ffmpeg结构体以及函数介绍（三）

1 AVPacket

typedef
struct AVPacket {

/**

* Presentation timestamp in AVStream->time_base units; the time at which

* the decompressed packet will be presented to the user.

* Can be AV_NOPTS_VALUE if it is not stored in the file.

* pts MUST be larger or equal to dts as presentation cannot happen before

* decompression, unless one wants to view hex dumps. Some formats misuse

* the terms dts and pts/cts to mean something different. Such timestamps

* must be converted to true pts/dts before they are stored in AVPacket.

*/

int64_t pts;

/**

* Decompression timestamp in AVStream->time_base units; the time at which

* the packet is decompressed.

* Can be AV_NOPTS_VALUE if it is not stored in the file.

*/

int64_t dts;

uint8_t *data;

int   size;

int   stream_index;

int   flags;

int   duration;

.

.

.

} AVPacket

// AVPacket是个很重要的结构,该结构在读媒体源文件和写输出文件时都需要用到

// int64_t pts; 显示时间戳

// int64_t dts; 解码时间戳

// uint8_t *data; 包数据

// int   size; 包数据长度

// int   stream_index; 包所属流序号

// int   duration; 时长

// 以上信息,如果是在读媒体源文件那么avcodec会初始化,如果是输出文件,用户需要对以上信息赋值

 

2
av_init_packet()

/**

* Initialize optional fields of a packet with default values.

*

* @param pkt packet

*/

void av_init_packet(AVPacket *pkt);

// 使用默认值初始化AVPacket

// 定义AVPacket对象后,请使用av_init_packet进行初始化

 

3
av_free_packet()

/**

* Free a packet.

*

* @param pkt packet to free

*/

void av_free_packet(AVPacket *pkt);

// 释放AVPacket对象

 

4
av_read_frame()

/**

* Return the next frame of a stream.

* This function returns what is stored in the file, and does not validate

* that what is there are valid frames for the decoder. It will split what is

* stored in the file into frames and return one for each call. It will not

* omit invalid data between valid frames so as to give the decoder the maximum

* information possible for decoding.

*

* The returned packet is valid

* until the next av_read_frame() or until av_close_input_file() and

* must be freed with av_free_packet. For video, the packet contains

* exactly one frame. For audio, it contains an integer number of

* frames if each frame has a known fixed size (e.g. PCM or ADPCM

* data). If the audio frames have a variable size (e.g. MPEG audio),

* then it contains one frame.

*

* pkt->pts, pkt->dts and pkt->duration are always set to correct

* values in AVStream.time_base units (and guessed if the format cannot

* provide them). pkt->pts can be AV_NOPTS_VALUE if the video format

* has B-frames, so it is better to rely on pkt->dts if you do not

* decompress the payload.

*

* @return 0 if OK, < 0 on error or end of file

*/

int av_read_frame(AVFormatContext *s, AVPacket *pkt);

// 从输入源文件容器中读取一个AVPacket数据包

// 该函数读出的包并不每次都是有效的,对于读出的包我们都应该进行相应的解码(视频解码/音频解码),

// 在返回值>=0时,循环调用该函数进行读取,循环调用之前请调用av_free_packet函数清理AVPacket

 

5
avcodec_decode_video2()

/**

* Decode the video frame of size avpkt->size from avpkt->data into picture.

* Some decoders may support multiple frames in a single AVPacket, such

* decoders would then just decode the first frame.

*

* @warning The input buffer must be FF_INPUT_BUFFER_PADDING_SIZE larger than

* the actual read bytes because some optimized bitstream readers read 32 or 64

* bits at once and could read over the end.

*

* @warning The end of the input buffer buf should be set to 0 to ensure that

* no overreading happens for damaged MPEG streams.

*

* @note You might have to align the input buffer avpkt->data.

* The alignment requirements depend on the CPU: on some CPUs it isn't

* necessary at all, on others it won't work at all if not aligned and on others

* it will work but it will have an impact on performance.

*

* In practice, avpkt->data should have 4 byte alignment at minimum.

*

* @note Some codecs have a delay between input and output, these need to be

* fed with avpkt->data=NULL, avpkt->size=0 at the end to return the remaining frames.

*

* @param avctx the codec context

* @param[out] picture The AVFrame in which the decoded video frame will be stored.

