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320 lines
11 KiB
320 lines
11 KiB
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。
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# 一、源码整理
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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)：
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```
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/* 雷霄骅
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* 中国传媒大学/数字电视技术
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* leixiaohua1020@126.com
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*
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*/
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/**
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* AVCodec.
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*/
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typedef struct AVCodec {
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/**
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* Name of the codec implementation.
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* The name is globally unique among encoders and among decoders (but an
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* encoder and a decoder can share the same name).
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* This is the primary way to find a codec from the user perspective.
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*/
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const char *name;
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/**
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* Descriptive name for the codec, meant to be more human readable than name.
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* You should use the NULL_IF_CONFIG_SMALL() macro to define it.
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*/
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const char *long_name;
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enum AVMediaType type;
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enum CodecID id;
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/**
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* Codec capabilities.
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* see CODEC_CAP_*
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*/
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int capabilities;
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const AVRational *supported_framerates; ///< array of supported framerates, or NULL if any, array is terminated by {0,0}
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const enum PixelFormat *pix_fmts; ///< array of supported pixel formats, or NULL if unknown, array is terminated by -1
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const int *supported_samplerates; ///< array of supported audio samplerates, or NULL if unknown, array is terminated by 0
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const enum AVSampleFormat *sample_fmts; ///< array of supported sample formats, or NULL if unknown, array is terminated by -1
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const uint64_t *channel_layouts; ///< array of support channel layouts, or NULL if unknown. array is terminated by 0
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uint8_t max_lowres; ///< maximum value for lowres supported by the decoder
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const AVClass *priv_class; ///< AVClass for the private context
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const AVProfile *profiles; ///< array of recognized profiles, or NULL if unknown, array is terminated by {FF_PROFILE_UNKNOWN}
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/*****************************************************************
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* No fields below this line are part of the public API. They
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* may not be used outside of libavcodec and can be changed and
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* removed at will.
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* New public fields should be added right above.
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*****************************************************************
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*/
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int priv_data_size;
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struct AVCodec *next;
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/**
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* @name Frame-level threading support functions
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* @{
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*/
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/**
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* If defined, called on thread contexts when they are created.
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* If the codec allocates writable tables in init(), re-allocate them here.
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* priv_data will be set to a copy of the original.
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*/
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int (*init_thread_copy)(AVCodecContext *);
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/**
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* Copy necessary context variables from a previous thread context to the current one.
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* If not defined, the next thread will start automatically; otherwise, the codec
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* must call ff_thread_finish_setup().
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*
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* dst and src will (rarely) point to the same context, in which case memcpy should be skipped.
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*/
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int (*update_thread_context)(AVCodecContext *dst, const AVCodecContext *src);
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/** @} */
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/**
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* Private codec-specific defaults.
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*/
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const AVCodecDefault *defaults;
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/**
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* Initialize codec static data, called from avcodec_register().
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*/
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void (*init_static_data)(struct AVCodec *codec);
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int (*init)(AVCodecContext *);
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int (*encode)(AVCodecContext *, uint8_t *buf, int buf_size, void *data);
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/**
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* Encode data to an AVPacket.
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*
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* @param avctx codec context
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* @param avpkt output AVPacket (may contain a user-provided buffer)
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* @param[in] frame AVFrame containing the raw data to be encoded
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* @param[out] got_packet_ptr encoder sets to 0 or 1 to indicate that a
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* non-empty packet was returned in avpkt.
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* @return 0 on success, negative error code on failure
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*/
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int (*encode2)(AVCodecContext *avctx, AVPacket *avpkt, const AVFrame *frame,
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int *got_packet_ptr);
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int (*decode)(AVCodecContext *, void *outdata, int *outdata_size, AVPacket *avpkt);
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int (*close)(AVCodecContext *);
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/**
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* Flush buffers.
