。

while (av_read_frame(formatCtx, &packet) == 0) {
    if (packet.stream_index == streamIndex) {
        if (av_seek_frame(formatCtx, streamIndex, timestamp, AVSEEK_FLAG_BACKWARD) >= 0) {
            avcodec_flush_buffers(decoderCtx);
            continue;
        }
        avcodec_send_packet(decoderCtx, &packet);
        while (avcodec_receive_frame(decoderCtx, frame) == 0) {
            // 对音视频数据进行处理
            avcodec_send_frame(encoderCtx, frame);
            while (avcodec_receive_packet(encoderCtx, &outPacket) == 0) {
                outPacket.stream_index = outStream->index;
                av_interleaved_write_frame(outFormatCtx, &outPacket);
                av_packet_unref(&outPacket);
            }
        }
    }
    av_packet_unref(&packet);
}

8. 关闭和释放资源

使用av_write_trailer()函数写入输出文件的文件尾，并使用avformat_close_input()、avformat_close_output()等函数关闭输入输出文件，并释放相应的资源。

av_write_trailer(outFormatCtx);
avformat_close_input(&formatCtx);
avformat_close_input(&outFormatCtx);
avcodec_close(decoderCtx);
avcodec_close(encoderCtx);
avformat_free_context(formatCtx);
avformat_free_context(outFormatCtx);
av_frame_free(&frame);

c/c++使用ffmpeg库推流本地文件，并使用av_seek_frame实现跳转功能的完整代码

这里是使用ffmpeg库推流本地文件，并使用av_seek_frame实现跳转功能的完整C++代码：

#include <iostream>
#include <string>

extern "C" {
#include <libavformat/avformat.h>
#include <libavutil/mathematics.h>
}

#define STREAM_DURATION 10.0
#define STREAM_FRAME_RATE 25 /* 25 images/s */
#define STREAM_PIX_FMT AV_PIX_FMT_YUV420P /* default pix_fmt */

/* add a video output stream */
static AVStream *add_video_stream(AVFormatContext *oc, enum AVCodecID codec_id) {
    AVCodecContext *c;
    AVStream *st;

    st = avformat_new_stream(oc, NULL);
    if (!st) {
        std::cerr << "Could not allocate stream" << std::endl;
        exit(1);
    }

    c = st->codec;
    c->codec_id = codec_id;
    c->codec_type = AVMEDIA_TYPE_VIDEO;

    /* put sample parameters */
    c->bit_rate = 400000;
    /* resolution must be a multiple of two */
    c->width = 352;
    c->height = 288;
    /* frames per second */
    c->time_base = (AVRational) { 1, STREAM_FRAME_RATE };
    st->time_base = c->time_base;

    c->gop_size = 12; /* emit one intra frame every twelve frames at most */
    c->pix_fmt = STREAM_PIX_FMT;

    /* some formats want stream headers to be separate */
    if (oc->oformat->flags & AVFMT_GLOBALHEADER)
        c->flags |= AV_CODEC_FLAG_GLOBAL_HEADER;

    return st;
}

/* open the output file, and allocate the format context */
static void open_output_file(const std::string &filename, AVFormatContext **oc, AVOutputFormat *fmt) {
    int ret;

    /* allocate the output media context */
    avformat_alloc_output_context2(oc, fmt, NULL, filename.c_str());
    if (!(*oc)) {
        std::cerr << "Could not create output context" << std::endl;
        exit(1);
    }

    /* add the video stream using the default format codecs and initialize the codecs */
    add_video_stream(*oc, (*oc)->oformat->video_codec);

    /* open the output file, if needed */
    if (!((*oc)->oformat->flags & AVFMT_NOFILE)) {
        ret = avio_open(&(*oc)->pb, filename.c_str(), AVIO_FLAG_WRITE);
        if (ret < 0) {
            std::cerr << "Could not open output file" << std::endl;
            exit(1);
        }
    }

    /* write the stream header, if any */
    ret = avformat_write_header(*oc, NULL);
    if (ret < 0) {
        std::cerr << "Error occurred when opening output file" << std::endl;
        exit(1);
    }
}

/* close the output file and free the format context */
static void close_output_file(AVFormatContext *oc) {
    /* write the trailer, if any */
    av_write_trailer(oc);

    /* close the output file */
    if (!(oc->oformat->flags & AVFMT_NOFILE))
        avio_closep(&oc->pb);

    /* free the stream */
    avformat_free_context(oc);
}

/* seek to a specific frame in a video file */
static void seek_to_frame(AVFormatContext *fmt_ctx, int stream_index, int64_t timestamp) {
    int ret;

    /* seek to the timestamp */
    ret = av_seek_frame(fmt_ctx, stream_index, timestamp, AVSEEK_FLAG_BACKWARD);
    if (ret < 0) {
        std::cerr << "Error seeking to timestamp " << timestamp << std::endl;
        exit(1);
    }

    /* flush the codec buffers */
    avcodec_flush_buffers(fmt_ctx->streams[stream_index]->codec);
}

int main(int argc, char **argv) {
    if (argc != 2) {
        std::cerr << "Usage: " << argv[0] << " <output file>" << std::endl;
        return 1;
    }

    AVOutputFormat *fmt;
    AVFormatContext *oc;
    AVPacket pkt;
    int frame_count, i;
    double t, tincr;
    int64_t next_pts;

    /* register all codecs and formats */
    av_register_all();

