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 AVFilterBuffer 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, and sample format (the sample packing is implied by the
  sample format).

  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 ends 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.


Buffer references ownership and permissions
===========================================

  TODO


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.

  start_frame / filter_samples
  ----------------------------

    These methods are called when a frame is pushed to the filter's input.
    They 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: framerate-doubling filter: start_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
  -------------

    This method is called when a frame is wanted on an output.

    For an input, it should directly call start_frame or filter_samples 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, 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;
        }
        while (!frame_pushed) {
            input = input_where_a_frame_is_most_needed();
            ret = avfilter_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, request_frame
    does not push frames: it requests them to its input, and as a reaction,
    the start_frame / filter_samples method will be called and do the work.
