FFmpeg编解码
FFmpeg支持绝大多数视频编解码格式，如何遍历FFmpeg编解码器？
编解码器以链表形式存储，使用av_codec_next() 函数可以获取编解码器指针，当参数为NULL时，获取第一个编解码器指针，循环遍历，获取所有编解码器信息
void avcodecInfo()
{
av_register_all();
AVCodec *c_temp = av_codec_next(NULL);
while(c_temp!=NULL){
if (c_temp->encode2!=NULL) {
switch (c_temp->type){
case AVMEDIA_TYPE_VIDEO:
printf("[video encode] %10s\n", c_temp->name);
break;
case AVMEDIA_TYPE_AUDIO:
printf("[audio encode] %10s\n", c_temp->name);
break;
default:
printf("[other encode] %10s\n", c_temp->name);
break;
}
}
c_temp=c_temp->next;
}
}
AVCodec登场
遍历FFmpeg编解码器的时候，出现了AVCodec，什么是AVCodec？
AVCodec是存储编解码器信息的结构体，包含了编解码器的基本信息，例如编解码器的名称，编解码类型(video or audio)，以及编解码的参数等。下面列举了常用字段：
- const char *name; //编解码器名字
- const char *long_name; //编解码器全名
- enum AVMediaType type; //编解码器类型
- enum AVCodecID id; //编解码器ID
- const AVRational *supported_framerates; //支持帧率（视频）
- const enum AVPixelFormat *pix_fmts; //支持像素格式(视频）
- const int *supported_samplerates; //支持音频采样率(音频）
- const enum AVSampleFormat *sample_fmts; //支持采样格式(音频)
- const uint64_t *channel_layouts; //支持声道数（音频）
- const AVClass *priv_class; //私有数据
AVCodec结构体
mpeg2video(video)编码器结构体
pcm_bluray(audio)解码器结构体
注意：在编解码的时候，要设置为编解码器支持的格式，例如给mpeg2video编码器设置pix_fmts为AV_PIX_FMT_NV12，则会报出以下错误：
Error：Specified pixel format nv12 is invalid or not supported
AVCodec结构体内容
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}
/*****************************************************************
* 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_sub)(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)(AVCodecContext *avctx, AVPacket *avpkt, const AVFrame *frame,
int *got_packet_ptr);
int (*decode)(AVCodecContext *, void *outdata, int *outdata_size, AVPacket *avpkt);
int (*close)(AVCodecContext *);
/**
* Encode API with decoupled packet/frame dataflow. The API is the
* same as the avcodec_ prefixed APIs (avcodec_send_frame() etc.), except
* that:
* - never called if the codec is closed or the wrong type,
* - if AV_CODEC_CAP_DELAY is not set, drain frames are never sent,
* - only one drain frame is ever passed down,
*/
int (*send_frame)(AVCodecContext *avctx, const AVFrame *frame);
int (*receive_packet)(AVCodecContext *avctx, 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)(AVCodecContext *avctx, AVFrame *frame);
/**
* Flush buffers.
* Will be called when seeking
*/
void (*flush)(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;
} AVCodec;
关联类型
从AVCodec中，字段除了基本数据类型，还涉及到了其它结构体和枚举，例如AVMediaType，AVCodecID，AVRational，AVPixelFormat，AVSampleFormat，AVClass等。
AVMediaType媒体类型，是视频，音频，字幕等。
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
};
AVCodecID编解码器唯一标识符
enum AVCodecID {
AV_CODEC_ID_NONE,
/* video codecs */
AV_CODEC_ID_MPEG1VIDEO,
AV_CODEC_ID_MPEG2VIDEO, ///< preferred ID for MPEG-1/2 video decoding
#if FF_API_XVMC
AV_CODEC_ID_MPEG2VIDEO_XVMC,
#endif /* FF_API_XVMC */
AV_CODEC_ID_H261,
AV_CODEC_ID_H263,
AV_CODEC_ID_RV10,
AV_CODEC_ID_RV20,
.......
}
AVPixelFormat视频格式，如RGB，YUV等
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 bits with AV_PIX_FMT_RGB32 palette
AV_PIX_FMT_YUVJ420P, ///< planar YUV 4:2:0, 12bpp, full scale (JPEG), deprecated in favor of AV_PIX_FMT_YUV420P and setting color_range
AV_PIX_FMT_YUVJ422P, ///< planar YUV 4:2:2, 16bpp, full scale (JPEG), deprecated in favor of AV_PIX_FMT_YUV422P and setting color_range
AV_PIX_FMT_YUVJ444P, ///< planar YUV 4:4:4, 24bpp, full scale (JPEG), deprecated in favor of AV_PIX_FMT_YUV444P and setting color_range
#if FF_API_XVMC
AV_PIX_FMT_XVMC_MPEG2_MC,///< XVideo Motion Acceleration via common packet passing
AV_PIX_FMT_XVMC_MPEG2_IDCT,
AV_PIX_FMT_XVMC = AV_PIX_FMT_XVMC_MPEG2_IDCT,
#endif /* FF_API_XVMC */
AV_PIX_FMT_UYVY422,
......
}
AVSampleFormat采样格式，其中带P结尾的为平面格式。例如对于双通道音频，设左通道为L，右通道为R，如果是平面格式，在内存中的排列为LLLLLL……RRRRRR……，对于非平面格式，内存中排列为LRLRLRLRLR……
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_S64, ///< signed 64 bits
AV_SAMPLE_FMT_S64P, ///< signed 64 bits, planar
AV_SAMPLE_FMT_NB ///< Number of sample formats. DO NOT USE if linking dynamically
};