AVOutputFormat 再分析

AVOutputFormat 结构体

/*** @addtogroup lavf_encoding* @{*/
typedef struct AVOutputFormat {const char *name;/*** Descriptive name for the format, meant to be more human-readable* than name. You should use the NULL_IF_CONFIG_SMALL() macro* to define it.*/const char *long_name;const char *mime_type;const char *extensions; /**< comma-separated filename extensions *//* output support */enum AVCodecID audio_codec;    /**< default audio codec */enum AVCodecID video_codec;    /**< default video codec */enum AVCodecID subtitle_codec; /**< default subtitle codec *//*** can use flags: AVFMT_NOFILE, AVFMT_NEEDNUMBER,* AVFMT_GLOBALHEADER, AVFMT_NOTIMESTAMPS, AVFMT_VARIABLE_FPS,* AVFMT_NODIMENSIONS, AVFMT_NOSTREAMS, AVFMT_ALLOW_FLUSH,* AVFMT_TS_NONSTRICT, AVFMT_TS_NEGATIVE*/int flags;/*** List of supported codec_id-codec_tag pairs, ordered by "better* choice first". The arrays are all terminated by AV_CODEC_ID_NONE.*/const struct AVCodecTag * const *codec_tag;const AVClass *priv_class; ///< AVClass for the private context/****************************************************************** No fields below this line are part of the public API. They* may not be used outside of libavformat and can be changed and* removed at will.* New public fields should be added right above.******************************************************************//*** The ff_const59 define is not part of the public API and will* be removed without further warning.*/
#if FF_API_AVIOFORMAT
#define ff_const59
#else
#define ff_const59 const
#endifff_const59 struct AVOutputFormat *next;/*** size of private data so that it can be allocated in the wrapper*/int priv_data_size;int (*write_header)(struct AVFormatContext *);/*** Write a packet. If AVFMT_ALLOW_FLUSH is set in flags,* pkt can be NULL in order to flush data buffered in the muxer.* When flushing, return 0 if there still is more data to flush,* or 1 if everything was flushed and there is no more buffered* data.*/int (*write_packet)(struct AVFormatContext *, AVPacket *pkt);int (*write_trailer)(struct AVFormatContext *);/*** A format-specific function for interleavement.* If unset, packets will be interleaved by dts.*/int (*interleave_packet)(struct AVFormatContext *, AVPacket *out,AVPacket *in, int flush);/*** Test if the given codec can be stored in this container.** @return 1 if the codec is supported, 0 if it is not.*         A negative number if unknown.*         MKTAG('A', 'P', 'I', 'C') if the codec is only supported as AV_DISPOSITION_ATTACHED_PIC*/int (*query_codec)(enum AVCodecID id, int std_compliance);void (*get_output_timestamp)(struct AVFormatContext *s, int stream,int64_t *dts, int64_t *wall);/*** Allows sending messages from application to device.*/int (*control_message)(struct AVFormatContext *s, int type,void *data, size_t data_size);/*** Write an uncoded AVFrame.** See av_write_uncoded_frame() for details.** The library will free *frame afterwards, but the muxer can prevent it* by setting the pointer to NULL.*/int (*write_uncoded_frame)(struct AVFormatContext *, int stream_index,AVFrame **frame, unsigned flags);/*** Returns device list with it properties.* @see avdevice_list_devices() for more details.*/int (*get_device_list)(struct AVFormatContext *s, struct AVDeviceInfoList *device_list);/*** Initialize device capabilities submodule.* @see avdevice_capabilities_create() for more details.*/int (*create_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);/*** Free device capabilities submodule.* @see avdevice_capabilities_free() for more details.*/int (*free_device_capabilities)(struct AVFormatContext *s, struct AVDeviceCapabilitiesQuery *caps);enum AVCodecID data_codec; /**< default data codec *//*** Initialize format. May allocate data here, and set any AVFormatContext or* AVStream parameters that need to be set before packets are sent.* This method must not write output.** Return 0 if streams were fully configured, 1 if not, negative AVERROR on failure** Any allocations made here must be freed in deinit().*/int (*init)(struct AVFormatContext *);/*** Deinitialize format. If present, this is called whenever the muxer is being* destroyed, regardless of whether or not the header has been written.** If a trailer is being written, this is called after write_trailer().** This is called if init() fails as well.*/void (*deinit)(struct AVFormatContext *);/*** Set up any necessary bitstream filtering and extract any extra data needed* for the global header.* Return 0 if more packets from this stream must be checked; 1 if not.*/int (*check_bitstream)(struct AVFormatContext *, const AVPacket *pkt);
} AVOutputFormat;

AVOutputFormat 是 FFmpeg 中用于管理输出文件容器格式的核心结构体，定义了封装（Muxing）行为的关键属性和操作方法14。以下是其核心功能和用法解析：

‌1. 功能与定义‌

‌核心作用‌
AVOutputFormat 描述输出容器格式的元数据和操作接口，支持 MP4、FLV、TS 等格式的封装逻辑实现14。
每个支持的格式对应一个独立的 AVOutputFormat 实例，并以链表形式组织存储14。
‌结构体定义关键字段

typedef struct AVOutputFormat {const char *name;           // 格式短名称（如 "flv"）const char *long_name;      // 易读的格式全称（如 "FLV (Flash Video)"）const char *mime_type;      // MIME 类型（如 "video/x-flv"）const char *extensions;     // 文件扩展名（如 "flv"）enum AVCodecID audio_codec; // 默认音频编码格式enum AVCodecID video_codec; // 默认视频编码格式// 函数指针（初始化、写头/数据包/尾部等）int (*write_header)(AVFormatContext *);int (*write_packet)(AVFormatContext *, AVPacket *);int (*write_trailer)(AVFormatContext *);// 其他字段（flags、私有数据大小等）// ...
} AVOutputFormat;

关键字段说明：

name/long_name：标识封装格式类型14；
audio_codec/video_codec：指定默认音视频编码格式4；
函数指针（如 write_header）：实现格式封装的具体逻辑48。

‌2. 标志位（flags）‌

通过 flags 字段控制封装行为，常见值包括：

‌AVFMT_GLOBALHEADER‌
要求编码器生成全局头信息（如 H.264 的 SPS/PPS），适用于 MP4 等格式6；
‌AVFMT_VARIABLE_FPS‌
允许可变帧率封装，适用于直播流场景6；
‌AVFMT_TS_NONSTRICT‌
允许时间戳非严格递增，增强容错性6。

‌3. 使用场景与示例‌

‌3.1 获取输出格式‌

通过格式名称、文件扩展名或 MIME 类型查找支持的封装器：

AVOutputFormat *fmt = av_guess_format("flv", NULL, NULL); // 获取 FLV 封装器

‌3.2 初始化封装上下文‌

将 AVOutputFormat 绑定到 AVFormatContext 并执行封装操作：

AVFormatContext *oc = NULL;
avformat_alloc_output_context2(&oc, fmt, NULL, "output.flv"); // 分配上下文并关联格式

‌3.3 自定义格式实现‌

定义新的封装器需实现 AVOutputFormat 的函数接口。以 FLV 为例：

AVOutputFormat ff_flv_muxer = {.name           = "flv",.long_name      = "FLV (Flash Video)",.mime_type      = "video/x-flv",.priv_data_size = sizeof(FLVContext), // 私有数据存储格式特定参数.audio_codec    = AV_CODEC_ID_MP3,.video_codec    = AV_CODEC_ID_FLV1,.write_header   = flv_write_header,   // 自定义头部写入逻辑.write_packet   = flv_write_packet,   // 数据包写入逻辑// ...
};