*             Use avcodec_alloc_frame to get an AVFrame, the codec will

*             allocate memory for the actual bitmap.

*             with default get/release_buffer(), the decoder frees/reuses the bitmap as it sees fit.

*             with overridden get/release_buffer() (needs CODEC_CAP_DR1) the user decides into what buffer the decoder

*                   decodes and the decoder tells the user once it does not need the data anymore,

*                   the user app can at this point free/reuse/keep the memory as it sees fit.

*

* @param[in] avpkt The input AVpacket containing the input buffer.

*            You can create such packet with av_init_packet() and by then setting

*            data and size, some decoders might in addition need other fields like

*            flags&AV_PKT_FLAG_KEY. All decoders are designed to use the least

*            fields possible.

* @param[in,out] got_picture_ptr Zero if no frame could be decompressed, otherwise, it is nonzero.

* @return On error a negative value is returned, otherwise the number of bytes

* used or zero if no frame could be decompressed.

*/

int avcodec_decode_video2(AVCodecContext *avctx, AVFrame *picture,

int *got_picture_ptr,

AVPacket *avpkt);

// 解码视频流AVPacket

// 使用av_read_frame读取媒体流后需要进行判断,如果为视频流则调用该函数解码

// 返回结果<0时失败,此时程序应该退出检查原因

// 返回>=0时正常,假设 读取包为:AVPacket vPacket 返回值为 int vLen; 每次解码正常时,对vPacket做

// 如下处理:

//   vPacket.size -= vLen;

//   vPacket.data += vLen;

// 如果 vPacket.size==0,则继续读下一流包,否则继续调度该方法进行解码,直到vPacket.size==0

// 返回 got_picture_ptr > 0 时,表示解码到了AVFrame *picture,其后可以对picture进程处理

 

6
avcodec_decode_audio3()

/**

* Decode the audio frame of size avpkt->size from avpkt->data into samples.

* Some decoders may support multiple frames in a single AVPacket, such

* decoders would then just decode the first frame. In this case,

* avcodec_decode_audio3 has to be called again with an AVPacket that contains

* the remaining data in order to decode the second frame etc.

* If no frame

* could be outputted, frame_size_ptr is zero. Otherwise, it is the

* decompressed frame size in bytes.

*

* @warning You must set frame_size_ptr to the allocated size of the

* output buffer before calling avcodec_decode_audio3().

*

* @warning The input buffer must be FF_INPUT_BUFFER_PADDING_SIZE larger than

* the actual read bytes because some optimized bitstream readers read 32 or 64

* bits at once and could read over the end.

*

* @warning The end of the input buffer avpkt->data should be set to 0 to ensure that

* no overreading happens for damaged MPEG streams.

*

* @note You might have to align the input buffer avpkt->data and output buffer

* samples. The alignment requirements depend on the CPU: On some CPUs it isn't

* necessary at all, on others it won't work at all if not aligned and on others

* it will work but it will have an impact on performance.

*

* In practice, avpkt->data should have 4 byte alignment at minimum and

* samples should be 16 byte aligned unless the CPU doesn't need it

* (AltiVec and SSE do).

*

* @param avctx the codec context

* @param[out] samples the output buffer, sample type in avctx->sample_fmt

* @param[in,out] frame_size_ptr the output buffer size in bytes

* @param[in] avpkt The input AVPacket containing the input buffer.

*            You can create such packet with av_init_packet() and by then setting

*            data and size, some decoders might in addition need other fields.

*            All decoders are designed to use the least fields possible though.

* @return On error a negative value is returned, otherwise the number of bytes

* used or zero if no frame data was decompressed (used) from the input AVPacket.

*/

int avcodec_decode_audio3(AVCodecContext *avctx, int16_t *samples,

int *frame_size_ptr,

AVPacket *avpkt);

// 解码音频流AVPacket

// 使用av_read_frame读取媒体流后需要进行判断,如果为音频流则调用该函数解码

// 返回结果<0时失败,此时程序应该退出检查原因

// 返回>=0时正常,假设 读取包为:AVPacket vPacket 返回值为 int vLen; 每次解码正常时,对vPacket做

// 如下处理:

//   vPacket.size -= vLen;

//   vPacket.data += vLen;

// 如果 vPacket.size==0,则继续读下一流包,否则继续调度该方法进行解码,直到vPacket.size==0

//