FFmpeg資料結構AVPacket

作者： 葉餘 來源： https://www.cnblogs.com/leisure_chn/p/10410320.html

1. 資料結構定義

struct AVPacket定義于<libavcodec/avcodec.h>

struct AVPacket packet;      

AVPacket中存儲的是經過編碼的壓縮資料。在解碼中，AVPacket由解複用器輸出到解碼器；在編碼中，AVPacket由編碼器輸出到複用器。下圖中，解複用器(demuxer)的輸出和複用器(muxer)的輸入“encoded data packets”的資料類型就是AVPacket：

_______              ______________
|       |            |              |
| input |  demuxer   | encoded data |   decoder
| file  | ---------> | packets      | -----+
|_______|            |______________|      |
                                           v
                                       _________
                                      |         |
                                      | decoded |
                                      | frames  |
                                      |_________|
 ________             ______________       |
|        |           |              |      |
| output | <-------- | encoded data | <----+
| file   |   muxer   | packets      |   encoder
|________|           |______________|      

對于視訊而言，一個AVPacket通常隻包含一個壓縮視訊幀。而對于音頻而言，一個AVPacket可能包含多個完整的音頻壓縮幀。AVPacket也可以不包含壓縮編碼資料，而隻包含side data，這種包可以稱為空packet。例如，編碼結束後隻需要更新一些參數時就可以發空packet。

AVPacket對象可以在棧上配置設定，注意此處指的是AVPacket對象本身。而AVPacket中包含的資料緩沖區是通過av_malloc()在堆上配置設定的。

TODO: AVPacket對象在棧上配置設定，原理不清楚，待研究

/**
 * This structure stores compressed data. It is typically exported by demuxers
 * and then passed as input to decoders, or received as output from encoders and
 * then passed to muxers.
 *
 * For video, it should typically contain one compressed frame. For audio it may
 * contain several compressed frames. Encoders are allowed to output empty
 * packets, with no compressed data, containing only side data
 * (e.g. to update some stream parameters at the end of encoding).
 *
 * AVPacket is one of the few structs in FFmpeg, whose size is a part of public
 * ABI. Thus it may be allocated on stack and no new fields can be added to it
 * without libavcodec and libavformat major bump.
 *
 * The semantics of data ownership depends on the buf field.
 * If it is set, the packet data is dynamically allocated and is
 * valid indefinitely until a call to av_packet_unref() reduces the
 * reference count to 0.
 *
 * If the buf field is not set av_packet_ref() would make a copy instead
 * of increasing the reference count.
 *
 * The side data is always allocated with av_malloc(), copied by
 * av_packet_ref() and freed by av_packet_unref().
 *
 * @see av_packet_ref
 * @see av_packet_unref
 */
typedef struct AVPacket {
    /**
     * A reference to the reference-counted buffer where the packet data is
     * stored.
     * May be NULL, then the packet data is not reference-counted.
     */
    AVBufferRef *buf;
    /**
     * Presentation timestamp in AVStream->time_base units; the time at which
     * the decompressed packet will be presented to the user.
     * Can be AV_NOPTS_VALUE if it is not stored in the file.
     * pts MUST be larger or equal to dts as presentation cannot happen before
     * decompression, unless one wants to view hex dumps. Some formats misuse
     * the terms dts and pts/cts to mean something different. Such timestamps
     * must be converted to true pts/dts before they are stored in AVPacket.
     */
    int64_t pts;
    /**
     * Decompression timestamp in AVStream->time_base units; the time at which
     * the packet is decompressed.
     * Can be AV_NOPTS_VALUE if it is not stored in the file.
     */
    int64_t dts;
    uint8_t *data;
    int   size;
    int   stream_index;
    /**
     * A combination of AV_PKT_FLAG values
     */
    int   flags;
    /**
     * Additional packet data that can be provided by the container.
     * Packet can contain several types of side information.
     */
    AVPacketSideData *side_data;
    int side_data_elems;

    /**
     * Duration of this packet in AVStream->time_base units, 0 if unknown.
     * Equals next_pts - this_pts in presentation order.
     */
    int64_t duration;

    int64_t pos;                            ///< byte position in stream, -1 if unknown

