Callbacks and properties to describe the steps of a format negotiation. More...

#include <formats.h>

Data Fields
unsigned	nb_mergers

const AVFilterFormatsMerger *	mergers

const char *	conversion_filter

unsigned	conversion_opts_offset

Detailed Description

Callbacks and properties to describe the steps of a format negotiation.

The steps are:

query_formats(): call the callbacks on all filter to set lists of supported formats. When links on a filter must eventually have the same format, the lists of supported formats are the same object in memory. See: http://www.normalesup.org/~george/articles/format_negotiation_in_libavfilter/#12
query_formats(): merge lists of supported formats or insert automatic conversion filters. Compute the intersection of the lists of supported formats on the ends of links. If it succeeds, replace both objects with the intersection everywhere they are referenced. If the intersection is empty, insert an automatic conversion filter. If several formats are negotiated at once (format, rate, layout), only merge if all three can be, since the conversion filter can convert all three at once. This process goes on as long as progress is made. See: http://www.normalesup.org/~george/articles/format_negotiation_in_libavfilter/#14 http://www.normalesup.org/~george/articles/format_negotiation_in_libavfilter/#29
reduce_formats(): try to reduce format conversion within filters. For each link where there is only one supported formats on output, for each output of the connected filter, if the media type is the same and said format is supported, keep only this one. This process goes on as long as progress is made. Rationale: conversion filters will set a large list of supported formats on outputs but users will expect the output to be as close as possible as the input (examples: scale without changing the pixel format, resample without changint the layout). FIXME: this can probably be done by merging the input and output lists instead of re-implementing the logic.
swap_sample_fmts(): swap_samplerates(): swap_channel_layouts(): For each filter with an input with only one supported format, when outputs have several supported formats, put the best one with reference to the input at the beginning of the list, to prepare it for being picked up by pick_formats(). The best format is the one that is most similar to the input while not losing too much information. This process need to run only once. FIXME: reduce_formats() operates on all inputs with a single format, swap_*() operates on the first one only: check if the difference makes sense. TODO: the swapping done for one filter can override the swapping done for another filter connected to the same list of formats, maybe it would be better to compute a total score for all connected filters and use the score to pick the format instead of just swapping. TODO: make the similarity logic available as public functions in libavutil.
pick_formats(): Choose one format from the lists of supported formats, use it for the link and reduce the list to a single element to force other filters connected to the same list to use it. First process all links where there is a single format and the output links of all filters with an input, trying to preserve similarity between input and outputs. Repeat as long as process is made. Then do a final run for the remaining filters. FIXME: the similarity logic (the ref argument to pick_format()) added in FFmpeg duplicates and overrides the swapping logic added in libav. Better merge them into a score system.