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概述
ffmpeg结构体（5）-之AVCodec及其相关函数
- 1、AVCodec数据结构定义
- 2、AVCodec常见变量及其作用
- 3、AVCodec关键函数
AVCodec是存储编解码器信息的结构体。本文将会详细分析一下该结构体里每个变量的含义和作用。
1、AVCodec数据结构定义
AVCodec位于libavcodeccodec.h文件中，结构体定义如下：
/**
* 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 AVCodecID id;
/**
* Codec capabilities.
* see AV_CODEC_CAP_*
*/
int capabilities;
const AVRational *supported_framerates; ///< array of supported framerates, or NULL if any, array is terminated by {0,0}
const enum AVPixelFormat *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}
/**
* Group name of the codec implementation.
* This is a short symbolic name of the wrapper backing this codec. A
* wrapper uses some kind of external implementation for the codec, such
* as an external library, or a codec implementation provided by the OS or
* the hardware.
* If this field is NULL, this is a builtin, libavcodec native codec.
* If non-NULL, this will be the suffix in AVCodec.name in most cases
* (usually AVCodec.name will be of the form "<codec_name>_<wrapper_name>").
*/
const char *wrapper_name;
/*****************************************************************
* 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;
#if FF_API_NEXT
struct AVCodec *next;
#endif
/**
* @name Frame-level threading support functions
* @{
*/
/**
* 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)(struct AVCodecContext *dst, const struct AVCodecContext *src);
/** @} */
/**
* Private codec-specific defaults.
*/
const AVCodecDefault *defaults;
/**
* Initialize codec static data, called from avcodec_register().
*
* This is not intended for time consuming operations as it is
* run for every codec regardless of that codec being used.
*/
void (*init_static_data)(struct AVCodec *codec);
int (*init)(struct AVCodecContext *);
int (*encode_sub)(struct AVCodecContext *, uint8_t *buf, int buf_size,
const struct AVSubtitle *sub);
/**
* 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)(struct AVCodecContext *avctx, struct AVPacket *avpkt,
const struct AVFrame *frame, int *got_packet_ptr);
int (*decode)(struct AVCodecContext *, void *outdata, int *outdata_size, struct AVPacket *avpkt);
int (*close)(struct AVCodecContext *);
/**
* Encode API with decoupled frame/packet dataflow. This function is called
* to get one output packet. It should call ff_encode_get_frame() to obtain
* input data.
*/
int (*receive_packet)(struct AVCodecContext *avctx, struct AVPacket *avpkt);
/**
* Decode API with decoupled packet/frame dataflow. This function is called
* to get one output frame. It should call ff_decode_get_packet() to obtain
* input data.
*/
int (*receive_frame)(struct AVCodecContext *avctx, struct AVFrame *frame);
/**
* Flush buffers.
* Will be called when seeking
*/
void (*flush)(struct AVCodecContext *);
/**
* Internal codec capabilities.
* See FF_CODEC_CAP_* in internal.h
*/
int caps_internal;
/**
* Decoding only, a comma-separated list of bitstream filters to apply to
* packets before decoding.
*/
const char *bsfs;
/**
* Array of pointers to hardware configurations supported by the codec,
* or NULL if no hardware supported. The array is terminated by a NULL
* pointer.
*
* The user can only access this field via avcodec_get_hw_config().
*/
const struct AVCodecHWConfigInternal *const *hw_configs;
/**
* List of supported codec_tags, terminated by FF_CODEC_TAGS_END.
*/
const uint32_t *codec_tags;
} AVCodec;
2、AVCodec常见变量及其作用
AVCodec结构体中的关键变量
const char *name;//编码器的名字，比较短。在编码器和解码器之间是全局唯一的。
const char *long_name;//编码器的名字，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;
int capabilities;
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
//初始化编解码器静态数据，从avcodec_register（）调用。
void (*init_static_data)(struct AVCodec *codec);
enum AVMediaType type;//指明了类型，是视频，音频还是字幕
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
};
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,
...（代码太长，略）
}
const enum AVPixelFormat *pix_fmts; //支持的像素格式，仅视频
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）
}
const enum AVSampleFormat *sample_fmts; //支持的采样格式，仅音频
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",
.long_name = NULL_IF_CONFIG_SMALL("H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
.type = AVMEDIA_TYPE_VIDEO,
.id = AV_CODEC_ID_H264,
.priv_data_size = sizeof(H264Context),
.init = h264_decode_init,
.close = h264_decode_end,
.decode = h264_decode_frame,
.capabilities = /*AV_CODEC_CAP_DRAW_HORIZ_BAND |*/ AV_CODEC_CAP_DR1 |
AV_CODEC_CAP_DELAY | AV_CODEC_CAP_SLICE_THREADS |
AV_CODEC_CAP_FRAME_THREADS,
.hw_configs = (const AVCodecHWConfigInternal *const []) {
#if CONFIG_H264_DXVA2_HWACCEL
HWACCEL_DXVA2(h264),
#endif
#if CONFIG_H264_D3D11VA_HWACCEL
HWACCEL_D3D11VA(h264),
#endif
#if CONFIG_H264_D3D11VA2_HWACCEL
HWACCEL_D3D11VA2(h264),
#endif
#if CONFIG_H264_NVDEC_HWACCEL
HWACCEL_NVDEC(h264),
#endif
#if CONFIG_H264_VAAPI_HWACCEL
HWACCEL_VAAPI(h264),
#endif
#if CONFIG_H264_VDPAU_HWACCEL
HWACCEL_VDPAU(h264),
#endif
#if CONFIG_H264_VIDEOTOOLBOX_HWACCEL
HWACCEL_VIDEOTOOLBOX(h264),
#endif
NULL
},
.caps_internal = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_EXPORTS_CROPPING |
FF_CODEC_CAP_ALLOCATE_PROGRESS | FF_CODEC_CAP_INIT_CLEANUP,
.flush = h264_decode_flush,
.update_thread_context = ONLY_IF_THREADS_ENABLED(ff_h264_update_thread_context),
.profiles = NULL_IF_CONFIG_SMALL(ff_h264_profiles),
.priv_class = &h264_class,
};
JPEG2000解码器结构体（j2kdec.c）
AVCodec ff_jpeg2000_decoder = {
.name = "jpeg2000",
.long_name = NULL_IF_CONFIG_SMALL("JPEG 2000"),
.type = AVMEDIA_TYPE_VIDEO,
.id = AV_CODEC_ID_JPEG2000,
.capabilities = AV_CODEC_CAP_SLICE_THREADS | AV_CODEC_CAP_FRAME_THREADS | AV_CODEC_CAP_DR1,
.priv_data_size = sizeof(Jpeg2000DecoderContext),
.init = jpeg2000_decode_init,
.decode = jpeg2000_decode_frame,
.priv_class = &jpeg2000_class,
.max_lowres = 5,
.profiles = NULL_IF_CONFIG_SMALL(ff_jpeg2000_profiles)
};
3、AVCodec关键函数
AVCodec结构体中的关键函数指针
//将数据编码到AVPacket。
int (*encode2)(AVCodecContext *avctx, AVPacket *avpkt, const AVFrame *frame, int *got_packet_ptr);
//解码数据到AVPacket。
int (*decode)(AVCodecContext *, void *outdata, int *outdata_size, AVPacket *avpkt);
//关闭编解码器。
int (*close)(AVCodecContext *);
//刷新缓冲区。当seek时会被调用。
void (*flush)(AVCodecContext *);
最后
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