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FFmpeg过滤器框架分析

ffmpeg的filter⽤起来是和Gstreamer的plugin是⼀样的概念,通过avfilter_link,将各个创建好的filter按 ⾃⼰想要的次序链接到⼀起,然后avfilter_graph_config之后,就可以正常使⽤。 ⽐较常⽤的滤镜有:scale、trim、overlay、rotate、movie、yadif。scale 滤镜⽤于缩放,trim 滤镜⽤ 于帧级剪切,overlay 滤镜⽤于视频叠加,rotate 滤镜实现旋转,movie 滤镜可以加载第三⽅的视频, yadif 滤镜可以去隔⾏。

主要结构体和API介绍

AVFilterGraph-对filters系统的整体管理

重点

struct AVFilterGraph 
{ 
	AVFilterContext **filters; 
	unsigned nb_filters; 
}

完整结构体

typedef struct AVFilterGraph {
    const AVClass *av_class;
    AVFilterContext **filters;
    unsigned nb_filters;

    char *scale_sws_opts; ///< sws options to use for the auto-inserted scale filters
#if FF_API_LAVR_OPTS
    attribute_deprecated char *resample_lavr_opts;   ///< libavresample options to use for the auto-inserted resample filters
#endif

    /**
     * Type of multithreading allowed for filters in this graph. A combination
     * of AVFILTER_THREAD_* flags.
     *
     * May be set by the caller at any point, the setting will apply to all
     * filters initialized after that. The default is allowing everything.
     *
     * When a filter in this graph is initialized, this field is combined using
     * bit AND with AVFilterContext.thread_type to get the final mask used for
     * determining allowed threading types. I.e. a threading type needs to be
     * set in both to be allowed.
     */
    int thread_type;

    /**
     * Maximum number of threads used by filters in this graph. May be set by
     * the caller before adding any filters to the filtergraph. Zero (the
     * default) means that the number of threads is determined automatically.
     */
    int nb_threads;

    /**
     * Opaque object for libavfilter internal use.
     */
    AVFilterGraphInternal *internal;

    /**
     * Opaque user data. May be set by the caller to an arbitrary value, e.g. to
     * be used from callbacks like @ref AVFilterGraph.execute.
     * Libavfilter will not touch this field in any way.
     */
    void *opaque;

    /**
     * This callback may be set by the caller immediately after allocating the
     * graph and before adding any filters to it, to provide a custom
     * multithreading implementation.
     *
     * If set, filters with slice threading capability will call this callback
     * to execute multiple jobs in parallel.
     *
     * If this field is left unset, libavfilter will use its internal
     * implementation, which may or may not be multithreaded depending on the
     * platform and build options.
     */
    avfilter_execute_func *execute;

    char *aresample_swr_opts; ///< swr options to use for the auto-inserted aresample filters, Access ONLY through AVOptions

    /**
     * Private fields
     *
     * The following fields are for internal use only.
     * Their type, offset, number and semantic can change without notice.
     */

    AVFilterLink **sink_links;
    int sink_links_count;

    unsigned disable_auto_convert;
} AVFilterGraph;

AVFilter-定义filter本身的能⼒

重点

const char *name; // overlay 
const AVFilterPad *inputs; 
const AVFilterPad *outputs;

⽐如:

AVFilter ff_vf_overlay = {
    .name          = "overlay",
    .description   = NULL_IF_CONFIG_SMALL("Overlay a video source on top of the input."),
    .preinit       = overlay_framesync_preinit,
    .init          = init,
    .uninit        = uninit,
    .priv_size     = sizeof(OverlayContext),
    .priv_class    = &overlay_class,
    .query_formats = query_formats,
    .activate      = activate,
    .process_command = process_command,
    .inputs        = avfilter_vf_overlay_inputs,
    .outputs       = avfilter_vf_overlay_outputs,
    .flags         = AVFILTER_FLAG_SUPPORT_TIMELINE_INTERNAL |
                     AVFILTER_FLAG_SLICE_THREADS,
};

// 定义filter本身的能⼒,拥有的pads,回调函数接⼝定义

typedef struct AVFilter {
    /**
     * Filter name. Must be non-NULL and unique among filters.
     */
    const char *name;

