Create FFmpeg 结构体学习(五)： AVCodec 分析.md

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+AVCodec是存储编解码器信息的结构体。下面我们来分析一下该结构体里重要变量的含义和作用。
+
+# 一、源码整理
+
+首先我们先看一下结构体AVCodec的定义的结构体源码(位于libavcodec/avcodec.h)：
+
+
+
+```
+/* 雷霄骅 
+ * 中国传媒大学/数字电视技术 
+ * leixiaohua1020@126.com 
+ * 
+ */
+ /**
+ * 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;
+```
+
+
+
+# 二、AVCodec 重点字段
+
+下面说一下最主要的几个变量：
+
+
+
+```
+const char *name：编解码器的名字，比较短
+
+const char *long_name：编解码器的名字，全称，比较长
+
+enum AVMediaType type：指明了类型，是视频，音频，还是字幕
+
+enum AVCodecID id：ID，不重复
+
+const AVRational *supported_framerates：支持的帧率（仅视频）
+
+const enum AVPixelFormat *pix_fmts：支持的像素格式（仅视频）
+
+const int *supported_samplerates：支持的采样率（仅音频）
+
+const enum AVSampleFormat *sample_fmts：支持的采样格式（仅音频）
+
+const uint64_t *channel_layouts：支持的声道数（仅音频）
+
+int priv_data_size：私有数据的大小
+```
+
+
+
+详细介绍几个变量：
+
+### 1.enum AVMediaType type
+
+AVMediaType定义如下：
+
+
+
+```
+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定义如下：
+
+
+
+```
+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,
+    ...
+}
+```
+
+
+
+### 3.const enum AVPixelFormat *pix_fmts
+
+AVPixelFormat定义如下：
+
+
+
+```
+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,
+    ...（代码太长，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
+};
+```
+
+
+
+ 每一个编解码器对应一个该结构体，查看一下ffmpeg的源代码，我们可以看一下H.264解码器的结构体如下所示（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解码器结构体（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}
+};
+```
+
+
+
+下面简单介绍一下遍历ffmpeg中的解码器信息的方法（这些解码器以一个链表的形式存储）： 
+
+1.注册所有编解码器：av_register_all();
+
+2.声明一个AVCodec类型的指针，比如说AVCodec* first_c;
+
+3.调用av_codec_next()函数，即可获得指向链表下一个解码器的指针，循环往复可以获得所有解码器的信息。注意，如果想要获得指向第一个解码器的指针，则需要将该函数的参数设置为NULL。