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* Will be called when seeking
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*/
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void (*flush)(AVCodecContext *);
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} AVCodec;
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```
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# 二、AVCodec 重点字段
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下面说一下最主要的几个变量：
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```
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const char *name：编解码器的名字，比较短
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const char *long_name：编解码器的名字，全称，比较长
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enum AVMediaType type：指明了类型，是视频，音频，还是字幕
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enum AVCodecID id：ID，不重复
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const AVRational *supported_framerates：支持的帧率（仅视频）
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const enum AVPixelFormat *pix_fmts：支持的像素格式（仅视频）
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const int *supported_samplerates：支持的采样率（仅音频）
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const enum AVSampleFormat *sample_fmts：支持的采样格式（仅音频）
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const uint64_t *channel_layouts：支持的声道数（仅音频）
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int priv_data_size：私有数据的大小
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```
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详细介绍几个变量：
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### 1.enum AVMediaType type
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AVMediaType定义如下：
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```
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enum AVMediaType {
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AVMEDIA_TYPE_UNKNOWN = -1, ///< Usually treated as AVMEDIA_TYPE_DATA
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AVMEDIA_TYPE_VIDEO,
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AVMEDIA_TYPE_AUDIO,
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AVMEDIA_TYPE_DATA, ///< Opaque data information usually continuous
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AVMEDIA_TYPE_SUBTITLE,
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AVMEDIA_TYPE_ATTACHMENT, ///< Opaque data information usually sparse
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AVMEDIA_TYPE_NB
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};
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```
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### 2.enum AVCodecID id
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AVCodecID定义如下：
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```
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enum AVCodecID {
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AV_CODEC_ID_NONE,
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/* video codecs */
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AV_CODEC_ID_MPEG1VIDEO,
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AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
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AV_CODEC_ID_MPEG2VIDEO_XVMC,
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AV_CODEC_ID_H261,
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AV_CODEC_ID_H263,
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AV_CODEC_ID_RV10,
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AV_CODEC_ID_RV20,
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AV_CODEC_ID_MJPEG,
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AV_CODEC_ID_MJPEGB,
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AV_CODEC_ID_LJPEG,
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AV_CODEC_ID_SP5X,
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AV_CODEC_ID_JPEGLS,
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AV_CODEC_ID_MPEG4,
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AV_CODEC_ID_RAWVIDEO,
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AV_CODEC_ID_MSMPEG4V1,
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AV_CODEC_ID_MSMPEG4V2,
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AV_CODEC_ID_MSMPEG4V3,
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AV_CODEC_ID_WMV1,
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AV_CODEC_ID_WMV2,
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AV_CODEC_ID_H263P,
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AV_CODEC_ID_H263I,
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AV_CODEC_ID_FLV1,
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AV_CODEC_ID_SVQ1,
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AV_CODEC_ID_SVQ3,
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AV_CODEC_ID_DVVIDEO,
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AV_CODEC_ID_HUFFYUV,
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AV_CODEC_ID_CYUV,
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AV_CODEC_ID_H264,
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...
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}
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```
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### 3.const enum AVPixelFormat *pix_fmts
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AVPixelFormat定义如下：
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```
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enum AVPixelFormat {
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AV_PIX_FMT_NONE = -1,
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AV_PIX_FMT_YUV420P, ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
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AV_PIX_FMT_YUYV422, ///< packed YUV 4:2:2, 16bpp, Y0 Cb Y1 Cr
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AV_PIX_FMT_RGB24, ///< packed RGB 8:8:8, 24bpp, RGBRGB...
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AV_PIX_FMT_BGR24, ///< packed RGB 8:8:8, 24bpp, BGRBGR...