    /**
     * A description of the filter. May be NULL.
     *
     * You should use the NULL_IF_CONFIG_SMALL() macro to define it.
     */
    const char *description;

    /**
     * List of inputs, terminated by a zeroed element.
     *
     * NULL if there are no (static) inputs. Instances of filters with
     * AVFILTER_FLAG_DYNAMIC_INPUTS set may have more inputs than present in
     * this list.
     */
    const AVFilterPad *inputs;
    /**
     * List of outputs, terminated by a zeroed element.
     *
     * NULL if there are no (static) outputs. Instances of filters with
     * AVFILTER_FLAG_DYNAMIC_OUTPUTS set may have more outputs than present in
     * this list.
     */
    const AVFilterPad *outputs;

    /**
     * A class for the private data, used to declare filter private AVOptions.
     * This field is NULL for filters that do not declare any options.
     *
     * If this field is non-NULL, the first member of the filter private data
     * must be a pointer to AVClass, which will be set by libavfilter generic
     * code to this class.
     */
    const AVClass *priv_class;

    /**
     * A combination of AVFILTER_FLAG_*
     */
    int flags;

    /*****************************************************************
     * 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.
     *****************************************************************
     */

    /**
     * Filter pre-initialization function
     *
     * This callback will be called immediately after the filter context is
     * allocated, to allow allocating and initing sub-objects.
     *
     * If this callback is not NULL, the uninit callback will be called on
     * allocation failure.
     *
     * @return 0 on success,
     *         AVERROR code on failure (but the code will be
     *           dropped and treated as ENOMEM by the calling code)
     */
    int (*preinit)(AVFilterContext *ctx);

    /**
     * Filter initialization function.
     *
     * This callback will be called only once during the filter lifetime, after
     * all the options have been set, but before links between filters are
     * established and format negotiation is done.
     *
     * Basic filter initialization should be done here. Filters with dynamic
     * inputs and/or outputs should create those inputs/outputs here based on
     * provided options. No more changes to this filter's inputs/outputs can be
     * done after this callback.
     *
     * This callback must not assume that the filter links exist or frame
     * parameters are known.
     *
     * @ref AVFilter.uninit "uninit" is guaranteed to be called even if
     * initialization fails, so this callback does not have to clean up on
     * failure.
     *
     * @return 0 on success, a negative AVERROR on failure
     */
    int (*init)(AVFilterContext *ctx);

    /**
     * Should be set instead of @ref AVFilter.init "init" by the filters that
     * want to pass a dictionary of AVOptions to nested contexts that are
     * allocated during init.
     *
     * On return, the options dict should be freed and replaced with one that
     * contains all the options which could not be processed by this filter (or
     * with NULL if all the options were processed).
     *
     * Otherwise the semantics is the same as for @ref AVFilter.init "init".
     */
    int (*init_dict)(AVFilterContext *ctx, AVDictionary **options);

    /**
     * Filter uninitialization function.
     *
     * Called only once right before the filter is freed. Should deallocate any
     * memory held by the filter, release any buffer references, etc. It does
     * not need to deallocate the AVFilterContext.priv memory itself.
     *
     * This callback may be called even if @ref AVFilter.init "init" was not
     * called or failed, so it must be prepared to handle such a situation.
     */
    void (*uninit)(AVFilterContext *ctx);

    /**
     * Query formats supported by the filter on its inputs and outputs.
     *
     * This callback is called after the filter is initialized (so the inputs
     * and outputs are fixed), shortly before the format negotiation. This
     * callback may be called more than once.
     *
     * This callback must set AVFilterLink.out_formats on every input link and
     * AVFilterLink.in_formats on every output link to a list of pixel/sample
     * formats that the filter supports on that link. For audio links, this
     * filter must also set @ref AVFilterLink.in_samplerates "in_samplerates" /
     * @ref AVFilterLink.out_samplerates "out_samplerates" and
     * @ref AVFilterLink.in_channel_layouts "in_channel_layouts" /
     * @ref AVFilterLink.out_channel_layouts "out_channel_layouts" analogously.
     *
     * This callback may be NULL for filters with one input, in which case
     * libavfilter assumes that it supports all input formats and preserves
     * them on output.
     *
     * @return zero on success, a negative value corresponding to an
     * AVERROR code otherwise
     */
    int (*query_formats)(AVFilterContext *);

    int priv_size;      ///< size of private data to allocate for the filter

    int flags_internal; ///< Additional flags for avfilter internal use only.

