ffmpeg#どのように制御してただカプセル化を回転して符号化を必要としません

flvからmkvへのようなパッケージを転送するだけで、トランスコード、すなわち復号および符号化のプロセスに関与しない場合がある.ffmpegはどうやって作ったのですか?答えは、InputStreamのdecoding_needed OutputStreamのencoding_neededとstream_copy.
decoding_needed
復号するかどうかを制御します.

typedef struct InputStream {
    int file_index;
    AVStream *st;
    int discard;             /* true if stream data should be discarded */
    int user_set_discard;
    int decoding_needed;     /* non zero if the packets must be decoded in 'raw_fifo', see DECODING_FOR_* */
#define DECODING_FOR_OST    1
#define DECODING_FOR_FILTER 2
...
}

encoding_needed

エンコードの有無を制御します.

typedef struct OutputStream {
    int file_index;          /* file index */
    int index;               /* stream index in the output file */
    int source_index;        /* InputStream index */
    AVStream *st;            /* stream in the output file */
    int encoding_needed;     /* true if encoding needed for this stream */
...
    int stream_copy;
...
}

/* pkt = NULL means EOF (needed to flush decoder buffers) */
static int process_input_packet(InputStream *ist, const AVPacket *pkt, int no_eof)
{
    int ret = 0, i;
    int repeating = 0;
    int eof_reached = 0;

    AVPacket avpkt;
...
    // while we have more to decode or while the decoder did output something on EOF
    while (ist->decoding_needed) {
        int64_t duration_dts = 0;
        int64_t duration_pts = 0;
        int got_output = 0;
        int decode_failed = 0;

        ist->pts = ist->next_pts;
        ist->dts = ist->next_dts;

        switch (ist->dec_ctx->codec_type) {
        case AVMEDIA_TYPE_AUDIO:
            ret = decode_audio    (ist, repeating ? NULL : &avpkt, &got_output,
                                   &decode_failed);
            break;
        case AVMEDIA_TYPE_VIDEO:
            ret = decode_video    (ist, repeating ? NULL : &avpkt, &got_output, &duration_pts, !pkt,
                                   &decode_failed);
            if (!repeating || !pkt || got_output) {
                if (pkt && pkt->duration) {
                    duration_dts = av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
                } else if(ist->dec_ctx->framerate.num != 0 && ist->dec_ctx->framerate.den != 0) {
                    int ticks= av_stream_get_parser(ist->st) ? av_stream_get_parser(ist->st)->repeat_pict+1 : ist->dec_ctx->ticks_per_frame;
                    duration_dts = ((int64_t)AV_TIME_BASE *
                                    ist->dec_ctx->framerate.den * ticks) /
                                    ist->dec_ctx->framerate.num / ist->dec_ctx->ticks_per_frame;
                }

                if(ist->dts != AV_NOPTS_VALUE && duration_dts) {
                    ist->next_dts += duration_dts;
                }else
                    ist->next_dts = AV_NOPTS_VALUE;
            }

            if (got_output) {
                if (duration_pts > 0) {
                    ist->next_pts += av_rescale_q(duration_pts, ist->st->time_base, AV_TIME_BASE_Q);
                } else {
                    ist->next_pts += duration_dts;
                }
            }
            break;
        case AVMEDIA_TYPE_SUBTITLE:
            if (repeating)
                break;
            ret = transcode_subtitles(ist, &avpkt, &got_output, &decode_failed);
            if (!pkt && ret >= 0)
                ret = AVERROR_EOF;
            break;
        default:
            return -1;
        }

        if (ret == AVERROR_EOF) {
            eof_reached = 1;
            break;
        }

        if (ret < 0) {
            if (decode_failed) {
                av_log(NULL, AV_LOG_ERROR, "Error while decoding stream #%d:%d: %s
",
                       ist->file_index, ist->st->index, av_err2str(ret));
            } else {
                av_log(NULL, AV_LOG_FATAL, "Error while processing the decoded "
                       "data for stream #%d:%d
", ist->file_index, ist->st->index);
            }
            if (!decode_failed || exit_on_error)
                exit_program(1);
            break;
        }

        if (got_output)
            ist->got_output = 1;

        if (!got_output)
            break;

        // During draining, we might get multiple output frames in this loop.
        // ffmpeg.c does not drain the filter chain on configuration changes,
        // which means if we send multiple frames at once to the filters, and
        // one of those frames changes configuration, the buffered frames will
        // be lost. This can upset certain FATE tests.
        // Decode only 1 frame per call on EOF to appease these FATE tests.
        // The ideal solution would be to rewrite decoding to use the new
        // decoding API in a better way.
        if (!pkt)
            break;

        repeating = 1;
    }
...

    for (i = 0; i < nb_output_streams; i++) {
        OutputStream *ost = output_streams[i];

        if (!check_output_constraints(ist, ost) || ost->encoding_needed)
            continue;

        do_streamcopy(ist, ost, pkt);
    }

    return !eof_reached;
}