#if FF_API_CONVERGENCE_DURATION
    /**
     * @deprecated Same as the duration field, but as int64_t. This was required
     * for Matroska subtitles, whose duration values could overflow when the
     * duration field was still an int.
     */
    attribute_deprecated
    int64_t convergence_duration;
#endif
} AVPacket;      

音視訊資料緩沖區

• uint8_t *data

  :

• int size

  資料緩沖區位址與大小。音視訊編碼壓縮資料存儲于此片記憶體區域。此記憶體區域由

  AVBufferRef *buf

  管理。

• AVBufferRef *buf

  資料緩沖區引用，也可叫引用計數緩沖區。對上一字段

  uint8_t *data

  指向的記憶體區域提供引用計數等管理機制。

  AVBufferRef對資料緩沖區提供了管理機制，使用者不應直接通路資料緩沖區。參考“

  FFmpeg資料結構AVBuffer

  ”

  如果

  buf

  值為NULL，則

  data

  指向的資料緩沖區不使用引用計數機制。

  av_packet_ref(dst, src)

  将執行資料緩沖區的拷貝，而非僅僅增加緩沖區引用計數。

  buf

  值非NULL，則

  data

  指向的資料緩沖區使用引用計數機制。

  av_packet_ref(dst, src)

  将不拷貝緩沖區，而僅增加緩沖區引用計數。

  av_packet_unref()

  将資料緩沖區引用計數減1，當緩沖區引用計數為0時，緩沖區記憶體被FFmpeg回收。

  對于

  struct AVPacket pkt

  對象，如果

  pkt.buf

  值非NULL，則有pkt.data == pkt.buf->data == pkt.buf->buffer.data

額外類型資料

• AVPacketSideData *side_data

• int side_data_elems

  由容器(container)提供的額外包資料。TODO: 待研究

packet屬性

• int64_t pts

  顯示時間戳。機關time_base，幀率的倒數。

• int64_t dts

  解碼時間戳。機關time_base，幀率的倒數。

• int stream_index

  目前包(packet)所有流(stream)的索引(index)。

• int flags

  packet标志位。比如是否關鍵幀等。

• int64_t duration

  目前包解碼後的幀播放持續的時長。機關timebase。值等于下一幀pts減目前幀pts。

• int64_t pos

  目前包在流中的位置，機關位元組。

2. 關鍵函數實作

這裡列出的幾個關鍵函數，主要是為了幫助了解

struct AVPacket

資料結構

2.1 av_packet_ref()

int av_packet_ref(AVPacket *dst, const AVPacket *src)
{
    int ret;

    ret = av_packet_copy_props(dst, src);
    if (ret < 0)
        return ret;

    if (!src->buf) {
        ret = packet_alloc(&dst->buf, src->size);
        if (ret < 0)
            goto fail;
        if (src->size)
            memcpy(dst->buf->data, src->data, src->size);

        dst->data = dst->buf->data;
    } else {
        dst->buf = av_buffer_ref(src->buf);
        if (!dst->buf) {
            ret = AVERROR(ENOMEM);
            goto fail;
        }
        dst->data = src->data;
    }

    dst->size = src->size;

    return 0;
fail:
    av_packet_free_side_data(dst);
    return ret;
}      

av_packet_ref()作了處理如下:

a) 如果src->buf為NULL，則将src緩沖區拷貝到新建立的dst緩沖區，注意src緩沖區不支援引用計數，但建立的dst緩沖區是支援引用計數的，因為dst->buf不為NULL。

b) 如果src->buf不為空，則dst與src共用緩沖區，調用

av_buffer_ref()

增加緩沖區引用計數即可。

av_buffer_ref()

分析參考“

2.2 av_packet_unref()

void av_packet_unref(AVPacket *pkt)
{
    av_packet_free_side_data(pkt);
    av_buffer_unref(&pkt->buf);
    av_init_packet(pkt);
    pkt->data = NULL;
    pkt->size = 0;
}      

av_packet_unref()

登出

AVPacket *pkt

對象，并調用

av_buffer_unref(&pkt->buf);

将緩沖區引用計數減1。

av_buffer_unref()

中将緩沖區引用計數減1後，若緩沖區引用計數變成0，則回收緩沖區記憶體。

av_buffer_unref()

3. 參考資料

[1]

FFmpeg資料結構：AVPacket解析

,

https://www.cnblogs.com/wangguchangqing/p/5790705.html

4. 修改記錄

2018-12-14 V1.0 初稿

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