FFMPEG結構體分析：AVCodec

注：寫了一系列的結構體的分析的文章，在這裏列一個列表：

FFMPEG結構體分析：AVFrame

FFMPEG結構體分析：AVFormatContext

FFMPEG結構體分析：AVCodecContext

FFMPEG結構體分析：AVIOContext

FFMPEG結構體分析：AVCodec

FFMPEG結構體分析：AVStream
FFMPEG結構體分析：AVPacket

FFMPEG有幾個最重要的結構體，包含了解協議，解封裝，解碼操作，此前已經進行過分析：

FFMPEG中最關鍵的結構體之間的關係

在此不再詳述，其中AVCodec是存儲編解碼器信息的結構體。本文將會詳細分析一下該結構體裏每個變量的含義和作用。

首先看一下結構體的定義（位於avcodec.h文件中）：

/* 雷霄驊
 * 中國傳媒大學/數字電視技術
 * [email protected]
 *
 */
 /**
 * AVCodec.
 */
typedef struct AVCodec {
    /**
     * Name of the codec implementation.
     * The name is globally unique among encoders and among decoders (but an
     * encoder and a decoder can share the same name).
     * This is the primary way to find a codec from the user perspective.
     */
    const char *name;
    /**
     * Descriptive name for the codec, meant to be more human readable than name.
     * You should use the NULL_IF_CONFIG_SMALL() macro to define it.
     */
    const char *long_name;
    enum AVMediaType type;
    enum CodecID id;
    /**
     * Codec capabilities.
     * see CODEC_CAP_*
     */
    int capabilities;
    const AVRational *supported_framerates; ///< array of supported framerates, or NULL if any, array is terminated by {0,0}
    const enum PixelFormat *pix_fmts;       ///< array of supported pixel formats, or NULL if unknown, array is terminated by -1
    const int *supported_samplerates;       ///< array of supported audio samplerates, or NULL if unknown, array is terminated by 0
    const enum AVSampleFormat *sample_fmts; ///< array of supported sample formats, or NULL if unknown, array is terminated by -1
    const uint64_t *channel_layouts;         ///< array of support channel layouts, or NULL if unknown. array is terminated by 0
    uint8_t max_lowres;                     ///< maximum value for lowres supported by the decoder
    const AVClass *priv_class;              ///< AVClass for the private context
    const AVProfile *profiles;              ///< array of recognized profiles, or NULL if unknown, array is terminated by {FF_PROFILE_UNKNOWN}

    /*****************************************************************
     * No fields below this line are part of the public API. They
     * may not be used outside of libavcodec and can be changed and
     * removed at will.
     * New public fields should be added right above.
     *****************************************************************
     */
    int priv_data_size;
    struct AVCodec *next;
    /**
     * @name Frame-level threading support functions
     * @{
     */
    /**
     * If defined, called on thread contexts when they are created.
     * If the codec allocates writable tables in init(), re-allocate them here.
     * priv_data will be set to a copy of the original.
     */
    int (*init_thread_copy)(AVCodecContext *);
    /**
     * Copy necessary context variables from a previous thread context to the current one.
     * If not defined, the next thread will start automatically; otherwise, the codec
     * must call ff_thread_finish_setup().
     *
     * dst and src will (rarely) point to the same context, in which case memcpy should be skipped.
     */
    int (*update_thread_context)(AVCodecContext *dst, const AVCodecContext *src);
    /** @} */

    /**
     * Private codec-specific defaults.
     */
    const AVCodecDefault *defaults;

    /**
     * Initialize codec static data, called from avcodec_register().
     */
    void (*init_static_data)(struct AVCodec *codec);

    int (*init)(AVCodecContext *);
    int (*encode)(AVCodecContext *, uint8_t *buf, int buf_size, void *data);
    /**
     * Encode data to an AVPacket.
     *
     * @param      avctx          codec context
     * @param      avpkt          output AVPacket (may contain a user-provided buffer)
     * @param[in]  frame          AVFrame containing the raw data to be encoded
     * @param[out] got_packet_ptr encoder sets to 0 or 1 to indicate that a
     *                            non-empty packet was returned in avpkt.
     * @return 0 on success, negative error code on failure
     */
    int (*encode2)(AVCodecContext *avctx, AVPacket *avpkt, const AVFrame *frame,
                   int *got_packet_ptr);
    int (*decode)(AVCodecContext *, void *outdata, int *outdata_size, AVPacket *avpkt);
    int (*close)(AVCodecContext *);
    /**
     * Flush buffers.
     * Will be called when seeking
     */
    void (*flush)(AVCodecContext *);
} AVCodec;

