FFmpeg4/doc/filter_design.txt

Filter design
=============

This document explains guidelines that should be observed (or ignored with
good reason) when writing filters for libavfilter.

In this document, the word “frame” indicates either a video frame or a group
of audio samples, as stored in an AVFrame structure.


Format negotiation
==================

  The query_formats method should set, for each input and each output links,
  the list of supported formats.

  For video links, that means pixel format. For audio links, that means
  channel layout, sample format (the sample packing is implied by the sample
  format) and sample rate.

  The lists are not just lists, they are references to shared objects. When
  the negotiation mechanism computes the intersection of the formats
  supported at each end of a link, all references to both lists are replaced
  with a reference to the intersection. And when a single format is
  eventually chosen for a link amongst the remaining list, again, all
  references to the list are updated.

  That means that if a filter requires that its input and output have the
  same format amongst a supported list, all it has to do is use a reference
  to the same list of formats.

  query_formats can leave some formats unset and return AVERROR(EAGAIN) to
  cause the negotiation mechanism to try again later. That can be used by
  filters with complex requirements to use the format negotiated on one link
  to set the formats supported on another.


Frame references ownership and permissions
==========================================

  Principle
  ---------

    Audio and video data are voluminous; the frame and frame reference
    mechanism is intended to avoid, as much as possible, expensive copies of
    that data while still allowing the filters to produce correct results.

    The data is stored in buffers represented by AVFrame structures.
    Several references can point to the same frame buffer; the buffer is
    automatically deallocated once all corresponding references have been
    destroyed.

    The characteristics of the data (resolution, sample rate, etc.) are
    stored in the reference; different references for the same buffer can
    show different characteristics. In particular, a video reference can
    point to only a part of a video buffer.

    A reference is usually obtained as input to the filter_frame method or
    requested using the ff_get_video_buffer or ff_get_audio_buffer
    functions. A new reference on an existing buffer can be created with
    av_frame_ref(). A reference is destroyed using
    the av_frame_free() function.

  Reference ownership
  -------------------

    At any time, a reference “belongs” to a particular piece of code,
    usually a filter. With a few caveats that will be explained below, only
    that piece of code is allowed to access it. It is also responsible for
    destroying it, although this is sometimes done automatically (see the
    section on link reference fields).

    Here are the (fairly obvious) rules for reference ownership:

    * A reference received by the filter_frame method belongs to the
      corresponding filter.

    * A reference passed to ff_filter_frame is given away and must no longer
      be used.

    * A reference created with av_frame_ref() belongs to the code that
      created it.

    * A reference obtained with ff_get_video_buffer or ff_get_audio_buffer
      belongs to the code that requested it.

    * A reference given as return value by the get_video_buffer or
      get_audio_buffer method is given away and must no longer be used.

  Link reference fields
  ---------------------

    The AVFilterLink structure has a few AVFrame fields.

    partial_buf is used by libavfilter internally and must not be accessed
    by filters.

    fifo contains frames queued in the filter's input. They belong to the
    framework until they are taken by the filter.

  Reference permissions
  ---------------------

    Since the same frame data can be shared by several frames, modifying may
    have unintended consequences. A frame is considered writable if only one
    reference to it exists. The code owning that reference it then allowed
    to modify the data.

    A filter can check if a frame is writable by using the
    av_frame_is_writable() function.

    A filter can ensure that a frame is writable at some point of the code
    by using the ff_inlink_make_frame_writable() function. It will duplicate
    the frame if needed.

    A filter can ensure that the frame passed to the filter_frame() callback
    is writable by setting the needs_writable flag on the corresponding
    input pad. It does not apply to the activate() callback.


Frame scheduling
================

  The purpose of these rules is to ensure that frames flow in the filter
  graph without getting stuck and accumulating somewhere.

  Simple filters that output one frame for each input frame should not have
  to worry about it.

  There are two design for filters: one using the filter_frame() and
  request_frame() callbacks and the other using the activate() callback.

  The design using filter_frame() and request_frame() is legacy, but it is
  suitable for filters that have a single input and process one frame at a
  time. New filters with several inputs, that treat several frames at a time
  or that require a special treatment at EOF should probably use the design
  using activate().

  activate
  --------

    This method is called when something must be done in a filter; the
    definition of that "something" depends on the semantic of the filter.

    The callback must examine the status of the filter's links and proceed
    accordingly.

    The status of output links is stored in the frame_wanted_out, status_in
    and status_out fields and tested by the ff_outlink_frame_wanted()
    function. If this function returns true, then the processing requires a
    frame on this link and the filter is expected to make efforts in that
    direction.

