ffmpeg # 怎么控制只是转封装而不需要转码

有时候可能只需要转下封装，比如从flv到mkv，这时候就不涉及转码，也就是不涉及解码和编码的过程。
ffmpeg是如何做的呢?
答案是:
通过以下几个参数来控制，分别是:
InputStream中的decoding_needed
OutputStream的encoding_needed和stream_copy。

decoding_needed

用于控制是否解码。

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

用于控制是否编码。

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\n",
                       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\n", 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;
}