下面說一下最主要的幾個變量：

const char *name：編解碼器的名字，比較短

const char *long_name：編解碼器的名字，全稱，比較長

enum AVMediaType type：指明瞭類型，是視頻，音頻，還是字幕

enum AVCodecID id：ID，不重複

const AVRational *supported_framerates：支持的幀率（僅視頻）

const enum AVPixelFormat *pix_fmts：支持的像素格式（僅視頻）

const int *supported_samplerates：支持的採樣率（僅音頻）

const enum AVSampleFormat *sample_fmts：支持的採樣格式（僅音頻）

const uint64_t *channel_layouts：支持的聲道數（僅音頻）

int priv_data_size：私有數據的大小

詳細介紹幾個變量：

1.enum AVMediaType type

AVMediaType定義如下：

enum AVMediaType {
    AVMEDIA_TYPE_UNKNOWN = -1,  ///< Usually treated as AVMEDIA_TYPE_DATA
    AVMEDIA_TYPE_VIDEO,
    AVMEDIA_TYPE_AUDIO,
    AVMEDIA_TYPE_DATA,          ///< Opaque data information usually continuous
    AVMEDIA_TYPE_SUBTITLE,
    AVMEDIA_TYPE_ATTACHMENT,    ///< Opaque data information usually sparse
    AVMEDIA_TYPE_NB
};

2.enum AVCodecID id

AVCodecID定義如下：

enum AVCodecID {
    AV_CODEC_ID_NONE,

    /* video codecs */
    AV_CODEC_ID_MPEG1VIDEO,
    AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
    AV_CODEC_ID_MPEG2VIDEO_XVMC,
    AV_CODEC_ID_H261,
    AV_CODEC_ID_H263,
    AV_CODEC_ID_RV10,
    AV_CODEC_ID_RV20,
    AV_CODEC_ID_MJPEG,
    AV_CODEC_ID_MJPEGB,
    AV_CODEC_ID_LJPEG,
    AV_CODEC_ID_SP5X,
    AV_CODEC_ID_JPEGLS,
    AV_CODEC_ID_MPEG4,
    AV_CODEC_ID_RAWVIDEO,
    AV_CODEC_ID_MSMPEG4V1,
    AV_CODEC_ID_MSMPEG4V2,
    AV_CODEC_ID_MSMPEG4V3,
    AV_CODEC_ID_WMV1,
    AV_CODEC_ID_WMV2,
    AV_CODEC_ID_H263P,
    AV_CODEC_ID_H263I,
    AV_CODEC_ID_FLV1,
    AV_CODEC_ID_SVQ1,
    AV_CODEC_ID_SVQ3,
    AV_CODEC_ID_DVVIDEO,
    AV_CODEC_ID_HUFFYUV,
    AV_CODEC_ID_CYUV,
    AV_CODEC_ID_H264,
    ...（代碼太長，略）
}

3.const enum AVPixelFormat *pix_fmts

AVPixelFormat定義如下：

enum AVPixelFormat {
    AV_PIX_FMT_NONE = -1,
    AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
    AV_PIX_FMT_YUYV422,   ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
    AV_PIX_FMT_RGB24,     ///< packed RGB 8:8:8, 24bpp, RGBRGB...
    AV_PIX_FMT_BGR24,     ///< packed RGB 8:8:8, 24bpp, BGRBGR...
    AV_PIX_FMT_YUV422P,   ///< planar YUV 4:2:2, 16bpp, (1 Cr & Cb sample per 2x1 Y samples)
    AV_PIX_FMT_YUV444P,   ///< planar YUV 4:4:4, 24bpp, (1 Cr & Cb sample per 1x1 Y samples)
    AV_PIX_FMT_YUV410P,   ///< planar YUV 4:1:0,  9bpp, (1 Cr & Cb sample per 4x4 Y samples)
    AV_PIX_FMT_YUV411P,   ///< planar YUV 4:1:1, 12bpp, (1 Cr & Cb sample per 4x1 Y samples)
    AV_PIX_FMT_GRAY8,     ///<        Y        ,  8bpp
    AV_PIX_FMT_MONOWHITE, ///<        Y        ,  1bpp, 0 is white, 1 is black, in each byte pixels are ordered from the msb to the lsb
    AV_PIX_FMT_MONOBLACK, ///<        Y        ,  1bpp, 0 is black, 1 is white, in each byte pixels are ordered from the msb to the lsb
    AV_PIX_FMT_PAL8,      ///< 8 bit with PIX_FMT_RGB32 palette
    AV_PIX_FMT_YUVJ420P,  ///< planar YUV 4:2:0, 12bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV420P and setting color_range
    AV_PIX_FMT_YUVJ422P,  ///< planar YUV 4:2:2, 16bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV422P and setting color_range
    AV_PIX_FMT_YUVJ444P,  ///< planar YUV 4:4:4, 24bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV444P and setting color_range
    AV_PIX_FMT_XVMC_MPEG2_MC,///< XVideo Motion Acceleration via common packet passing
    AV_PIX_FMT_XVMC_MPEG2_IDCT,
    ...（代碼太長，略）
}

