FFMPEG trim by remux doesn't write a keyframe

I am using the FFMPEG libraries to trim video files. I do this all as a remux, with no encoding or decoding.

Trimming currently works correctly with audio, but the trimmed video data appears as a solid color, with small squares of pixels changing. I believe this is because I am not catching/writing a keyframe. It is my understanding that av_seek_frame will seek to a keyframe, which does not seem to be the case..

If need be, can I decode and then reencode just the first video frame I read after seeking? This will probably be more code than reencoding every frame, but speed is the primary issue here, not complexity.

Thank you for any help. Also I apologize if I am misunderstanding something to do with video files, I'm still new to this.

Example output frame:

Code, adapted from the remux example provided with ffmpeg:

const char *out_filename = "aaa.mp4";

FILE *fp = fdopen(fd, "r");
fseek(fp, 0, SEEK_SET);

if ( fp ) {

    // Build an ffmpeg file
    char path[512];
    sprintf(path, "pipe:%d", fileno(fp));

    // Turn on verbosity
    av_log_set_level( AV_LOG_DEBUG );
    av_log_set_callback( avLogCallback );


    av_register_all();
    avcodec_register_all();


    AVOutputFormat *ofmt = NULL;
    AVFormatContext *ifmt_ctx = avformat_alloc_context(), *ofmt_ctx = NULL;
    AVPacket pkt;
    int ret, i;


    if ((ret = avformat_open_input(&ifmt_ctx, path, av_find_input_format("mp4"), NULL)) < 0) {
        LOG("Could not open input file '%s'", path);
        goto end;
    }

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


    avformat_alloc_output_context2(&ofmt_ctx, NULL, NULL, out_filename);
    if (!ofmt_ctx) {
        LOG("Could not create output context\n");
        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, NULL);

        if (!out_stream) {
            LOG("Failed allocating output stream\n");
            goto end;
        }

        ret = avcodec_parameters_copy(out_stream->codecpar, in_stream->codecpar);
        if (ret < 0) {
            LOG("Failed to copy context from input to output stream codec context\n");
            goto end;
        }
        out_stream->codecpar->codec_tag = 0;
    }

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

    ret = avformat_write_header(ofmt_ctx, NULL);
    if (ret < 0) {
        LOG("Error occurred when writing headers\n");
        goto end;
    }

    ret = av_seek_frame(ifmt_ctx, -1, from_seconds * AV_TIME_BASE, AVSEEK_FLAG_ANY);
    if (ret < 0) {
        LOG("Error seek\n");
        goto end;
    }

    int64_t *dts_start_from;
    int64_t *pts_start_from;
    dts_start_from = (int64_t *) malloc(sizeof(int64_t) * ifmt_ctx->nb_streams);
    memset(dts_start_from, 0, sizeof(int64_t) * ifmt_ctx->nb_streams);
    pts_start_from = (int64_t *) 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);
        LOG("while %d", ret);
        LOG("Packet size: %d", pkt.size);
        LOG("Packet stream: %d", pkt.stream_index);
        if (ret < 0)
            break;

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

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

        if (dts_start_from[pkt.stream_index] == 0) {
            dts_start_from[pkt.stream_index] = pkt.dts;
            printf("dts_start_from: %s\n", av_ts_make_string((char[AV_TS_MAX_STRING_SIZE]){0},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\n", av_ts_make_string((char[AV_TS_MAX_STRING_SIZE]){0},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, (AVRounding) (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, (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, in_stream->time_base, out_stream->time_base);
        pkt.pos = -1;
        printf("\n");

        ret = av_interleaved_write_frame(ofmt_ctx, &pkt);
        if (ret < 0) {
            LOG("Error muxing packet\n");
            break;
        }
        av_packet_unref(&pkt);
    }
    free(dts_start_from);
    free(pts_start_from);

    av_write_trailer(ofmt_ctx);

    end:
    LOG("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) {
        LOG("-- Error occurred: %s\n", av_err2str(ret));
        return 1;
    }
}