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在FFmpeg里面封装、也叫复用、也叫Muxer的时候，主要是用的是AVOutputFormat作为封装模块的机构，抽象了几个通用的接口，下面看一下代码注释
文件： libavformat/avformat.h
[C] 纯文本查看 复制代码 /**
* @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.
*****************************************************************
*/
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 *);
/**
* Currently only used to set pixel format if not YUV420P.
*/
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;
/**
* @}
*/
注释写得很详细，下面来简单解析一下：
name 为一个常量，也就是这个模块的名称，例如图片的一般为“image2”， HLS 的为“hls”
long_name 也是一个常量，也就是关于模块的描述，例如图片一般为“image2 sequence”， HLS的为“Apple HTTP Live Streaming”
mime_type 也是一个常量，比如flv，一般为"video/x-flv"， ts一般为"video/MP2T"， 这类mime_type的理解方式与http服务器中配置的相同
extensions 也是一个常量，是媒体的扩展名描述，比如flv一般是.flv, HLS是.m3u8
然后是三个常见的枚举变量， audio_codec,video_codec,subtitle_codec 分别是音频的编码标识，视频的编码标识，字母的编码标识，类型是 AVCodecID 定义如下：文件：libavformat/internal.h
[C] 纯文本查看 复制代码 /**
* Identify the syntax and semantics of the bitstream.
* The principle is roughly:
* Two decoders with the same ID can decode the same streams.
* Two encoders with the same ID can encode compatible streams.
* There may be slight deviations from the principle due to implementation
* details.
*
* If you add a codec ID to this list, add it so that
* 1. no value of an existing codec ID changes (that would break ABI),
* 2. it is as close as possible to similar codecs
*
* After adding new codec IDs, do not forget to add an entry to the codec
* descriptor list and bump libavcodec minor version.
*/
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_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,
省略剩余部分
接着是封装相关的flags：
[AppleScript] 纯文本查看 复制代码 /**
* 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;
例如设置global_header，或者notimestamps, 或者vfps相关的设置。
codec_tag, codec的标签变量，定义如下：
文件：libavformat/internal.h
[C] 纯文本查看 复制代码 typedef struct AVCodecTag {
enum AVCodecID id;
unsigned int tag;
} AVCodecTag;
偶尔也许会用到，例如wav封装：
[C] 纯文本查看 复制代码 .codec_tag = (const AVCodecTag* const []){ ff_codec_wav_tags, 0 },
而ff_codec_wav_tags的定义如下：
文件：libavformat/riff.c
[C] 纯文本查看 复制代码 const AVCodecTag ff_codec_wav_tags[] = {
{ AV_CODEC_ID_PCM_S16LE, 0x0001 },
/* must come after s16le in this list */
{ AV_CODEC_ID_PCM_U8, 0x0001 },
省略一部分内容
priv_class， 模块本身内容私有类
接下来是一系列的抽象接口：
write_header 写封装文件头部分。
write_packet 写封装文件的body内容部分
write_trailer 写封装文件的末尾部分，例如mp4的moov，一般是在这里面写入的
query_codec 查找codec信息
get_output_timestamp 获得写入的时间戳
control_message 做设备操作时从应用发消息到设备
write_uncoded_frame 写未经过编码的帧数居
get_device_list 获得设备列表
create_device_capabilities 初始化设备的功能
free_device_capabilities 释放设备
init 初始化封装，以前没有这个抽象接口，后来将write_header拆分成两部分，一部分是初始化，一部分是写文件头
deinit 结束封装，这个用到的并不多，与init基本对应
check_bitstream， 这个接口比较关键，在旧版本中，从mp4转封装到ts或者hls的时候，需要加上滤镜bsf:v mp4toannexb ，调用api操作的时候也需要用filter来处理一下，封装的时候加上这个接口处理mp4annexb操作的话，就无需那么多步骤进行操作了，因为在ffmpeg的write_frame的时候自动就check这个接口了：
文件：libavformat/mux.c
[AppleScript] 纯文本查看 复制代码 static int do_packet_auto_bsf(AVFormatContext *s, AVPacket *pkt) {
AVStream *st = s->streams[pkt->stream_index];
int i, ret;
if (!(s->flags & AVFMT_FLAG_AUTO_BSF))
return 1;
if (s->oformat->check_bitstream) {
if (!st->internal->bitstream_checked) {
if ((ret = s->oformat->check_bitstream(s, pkt)) < 0)
return ret;
else if (ret == 1)
st->internal->bitstream_checked = 1;
}
}
for (i = 0; i < st->internal->nb_bsfcs; i++) {
AVBSFContext *ctx = st->internal->bsfcs[i];
// TODO: when any bitstream filter requires flushing at EOF, we'll need to
// flush each stream's BSF chain on write_trailer.
if ((ret = av_bsf_send_packet(ctx, pkt)) < 0) {
av_log(ctx, AV_LOG_ERROR,
"Failed to send packet to filter %s for stream %d\n",
ctx->filter->name, pkt->stream_index);
return ret;
}
// TODO: when any automatically-added bitstream filter is generating multiple
// output packets for a single input one, we'll need to call this in a loop
// and write each output packet.
if ((ret = av_bsf_receive_packet(ctx, pkt)) < 0) {
if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
return 0;
av_log(ctx, AV_LOG_ERROR,
"Failed to send packet to filter %s for stream %d\n",
ctx->filter->name, pkt->stream_index);
if (s->error_recognition & AV_EF_EXPLODE)
return ret;
return 0;
}
}
return 1;
}
int av_write_frame(AVFormatContext *s, AVPacket *pkt)
{
int ret;
ret = prepare_input_packet(s, pkt);
if (ret < 0)
return ret;
if (!pkt) {
if (s->oformat->flags & AVFMT_ALLOW_FLUSH) {
ret = s->oformat->write_packet(s, NULL);
flush_if_needed(s);
if (ret >= 0 && s->pb && s->pb->error < 0)
ret = s->pb->error;
return ret;
}
return 1;
}
ret = do_packet_auto_bsf(s, pkt);
if (ret <= 0)
return ret;
#if FF_API_COMPUTE_PKT_FIELDS2 && FF_API_LAVF_AVCTX
ret = compute_muxer_pkt_fields(s, s->streams[pkt->stream_index], pkt);
if (ret < 0 && !(s->oformat->flags & AVFMT_NOTIMESTAMPS))
return ret;
#endif
ret = write_packet(s, pkt);
if (ret >= 0 && s->pb && s->pb->error < 0)
ret = s->pb->error;
if (ret >= 0)
s->streams[pkt->stream_index]->nb_frames++;
return ret;
}
从上面的代码实现中可以看到，已经自动处理了。
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发表于 2018-1-19 19:38:22
发表于 2018-1-19 19:40:51