FFmpeg之AVFilterLink

这个结构体主要是用来link两个filter的,它存在于每个AVFilterContext中

struct AVFilterContext {
    const AVClass *av_class;        ///< needed for av_log() and filters common options

    const AVFilter *filter;         ///< the AVFilter of which this is an instance

    char *name;                     ///< name of this filter instance

    AVFilterPad   *input_pads;      ///< array of input pads
    AVFilterLink **inputs;          ///< array of pointers to input links
    unsigned    nb_inputs;          ///< number of input pads

    AVFilterPad   *output_pads;     ///< array of output pads
    AVFilterLink **outputs;         ///< array of pointers to output links
    unsigned    nb_outputs;         ///< number of output pads

    void *priv;                     ///< private data for use by the filter

    struct AVFilterGraph *graph;    ///< filtergraph this filter belongs to
    ...
  }

结构体

先看结构体定义


/**
 * A link between two filters. This contains pointers to the source and
 * destination filters between which this link exists, and the indexes of
 * the pads involved. In addition, this link also contains the parameters
 * which have been negotiated and agreed upon between the filter, such as
 * image dimensions, format, etc.
 *
 * Applications must not normally access the link structure directly.
 * Use the buffersrc and buffersink API instead.
 * In the future, access to the header may be reserved for filters
 * implementation.
 */
struct AVFilterLink {
    AVFilterContext *src;       ///< source filter
    AVFilterPad *srcpad;        ///< output pad on the source filter

    AVFilterContext *dst;       ///< dest filter
    AVFilterPad *dstpad;        ///< input pad on the dest filter

    enum AVMediaType type;      ///< filter media type

    /* These parameters apply only to video */
    int w;                      ///< agreed upon image width
    int h;                      ///< agreed upon image height
    AVRational sample_aspect_ratio; ///< agreed upon sample aspect ratio
    /* These parameters apply only to audio */
    uint64_t channel_layout;    ///< channel layout of current buffer (see libavutil/channel_layout.h)
    int sample_rate;            ///< samples per second

    int format;                 ///< agreed upon media format

    /**
     * Define the time base used by the PTS of the frames/samples
     * which will pass through this link.
     * During the configuration stage, each filter is supposed to
     * change only the output timebase, while the timebase of the
     * input link is assumed to be an unchangeable property.
     */
    AVRational time_base;

    /*****************************************************************
     * All fields below this line are not part of the public API. They
     * may not be used outside of libavfilter and can be changed and
     * removed at will.
     * New public fields should be added right above.
     *****************************************************************
     */

    /**
     * Lists of supported formats / etc. supported by the input filter.
     */
    AVFilterFormatsConfig incfg;

    /**
     * Lists of supported formats / etc. supported by the output filter.
     */
    AVFilterFormatsConfig outcfg;

    /** stage of the initialization of the link properties (dimensions, etc) */
    enum {
        AVLINK_UNINIT = 0,      ///< not started
        AVLINK_STARTINIT,       ///< started, but incomplete
        AVLINK_INIT             ///< complete
    } init_state;

    /**
     * Graph the filter belongs to.
     */
    struct AVFilterGraph *graph;

    /**
     * Current timestamp of the link, as defined by the most recent
     * frame(s), in link time_base units.
     */
    int64_t current_pts;

    /**
     * Current timestamp of the link, as defined by the most recent
     * frame(s), in AV_TIME_BASE units.
     */
    int64_t current_pts_us;

    /**
     * Index in the age array.
     */
    int age_index;

    /**
     * Frame rate of the stream on the link, or 1/0 if unknown or variable;
     * if left to 0/0, will be automatically copied from the first input
     * of the source filter if it exists.
     *
     * Sources should set it to the best estimation of the real frame rate.
     * If the source frame rate is unknown or variable, set this to 1/0.
     * Filters should update it if necessary depending on their function.
     * Sinks can use it to set a default output frame rate.
     * It is similar to the r_frame_rate field in AVStream.
     */
    AVRational frame_rate;

    /**
     * Minimum number of samples to filter at once. If filter_frame() is
     * called with fewer samples, it will accumulate them in fifo.
     * This field and the related ones must not be changed after filtering
     * has started.
     * If 0, all related fields are ignored.
     */
    int min_samples;

    /**
     * Maximum number of samples to filter at once. If filter_frame() is
     * called with more samples, it will split them.
     */
    int max_samples;

    /**
     * Number of channels.
     */
    int channels;

    /**
     * Number of past frames sent through the link.
     */
    int64_t frame_count_in, frame_count_out;

    /**
     * Number of past samples sent through the link.
     */
    int64_t sample_count_in, sample_count_out;

    /**
     * A pointer to a FFFramePool struct.
     */
    void *frame_pool;

    /**
     * True if a frame is currently wanted on the output of this filter.
     * Set when ff_request_frame() is called by the output,
     * cleared when a frame is filtered.
     */
    int frame_wanted_out;

    /**
     * For hwaccel pixel formats, this should be a reference to the
     * AVHWFramesContext describing the frames.
     */
    AVBufferRef *hw_frames_ctx;

#ifndef FF_INTERNAL_FIELDS

    /**
     * Internal structure members.
     * The fields below this limit are internal for libavfilter's use
     * and must in no way be accessed by applications.
     */
    char reserved[0xF000];

#else /* FF_INTERNAL_FIELDS */

    /**
     * Queue of frames waiting to be filtered.
     */
    FFFrameQueue fifo;

    /**
     * If set, the source filter can not generate a frame as is.
     * The goal is to avoid repeatedly calling the request_frame() method on
     * the same link.
     */
    int frame_blocked_in;

    /**
     * Link input status.
     * If not zero, all attempts of filter_frame will fail with the
     * corresponding code.
     */
    int status_in;

    /**
     * Timestamp of the input status change.
     */
    int64_t status_in_pts;

    /**
     * Link output status.
     * If not zero, all attempts of request_frame will fail with the
     * corresponding code.
     */
    int status_out;

#endif /* FF_INTERNAL_FIELDS */

};

操作函数


/**
 * Link two filters together.
 *
 * @param src    the source filter
 * @param srcpad index of the output pad on the source filter
 * @param dst    the destination filter
 * @param dstpad index of the input pad on the destination filter
 * @return       zero on success
 */
int avfilter_link(AVFilterContext *src, unsigned srcpad,
                  AVFilterContext *dst, unsigned dstpad);

/**
 * Free the link in *link, and set its pointer to NULL.
 */
void avfilter_link_free(AVFilterLink **link);

/**
 * Negotiate the media format, dimensions, etc of all inputs to a filter.
 *
 * @param filter the filter to negotiate the properties for its inputs
 * @return       zero on successful negotiation
 */
int avfilter_config_links(AVFilterContext *filter);

说说什么是LINK,就是link指针的drcpad和dstpad的赋值

int avfilter_link(AVFilterContext *src, unsigned srcpad,
                  AVFilterContext *dst, unsigned dstpad)
{
    AVFilterLink *link;
...

    link->src     = src;
    link->dst     = dst;
    link->srcpad  = &src->output_pads[srcpad];
    link->dstpad  = &dst->input_pads[dstpad];
    link->type    = src->output_pads[srcpad].type;
    av_assert0(AV_PIX_FMT_NONE == -1 && AV_SAMPLE_FMT_NONE == -1);
    link->format  = -1;
    ff_framequeue_init(&link->fifo, &src->graph->internal->frame_queues);

    return 0;
}

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