    /**
     * Used by the filter registration system. Must not be touched by any other
     * code.
     */
    struct AVFilter *next;

    /**
     * Make the filter instance process a command.
     *
     * @param cmd    the command to process, for handling simplicity all commands must be alphanumeric only
     * @param arg    the argument for the command
     * @param res    a buffer with size res_size where the filter(s) can return a response. This must not change when the command is not supported.
     * @param flags  if AVFILTER_CMD_FLAG_FAST is set and the command would be
     *               time consuming then a filter should treat it like an unsupported command
     *
     * @returns >=0 on success otherwise an error code.
     *          AVERROR(ENOSYS) on unsupported commands
     */
    int (*process_command)(AVFilterContext *, const char *cmd, const char *arg, char *res, int res_len, int flags);

    /**
     * Filter initialization function, alternative to the init()
     * callback. Args contains the user-supplied parameters, opaque is
     * used for providing binary data.
     */
    int (*init_opaque)(AVFilterContext *ctx, void *opaque);

    /**
     * Filter activation function.
     *
     * Called when any processing is needed from the filter, instead of any
     * filter_frame and request_frame on pads.
     *
     * The function must examine inlinks and outlinks and perform a single
     * step of processing. If there is nothing to do, the function must do
     * nothing and not return an error. If more steps are or may be
     * possible, it must use ff_filter_set_ready() to schedule another
     * activation.
     */
    int (*activate)(AVFilterContext *ctx);
} AVFilter;

AVFilterContext-filter实例,管理filter与外部的联系

重点

struct AVFilterContext 
{ 
const AVFilter *filter; 
char *name; AVFilterPad *input_pads; 
AVFilterLink **inputs; 
unsigned nb_inputs AVFilterPad *output_pads; 
AVFilterLink **outputs; 
unsigned nb_outputs; 
struct AVFilterGraph *graph; // 从属于哪个AVFilterGraph 
}

完整的结构体

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

    /**
     * Type of multithreading being allowed/used. A combination of
     * AVFILTER_THREAD_* flags.
     *
     * May be set by the caller before initializing the filter to forbid some
     * or all kinds of multithreading for this filter. The default is allowing
     * everything.
     *
     * When the filter is initialized, this field is combined using bit AND with
     * AVFilterGraph.thread_type to get the final mask used for determining
     * allowed threading types. I.e. a threading type needs to be set in both
     * to be allowed.
     *
     * After the filter is initialized, libavfilter sets this field to the
     * threading type that is actually used (0 for no multithreading).
     */
    int thread_type;

    /**
     * An opaque struct for libavfilter internal use.
     */
    AVFilterInternal *internal;

    struct AVFilterCommand *command_queue;

    char *enable_str;               ///< enable expression string
    void *enable;                   ///< parsed expression (AVExpr*)
    double *var_values;             ///< variable values for the enable expression
    int is_disabled;                ///< the enabled state from the last expression evaluation

    /**
     * For filters which will create hardware frames, sets the device the
     * filter should create them in.  All other filters will ignore this field:
     * in particular, a filter which consumes or processes hardware frames will
     * instead use the hw_frames_ctx field in AVFilterLink to carry the
     * hardware context information.
     */
    AVBufferRef *hw_device_ctx;

    /**
     * Max number of threads allowed in this filter instance.
     * If <= 0, its value is ignored.
     * Overrides global number of threads set per filter graph.
     */
    int nb_threads;

    /**
     * Ready status of the filter.
     * A non-0 value means that the filter needs activating;
     * a higher value suggests a more urgent activation.
     */
    unsigned ready;

    /**
     * Sets the number of extra hardware frames which the filter will
     * allocate on its output links for use in following filters or by
     * the caller.
     *
     * Some hardware filters require all frames that they will use for
     * output to be defined in advance before filtering starts.  For such
     * filters, any hardware frame pools used for output must therefore be
     * of fixed size.  The extra frames set here are on top of any number
     * that the filter needs internally in order to operate normally.
     *
     * This field must be set before the graph containing this filter is
     * configured.
     */
    int extra_hw_frames;
};