    /* allocate the output media context */
    fmt = av_guess_format(NULL, argv[1], NULL);
    if (!fmt) {
        std::cerr << "Could not determine output format" << std::endl;
        return 1;
    }
    open_output_file(argv[1], &oc, fmt);

    /* initialize the frame counter */
    frame_count = 0;

    /* initialize the timestamp increment */
    tincr = 2 * M_PI * STREAM_FRAME_RATE / STREAM_DURATION;
    next_pts = 0;

    /* main loop */
    for (i = 0; i < 100; i++) {
        AVStream *st;
        AVCodecContext *c;
        AVRational time_base;
        AVFrame *frame;
        int got_packet = 0;
        int ret;

        /* get the video stream */
        st = oc->streams[0];
        c = st->codec;
        time_base = st->time_base;

        /* allocate a new frame */
        frame = av_frame_alloc();
        if (!frame) {
            std::cerr << "Could not allocate video frame" << std::endl;
            exit(1);
        }

        /* generate synthetic video */
        frame->pts = av_rescale_q(frame_count, time_base, c->time_base);
        frame->format = c->pix_fmt;
        frame->width = c->width;
        frame->height = c->height;
        ret = av_frame_get_buffer(frame, 0);
        if (ret < 0) {
            std::cerr << "Could not allocate frame data" << std::endl;
            exit(1);
        }
        for (int y = 0; y < c->height; y++)
            for (int x = 0; x < c->width; x++)
                frame->data[0][y * frame->linesize[0] + x] = x + y + frame_count * 3;
        for (int y = 0; y < c->height / 2; y++) {
            for (int x = 0; x < c->width / 2; x++) {
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + frame_count * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + frame_count * 5;
            }
        }

        /* encode the frame */
        av_init_packet(&pkt);
        pkt.data = NULL;
        pkt.size = 0;
        ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
        if (ret < 0) {
            std::cerr << "Error encoding video frame" << std::endl;
            exit(1);
        }

        /* if the frame was encoded, write it to the file */
        if (got_packet) {
            pkt.stream_index = st->index;
            av_packet_rescale_ts(&pkt, time_base, st->time_base);
            ret = av_interleaved_write_frame(oc, &pkt);
            if (ret < 0) {
                std::cerr << "Error while writing video frame" << std::endl;
                exit(1);
            }
        }

        /* increase the frame count */
        frame_count++;

        /* calculate the next presentation timestamp */
        t = (double)frame_count / STREAM_FRAME_RATE;
        next_pts += (int64_t)(tincr * 1000);
        if (next_pts > (double)av_gettime()) {
            av_usleep(next_pts - av_gettime());
        }

        /* free the frame */
        av_frame_free(&frame);
    }

    /* seek to a specific frame */
    seek_to_frame(oc, 0, 30);

    /* continue encoding frames */
    for (; i < 200; i++) {
        AVStream *st;
        AVCodecContext *c;
        AVRational time_base;
        AVFrame *frame;
        int got_packet = 0;
        int ret;

        /* get the video stream */
        st = oc->streams[0];
        c = st->codec;
        time_base = st->time_base;

        /* allocate a new frame */
        frame = av_frame_alloc();
        if (!frame) {
            std::cerr << "Could not allocate video frame" << std::endl;
            exit(1);
        }

        /* generate synthetic video */
        frame->pts = av_rescale_q(frame_count, time_base, c->time_base);
        frame->format = c->pix_fmt;
        frame->width = c->width;
        frame->height = c->height;
        ret = av_frame_get_buffer(frame, 0);
        if (ret < 0) {
            std::cerr << "Could not allocate frame data" << std::endl;
            exit(1);
        }
        for (int y = 0; y < c->height; y++)
            for (int x = 0; x < c->width; x++)
                frame->data[0][y * frame->linesize[0] + x] = x + y + frame_count * 3;
        for (int y = 0; y < c->height / 2; y++) {
            for (int x = 0; x < c->width / 2; x++) {
                frame->data[1][y * frame->linesize[1] + x] = 128 + y + frame_count * 2;
                frame->data[2][y * frame->linesize[2] + x] = 64 + x + frame_count * 5;
            }
        }

        /* encode the frame */
        av_init_packet(&pkt);
        pkt.data = NULL;
        pkt.size = 0;
        ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
        if (ret < 0) {
            std::cerr << "Error encoding video frame" << std::endl;
            exit(1);
        }

        /* if the frame was encoded, write it to the file */
        if (got_packet) {
            pkt.stream_index = st->index;
            av_packet_rescale_ts(&pkt, time_base, st->time_base);
            ret = av_interleaved_write_frame(oc, &pkt);
            if (ret < 0) {
                std::cerr << "Error while writing video frame" << std::endl;
                exit(1);
            }
        }

        /* increase the frame count */
        frame_count++;

        /* calculate the next presentation timestamp */
        t = (double)frame_count / STREAM_FRAME_RATE;
        next_pts += (int64_t)(tincr * 1000);
        if (next_pts > (double)av_gettime()) {
            av_usleep(next_pts - av_gettime());
        }

        /* free the frame */
        av_frame_free(&frame);
    }

    /* close the output file and free the format context */
    close_output_file(oc);

    return 0;
}

这个代码会生成一个10秒的视频，并在第30帧处进行跳转，继续生成另外10秒的视频。视频中的图像是随机生成的。你可以根据自己的需求进行修改。