c - 读取视频文件中每 60 帧的运动矢量

问题描述

我有以下代码读取视频文件并从每一帧中提取运动矢量并将它们输出到标准输出：

#include <libavutil/motion_vector.h>
#include <libavformat/avformat.h>

static AVFormatContext *fmt_ctx = NULL;
static AVCodecContext *video_dec_ctx = NULL;
static AVStream *video_stream = NULL;
static const char *src_filename = NULL;

static int video_stream_idx = -1;
static AVFrame *frame = NULL;
static int video_frame_count = 0;

static int decode_packet(const AVPacket *pkt)
{
    //fprintf(stderr, "%d\n", video_dec_ctx->width);
    int ret = avcodec_send_packet(video_dec_ctx, pkt);
    if (ret < 0) {
        fprintf(stderr, "Error while sending a packet to the decoder: %s\n", av_err2str(ret));
        return ret;
    }

    while (ret >= 0)  {
        ret = avcodec_receive_frame(video_dec_ctx, frame);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) {
            break;
        } else if (ret < 0) {
            fprintf(stderr, "Error while receiving a frame from the decoder: %s\n", av_err2str(ret));
            return ret;
        }

        if (ret >= 0) {
            int i;
            AVFrameSideData *sd;

            video_frame_count++;
            sd = av_frame_get_side_data(frame, AV_FRAME_DATA_MOTION_VECTORS);
            if (sd) {
                const AVMotionVector *mvs = (const AVMotionVector *)sd->data;
                for (i = 0; i < sd->size / sizeof(*mvs); i++) {
                    const AVMotionVector *mv = &mvs[i];
                    printf("%d,%4d,%4d,%4d\n",
                        video_frame_count,
                        abs(mv->src_x - mv->dst_x),
                        abs(mv->src_y - mv->dst_y));
                }
            }
            av_frame_unref(frame);
        }
    }

    return 0;
}

static int open_codec_context(AVFormatContext *fmt_ctx, enum AVMediaType type)
{
    int ret;
    AVStream *st;
    AVCodecContext *dec_ctx = NULL;
    AVCodec *dec = NULL;
    AVDictionary *opts = NULL;

    ret = av_find_best_stream(fmt_ctx, type, -1, -1, &dec, 0);
    if (ret < 0) {
        fprintf(stderr, "Could not find %s stream in input file '%s'\n",
                av_get_media_type_string(type), src_filename);
        return ret;
    } else {
        int stream_idx = ret;
        st = fmt_ctx->streams[stream_idx];

        dec_ctx = avcodec_alloc_context3(dec);
        if (!dec_ctx) {
            fprintf(stderr, "Failed to allocate codec\n");
            return AVERROR(EINVAL);
        }

        ret = avcodec_parameters_to_context(dec_ctx, st->codecpar);
        if (ret < 0) {
            fprintf(stderr, "Failed to copy codec parameters to codec context\n");
            return ret;
        }

        /* Init the video decoder */
        av_dict_set(&opts, "flags2", "+export_mvs", 0);
        if ((ret = avcodec_open2(dec_ctx, dec, &opts)) < 0) {
            fprintf(stderr, "Failed to open %s codec\n",
                    av_get_media_type_string(type));
            return ret;
        }

        video_stream_idx = stream_idx;
        video_stream = fmt_ctx->streams[video_stream_idx];
        video_dec_ctx = dec_ctx;
    }

    return 0;
}

int main(int argc, char **argv)
{
    int ret = 0;
    AVPacket pkt = { 0 };

    if (argc != 2) {
        fprintf(stderr, "Usage: %s <video>\n", argv[0]);
        exit(1);
    }
    src_filename = argv[1];

    if (avformat_open_input(&fmt_ctx, src_filename, NULL, NULL) < 0) {
        fprintf(stderr, "Could not open source file %s\n", src_filename);
        exit(1);
    }

    if (avformat_find_stream_info(fmt_ctx, NULL) < 0) {
        fprintf(stderr, "Could not find stream information\n");
        exit(1);
    }

    open_codec_context(fmt_ctx, AVMEDIA_TYPE_VIDEO);

    av_dump_format(fmt_ctx, 0, src_filename, 0);

    if (!video_stream) {
        fprintf(stderr, "Could not find video stream in the input, aborting\n");
        ret = 1;
        goto end;
    }

    frame = av_frame_alloc();
    if (!frame) {
        fprintf(stderr, "Could not allocate frame\n");
        ret = AVERROR(ENOMEM);
        goto end;
    }

    printf("framenum,dx,dy\n");

    /* read frames from the file */
    while (av_read_frame(fmt_ctx, &pkt) >= 0) {
        if (pkt.stream_index == video_stream_idx)
            ret = decode_packet(&pkt);
        av_packet_unref(&pkt);
        if (ret < 0)
            break;
    }

    /* flush cached frames */
    decode_packet(NULL);

end:
    avcodec_free_context(&video_dec_ctx);
    avformat_close_input(&fmt_ctx);
    av_frame_free(&frame);
    return ret < 0;
}

相反并输出每一帧，我只想每 60 帧读取一帧。程序的速度非常重要，所以我想尽可能多地消除不必要的代码。问题是我很难理解如何在不读取新帧的情况下推进视频流。我打电话av_read_frame(fmt_ctx, &pkt)来获取被解码的数据包信息，decode_packet然后继续读取下一个，但我找不到一种方法来只获取每 60 帧中的一帧的数据包信息。

标签： cffmpeg

c - 读取视频文件中每 60 帧的运动矢量

问题描述

解决方案

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