4.const enum AVSampleFormat *sample_fmts

enum AVSampleFormat {
    AV_SAMPLE_FMT_NONE = -1,
    AV_SAMPLE_FMT_U8,          ///< unsigned 8 bits
    AV_SAMPLE_FMT_S16,         ///< signed 16 bits
    AV_SAMPLE_FMT_S32,         ///< signed 32 bits
    AV_SAMPLE_FMT_FLT,         ///< float
    AV_SAMPLE_FMT_DBL,         ///< double

    AV_SAMPLE_FMT_U8P,         ///< unsigned 8 bits, planar
    AV_SAMPLE_FMT_S16P,        ///< signed 16 bits, planar
    AV_SAMPLE_FMT_S32P,        ///< signed 32 bits, planar
    AV_SAMPLE_FMT_FLTP,        ///< float, planar
    AV_SAMPLE_FMT_DBLP,        ///< double, planar

    AV_SAMPLE_FMT_NB           ///< Number of sample formats. DO NOT USE if linking dynamically
};

每一個編解碼器對應一個該結構體，查看一下ffmpeg的源代碼，我們可以看一下H.264解碼器的結構體如下所示（h264.c）：

AVCodec ff_h264_decoder = {
    .name           = "h264",
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = CODEC_ID_H264,
    .priv_data_size = sizeof(H264Context),
    .init           = ff_h264_decode_init,
    .close          = ff_h264_decode_end,
    .decode         = decode_frame,
    .capabilities   = /*CODEC_CAP_DRAW_HORIZ_BAND |*/ CODEC_CAP_DR1 | CODEC_CAP_DELAY |
                      CODEC_CAP_SLICE_THREADS | CODEC_CAP_FRAME_THREADS,
    .flush= flush_dpb,
    .long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .init_thread_copy      = ONLY_IF_THREADS_ENABLED(decode_init_thread_copy),
    .update_thread_context = ONLY_IF_THREADS_ENABLED(decode_update_thread_context),
    .profiles = NULL_IF_CONFIG_SMALL(profiles),
    .priv_class     = &h264_class,
};

JPEG2000解碼器結構體（j2kdec.c）

AVCodec ff_jpeg2000_decoder = {
    .name           = "j2k",
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = CODEC_ID_JPEG2000,
    .priv_data_size = sizeof(J2kDecoderContext),
    .init           = j2kdec_init,
    .close          = decode_end,
    .decode         = decode_frame,
    .capabilities = CODEC_CAP_EXPERIMENTAL,
    .long_name = NULL_IF_CONFIG_SMALL("JPEG 2000"),
    .pix_fmts =
        (const enum PixelFormat[]) {PIX_FMT_GRAY8, PIX_FMT_RGB24, PIX_FMT_NONE}
};

下面簡單介紹一下遍歷ffmpeg中的解碼器信息的方法（這些解碼器以一個鏈表的形式存儲）：

1.註冊所有編解碼器：av_register_all();

2.聲明一個AVCodec類型的指針，比如說AVCodec* first_c;

3.調用av_codec_next()函數，即可獲得指向鏈表下一個解碼器的指針，循環往復可以獲得所有解碼器的信息。注意，如果想要獲得指向第一個解碼器的指針，則需要將該函數的參數設置爲NULL。

原文地址：http://blog.csdn.net/leixiaohua1020/article/details/14215833