FFMPEG解码学习笔记

Posted 2020-08-19 绝望的老猫

FFMPEG的基本处理流程

 

音频视频同步资料

av_frame_get_pkt_duration(frame)

获取当前帧的持续时间

av_frame_get_pkt_pos(frame)

从最后一个AVPacket进入解码重新排序的偏移量

av_frame_get_best_effort_timestamp(frame)

已流中的时间为基础预估的时间戳

    /**
     * When decoding, this signals how much the picture must be delayed.
     * extra_delay = repeat_pict / (2*fps)
     */
    int repeat_pict;

当解码时，这个信号告诉你这张图片需要要延迟多少久。

需要求出扩展延时：

extra_delay = repeat_pict / (2*fps)

    /**
     * This is the fundamental unit of time (in seconds) in terms
     * of which frame timestamps are represented. For fixed-fps content,
     * timebase should be 1/framerate and timestamp increments should be
     * identically 1.
     * This often, but not always is the inverse of the frame rate or field rate
     * for video. 1/time_base is not the average frame rate if the frame rate is not
     * constant.
     *
     * Like containers, elementary streams also can store timestamps, 1/time_base
     * is the unit in which these timestamps are specified.
     * As example of such codec time base see ISO/IEC 14496-2:2001(E)
     * vop_time_increment_resolution and fixed_vop_rate
     * (fixed_vop_rate == 0 implies that it is different from the framerate)
     *
     * - encoding: MUST be set by user.
     * - decoding: the use of this field for decoding is deprecated.
     *             Use framerate instead.
     */
    AVRational time_base;

当前帧需要耗时多少秒，每一帧显示的时间

由以上两个定义可以推导：

extra_delay = repeat_pict / (2*fps)

fps=1/time_base

那么

extra_delay= repeat_pict*time_base*0.5

AVCodecContent

/**
 * Allocate an AVCodecContext and set its fields to default values. The
 * resulting struct should be freed with avcodec_free_context().
 *
 * @param codec if non-NULL, allocate private data and initialize defaults
 *              for the given codec. It is illegal to then call avcodec_open2()
 *              with a different codec.
 *              If NULL, then the codec-specific defaults won\'t be initialized,
 *              which may result in suboptimal default settings (this is
 *              important mainly for encoders, e.g. libx264).
 *
 * @return An AVCodecContext filled with default values or NULL on failure.
 */
AVCodecContext *avcodec_alloc_context3(const AVCodec *codec);

分配一个AVCodecContent并为其分配默认值。（可理解为一个AVCodecContent分配内存空间）

释放它的资源使用avcodec_free_context()。

int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

/**
 * Initialize the AVCodecContext to use the given AVCodec. Prior to using this
 * function the context has to be allocated with avcodec_alloc_context3().
 *
 * The functions avcodec_find_decoder_by_name(), avcodec_find_encoder_by_name(),
 * avcodec_find_decoder() and avcodec_find_encoder() provide an easy way for
 * retrieving a codec.
 *
 * @warning This function is not thread safe!
 *
 * @note Always call this function before using decoding routines (such as
 * @ref avcodec_receive_frame()).
 *
 * @code
 * avcodec_register_all();
 * av_dict_set(&opts, "b", "2.5M", 0);
 * codec = avcodec_find_decoder(AV_CODEC_ID_H264);
 * if (!codec)
 *     exit(1);
 *
 * context = avcodec_alloc_context3(codec);
 *
 * if (avcodec_open2(context, codec, opts) < 0)
 *     exit(1);
 * @endcode
 *
 * @param avctx The context to initialize.
 * @param codec The codec to open this context for. If a non-NULL codec has been
 *              previously passed to avcodec_alloc_context3() or
 *              for this context, then this parameter MUST be either NULL or
 *              equal to the previously passed codec.
 * @param options A dictionary filled with AVCodecContext and codec-private options.
 *                On return this object will be filled with options that were not found.
 *
 * @return zero on success, a negative value on error
 * @see avcodec_alloc_context3(), avcodec_find_decoder(), avcodec_find_encoder(),
 *      av_dict_set(), av_opt_find().
 */
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options);

使用AVCodec初始化AVCodecContext。在初始化之前需要为其分配内存。

/**
 * Copy the settings of the source AVCodecContext into the destination
 * AVCodecContext. The resulting destination codec context will be
 * unopened, i.e. you are required to call avcodec_open2() before you
 * can use this AVCodecContext to decode/encode video/audio data.
 *
 * @param dest target codec context, should be initialized with
 *             avcodec_alloc_context3(NULL), but otherwise uninitialized
 * @param src source codec context
 * @return AVERROR() on error (e.g. memory allocation error), 0 on success
 *
 * @deprecated The semantics of this function are ill-defined and it should not
 * be used. If you need to transfer the stream parameters from one codec context
 * to another, use an intermediate AVCodecParameters instance and the
 * avcodec_parameters_from_context() / avcodec_parameters_to_context()
 * functions.
 */
attribute_deprecated
int avcodec_copy_context(AVCodecContext *dest, const AVCodecContext *src);

该函数已不建议使用。它可以将原AVCodecContext的设置拷贝到目标AVCodecContent。

如果在使用这个数据之前调用avcodec_open2()，会导致无法打开。

建议使用avcodec_parameters_from_context() / avcodec_parameters_to_context()传递参数。

/**
 * Fill the parameters struct based on the values from the supplied codec
 * context. Any allocated fields in par are freed and replaced with duplicates
 * of the corresponding fields in codec.
 *
 * @return >= 0 on success, a negative AVERROR code on failure
 */
int avcodec_parameters_from_context(AVCodecParameters *par,
                                    const AVCodecContext *codec);

从一个AVCodecContent中拷贝出AVCodecParameters

/**
 * Fill the codec context based on the values from the supplied codec
 * parameters. Any allocated fields in codec that have a corresponding field in
 * par are freed and replaced with duplicates of the corresponding field in par.
 * Fields in codec that do not have a counterpart in par are not touched.
 *
 * @return >= 0 on success, a negative AVERROR code on failure.
 */
int avcodec_parameters_to_context(AVCodecContext *codec,
                                  const AVCodecParameters *par);

向一个AVCodecContent拷贝一个AVCodecParameters

/**
 * 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.
 *
 * If pkt->buf is NULL, then the packet is valid until the next
 * av_read_frame() or until avformat_close_input(). Otherwise the packet
 * is valid indefinitely. In both cases the packet must be freed with
 * av_packet_unref when it is no longer needed. 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);

返回下一帧的流

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