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AV_PIX_FMT_YUV422P, ///< planar YUV 4:2:2, 16bpp, (1 Cr & Cb sample per 2x1 Y samples)
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AV_PIX_FMT_YUV444P, ///< planar YUV 4:4:4, 24bpp, (1 Cr & Cb sample per 1x1 Y samples)
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AV_PIX_FMT_YUV410P, ///< planar YUV 4:1:0, 9bpp, (1 Cr & Cb sample per 4x4 Y samples)
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AV_PIX_FMT_YUV411P, ///< planar YUV 4:1:1, 12bpp, (1 Cr & Cb sample per 4x1 Y samples)
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AV_PIX_FMT_GRAY8, ///< Y , 8bpp
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AV_PIX_FMT_MONOWHITE, ///< Y , 1bpp, 0 is white, 1 is black, in each byte pixels are ordered from the msb to the lsb
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AV_PIX_FMT_MONOBLACK, ///< Y , 1bpp, 0 is black, 1 is white, in each byte pixels are ordered from the msb to the lsb
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AV_PIX_FMT_PAL8, ///< 8 bit with PIX_FMT_RGB32 palette
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AV_PIX_FMT_YUVJ420P, ///< planar YUV 4:2:0, 12bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV420P and setting color_range
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AV_PIX_FMT_YUVJ422P, ///< planar YUV 4:2:2, 16bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV422P and setting color_range
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AV_PIX_FMT_YUVJ444P, ///< planar YUV 4:4:4, 24bpp, full scale (JPEG), deprecated in favor of PIX_FMT_YUV444P and setting color_range
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AV_PIX_FMT_XVMC_MPEG2_MC,///< XVideo Motion Acceleration via common packet passing
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AV_PIX_FMT_XVMC_MPEG2_IDCT,
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...（代码太长，略）
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}
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```
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### 4.const enum AVSampleFormat *sample_fmts
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```
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enum AVSampleFormat {
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AV_SAMPLE_FMT_NONE = -1,
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AV_SAMPLE_FMT_U8, ///< unsigned 8 bits
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AV_SAMPLE_FMT_S16, ///< signed 16 bits
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AV_SAMPLE_FMT_S32, ///< signed 32 bits
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AV_SAMPLE_FMT_FLT, ///< float
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AV_SAMPLE_FMT_DBL, ///< double
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AV_SAMPLE_FMT_U8P, ///< unsigned 8 bits, planar
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AV_SAMPLE_FMT_S16P, ///< signed 16 bits, planar
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AV_SAMPLE_FMT_S32P, ///< signed 32 bits, planar
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AV_SAMPLE_FMT_FLTP, ///< float, planar
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AV_SAMPLE_FMT_DBLP, ///< double, planar
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AV_SAMPLE_FMT_NB ///< Number of sample formats. DO NOT USE if linking dynamically
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};
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```
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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）：
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```
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AVCodec ff_h264_decoder = {
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.name = "h264",
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.type = AVMEDIA_TYPE_VIDEO,
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.id = CODEC_ID_H264,
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.priv_data_size = sizeof(H264Context),
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.init = ff_h264_decode_init,
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.close = ff_h264_decode_end,
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.decode = decode_frame,
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.capabilities = /*CODEC_CAP_DRAW_HORIZ_BAND |*/ CODEC_CAP_DR1 | CODEC_CAP_DELAY |
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CODEC_CAP_SLICE_THREADS | CODEC_CAP_FRAME_THREADS,
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.flush= flush_dpb,
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.long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
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.init_thread_copy = ONLY_IF_THREADS_ENABLED(decode_init_thread_copy),
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.update_thread_context = ONLY_IF_THREADS_ENABLED(decode_update_thread_context),
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.profiles = NULL_IF_CONFIG_SMALL(profiles),
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.priv_class = &h264_class,
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};
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```
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JPEG2000解码器结构体（j2kdec.c）：
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```
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AVCodec ff_jpeg2000_decoder = {
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.name = "j2k",
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.type = AVMEDIA_TYPE_VIDEO,
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.id = CODEC_ID_JPEG2000,
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.priv_data_size = sizeof(J2kDecoderContext),
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.init = j2kdec_init,
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.close = decode_end,
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.decode = decode_frame,
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.capabilities = CODEC_CAP_EXPERIMENTAL,
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.long_name = NULL_IF_CONFIG_SMALL("JPEG 2000"),
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.pix_fmts =
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(const enum PixelFormat[]) {PIX_FMT_GRAY8, PIX_FMT_RGB24, PIX_FMT_NONE}
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};
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```
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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）：
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1.注册所有编解码器：av_register_all();
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2.声明一个AVCodec类型的指针，比如说AVCodec* first_c;
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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。
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