AVFilterLink-定义两个filters之间的联接

重点

struct AVFilterLink 
{ 
AVFilterContext *src; 
AVFilterPad *srcpad; 
AVFilterContext *dst; 
AVFilterPad *dstpad; 
struct AVFilterGraph *graph;	
}

完整结构体

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 formats and channel layouts supported by the input and output
     * filters respectively. These lists are used for negotiating the format
     * to actually be used, which will be loaded into the format and
     * channel_layout members, above, when chosen.
     *
     */
    AVFilterFormats *in_formats;
    AVFilterFormats *out_formats;

    /**
     * Lists of channel layouts and sample rates used for automatic
     * negotiation.
     */
    AVFilterFormats  *in_samplerates;
    AVFilterFormats *out_samplerates;
    struct AVFilterChannelLayouts  *in_channel_layouts;
    struct AVFilterChannelLayouts *out_channel_layouts;

    /**
     * Audio only, the destination filter sets this to a non-zero value to
     * request that buffers with the given number of samples should be sent to
     * it. AVFilterPad.needs_fifo must also be set on the corresponding input
     * pad.
     * Last buffer before EOF will be padded with silence.
     */
    int request_samples;

    /** 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;

    /**
     * Buffer partially filled with samples to achieve a fixed/minimum size.
     */
    AVFrame *partial_buf;

    /**
     * Size of the partial buffer to allocate.
     * Must be between min_samples and max_samples.
     */
    int partial_buf_size;

    /**
     * Minimum number of samples to filter at once. If filter_frame() is
     * called with fewer samples, it will accumulate them in partial_buf.
     * 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;

    /**
     * Link processing flags.
     */
    unsigned flags;

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

};

AVFilterPad-定义filter的输⼊/输出接⼝

重点

struct AVFilterPad 
{ 
const char *name; 
AVFrame *(*get_video_buffer)(AVFilterLink *link, int w, int h); 
AVFrame *(*get_audio_buffer)(AVFilterLink *link, int nb_samples); 
int (*filter_frame)(AVFilterLink *link, AVFrame *frame);
int (*request_frame)(AVFilterLink *link); 
}

完整结构体

struct AVFilterPad {
    /**
     * Pad name. The name is unique among inputs and among outputs, but an
     * input may have the same name as an output. This may be NULL if this
     * pad has no need to ever be referenced by name.
     */
    const char *name;

    /**
     * AVFilterPad type.
     */
    enum AVMediaType type;

    /**
     * Callback function to get a video buffer. If NULL, the filter system will
     * use ff_default_get_video_buffer().
     *
     * Input video pads only.
     */
    AVFrame *(*get_video_buffer)(AVFilterLink *link, int w, int h);

    /**
     * Callback function to get an audio buffer. If NULL, the filter system will
     * use ff_default_get_audio_buffer().
     *
     * Input audio pads only.
     */
    AVFrame *(*get_audio_buffer)(AVFilterLink *link, int nb_samples);

    /**
     * Filtering callback. This is where a filter receives a frame with
     * audio/video data and should do its processing.
     *
     * Input pads only.
     *
     * @return >= 0 on success, a negative AVERROR on error. This function
     * must ensure that frame is properly unreferenced on error if it
     * hasn't been passed on to another filter.
     */
    int (*filter_frame)(AVFilterLink *link, AVFrame *frame);

    /**
     * Frame poll callback. This returns the number of immediately available
     * samples. It should return a positive value if the next request_frame()
     * is guaranteed to return one frame (with no delay).
     *
     * Defaults to just calling the source poll_frame() method.
     *
     * Output pads only.
     */
    int (*poll_frame)(AVFilterLink *link);

    /**
     * Frame request callback. A call to this should result in some progress
     * towards producing output over the given link. This should return zero
     * on success, and another value on error.
     *
     * Output pads only.
     */
    int (*request_frame)(AVFilterLink *link);

