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
//

转载自：http://blog.chinaunix.net/uid-20718335-id-3026906.html