    The status of input links is stored by the status_in, fifo and
    status_out fields; they must not be accessed directly. The fifo field
    contains the frames that are queued in the input for processing by the
    filter. The status_in and status_out fields contains the queued status
    (EOF or error) of the link; status_in is a status change that must be
    taken into account after all frames in fifo have been processed;
    status_out is the status that have been taken into account, it is final
    when it is not 0.

    The typical task of an activate callback is to first check the backward
    status of output links, and if relevant forward it to the corresponding
    input. Then, if relevant, for each input link: test the availability of
    frames in fifo and process them;  if no frame is available, test and
    acknowledge a change of status using ff_inlink_acknowledge_status(); and
    forward the result (frame or status change) to the corresponding input.
    If nothing is possible, test the status of outputs and forward it to the
    corresponding input(s). If still not possible, return FFERROR_NOT_READY.

    If the filters stores internally one or a few frame for some input, it
    can consider them to be part of the FIFO and delay acknowledging a
    status change accordingly.

    Example code:

    ret = ff_outlink_get_status(outlink);
    if (ret) {
        ff_inlink_set_status(inlink, ret);
        return 0;
    }
    if (priv->next_frame) {
        /* use it */
        return 0;
    }
    ret = ff_inlink_consume_frame(inlink, &frame);
    if (ret < 0)
        return ret;
    if (ret) {
        /* use it */
        return 0;
    }
    ret = ff_inlink_acknowledge_status(inlink, &status, &pts);
    if (ret) {
        /* flush */
        ff_outlink_set_status(outlink, status, pts);
        return 0;
    }
    if (ff_outlink_frame_wanted(outlink)) {
        ff_inlink_request_frame(inlink);
        return 0;
    }
    return FFERROR_NOT_READY;

    The exact code depends on how similar the /* use it */ blocks are and
    how related they are to the /* flush */ block, and needs to apply these
    operations to the correct inlink or outlink if there are several.

    Macros are available to factor that when no extra processing is needed:

    FF_FILTER_FORWARD_STATUS_BACK(outlink, inlink);
    FF_FILTER_FORWARD_STATUS_ALL(outlink, filter);
    FF_FILTER_FORWARD_STATUS(inlink, outlink);
    FF_FILTER_FORWARD_STATUS_ALL(inlink, filter);
    FF_FILTER_FORWARD_WANTED(outlink, inlink);

  filter_frame
  ------------

    For filters that do not use the activate() callback, this method is
    called when a frame is pushed to the filter's input. It can be called at
    any time except in a reentrant way.

    If the input frame is enough to produce output, then the filter should
    push the output frames on the output link immediately.

    As an exception to the previous rule, if the input frame is enough to
    produce several output frames, then the filter needs output only at
    least one per link. The additional frames can be left buffered in the
    filter; these buffered frames must be flushed immediately if a new input
    produces new output.

    (Example: frame rate-doubling filter: filter_frame must (1) flush the
    second copy of the previous frame, if it is still there, (2) push the
    first copy of the incoming frame, (3) keep the second copy for later.)

    If the input frame is not enough to produce output, the filter must not
    call request_frame to get more. It must just process the frame or queue
    it. The task of requesting more frames is left to the filter's
    request_frame method or the application.

    If a filter has several inputs, the filter must be ready for frames
    arriving randomly on any input. Therefore, any filter with several inputs
    will most likely require some kind of queuing mechanism. It is perfectly
    acceptable to have a limited queue and to drop frames when the inputs
    are too unbalanced.

  request_frame
  -------------

    For filters that do not use the activate() callback, this method is
    called when a frame is wanted on an output.

    For a source, it should directly call filter_frame on the corresponding
    output.

    For a filter, if there are queued frames already ready, one of these
    frames should be pushed. If not, the filter should request a frame on
    one of its inputs, repeatedly until at least one frame has been pushed.

    Return values:
    if request_frame could produce a frame, or at least make progress
    towards producing a frame, it should return 0;
    if it could not for temporary reasons, it should return AVERROR(EAGAIN);
    if it could not because there are no more frames, it should return
    AVERROR_EOF.

    The typical implementation of request_frame for a filter with several
    inputs will look like that:

        if (frames_queued) {
            push_one_frame();
            return 0;
        }
        input = input_where_a_frame_is_most_needed();
        ret = ff_request_frame(input);
        if (ret == AVERROR_EOF) {
            process_eof_on_input();
        } else if (ret < 0) {
            return ret;
        }
        return 0;

    Note that, except for filters that can have queued frames and sources,
    request_frame does not push frames: it requests them to its input, and
    as a reaction, the filter_frame method possibly will be called and do
    the work.