    /**
     * Link configuration callback.
     *
     * For output pads, this should set the link properties such as
     * width/height. This should NOT set the format property - that is
     * negotiated between filters by the filter system using the
     * query_formats() callback before this function is called.
     *
     * For input pads, this should check the properties of the link, and update
     * the filter's internal state as necessary.
     *
     * For both input and output filters, this should return zero on success,
     * and another value on error.
     */
    int (*config_props)(AVFilterLink *link);

    /**
     * The filter expects a fifo to be inserted on its input link,
     * typically because it has a delay.
     *
     * input pads only.
     */
    int needs_fifo;

    /**
     * The filter expects writable frames from its input link,
     * duplicating data buffers if needed.
     *
     * input pads only.
     */
    int needs_writable;
};

AVFilterInOut-过滤器链输⼊/输出的链接列表

/**
 * A linked-list of the inputs/outputs of the filter chain.
 *
 * This is mainly useful for avfilter_graph_parse() / avfilter_graph_parse2(),
 * where it is used to communicate open (unlinked) inputs and outputs from and
 * to the caller.
 * This struct specifies, per each not connected pad contained in the graph, the
 * filter context and the pad index required for establishing a link.
 */
typedef struct AVFilterInOut {
    /** unique name for this input/output in the list */
    char *name;

    /** filter context associated to this input/output */
    AVFilterContext *filter_ctx;

    /** index of the filt_ctx pad to use for linking */
    int pad_idx;

    /** next input/input in the list, NULL if this is the last */
    struct AVFilterInOut *next;
} AVFilterInOut;

在AVFilter模块中定义了AVFilter结构,很个AVFilter都是具有独⽴功能的节点,如scale filter的作⽤就是 进⾏图像尺⼨变换,overlay filter的作⽤就是进⾏图像的叠加。
这⾥需要重点提的是两个特别的filter,⼀个是buffer,⼀个是buffersink,

  • 滤波器buffer代表filter graph中的源头,原始数据就往这个filter节点输⼊的;
  • ⽽滤波器buffersink代表filter graph中的输出节点,处理完成的数据从这个filter节点输出。

函数使⽤

// 获取FFmpeg中定义的filter,调⽤该⽅法前需要先调⽤avfilter_register_all();进⾏滤波器注册
AVFilter avfilter_get_by_name(const char name);

// 往源滤波器buffer中输⼊待处理的数据
int av_buffersrc_add_frame(AVFilterContext ctx, AVFrame frame);

// 从⽬的滤波器buffersink中获取处理完的数据
int av_buffersink_get_frame(AVFilterContext ctx, AVFrame frame);

// 创建⼀个滤波器图filter graph
AVFilterGraph *avfilter_graph_alloc(void);

// 创建⼀个滤波器实例AVFilterContext,并添加到AVFilterGraph中
int avfilter_graph_create_filter(AVFilterContext **filt_ctx, const AVFilter *filt, const char name, const char args, void *opaque, AVFilterGraph *graph_ctx);

// 连接两个滤波器节点
int avfilter_link(AVFilterContext *src, unsigned srcpad, AVFilterContext *dst, unsigned dstpad);

AVFilter主体框架流程

在利⽤AVFilter进⾏⾳视频数据处理前先将在进⾏的处理流程绘制出来,现在以FFmpeg filter官⽅⽂档中 的⼀个例⼦为例进⾏说明。

 				[main] 
 input --> split ---------------------> overlay --> output 
			 | 								^ 
			 |[tmp]						 [flip]| 
			 +-----> crop --> vflip -------+

这个例⼦的处理流程如上所示,⾸先使⽤split滤波器将input流分成两路流(main和tmp),然后分别对两 路流进⾏处理。对于tmp流,先经过crop滤波器进⾏裁剪处理,再经过flip滤波器进⾏垂直⽅向上的翻转操 作,输出的结果命名为flip流。再将main流和flip流输⼊到overlay滤波器进⾏合成操作。上图的input就是上⾯提过的buffer源滤波器,output就是上⾯的提过的buffersink滤波器。上图中每个节点都是⼀个 AVFilterContext,每个连线就是AVFliterLink。所有这些信息都统⼀由AVFilterGraph来管理。
AVFilterInOut AVFilterGraph


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