[FFmpeg-devel] [PATCH 2/3] avfiltergraph: don't query formats if filter has uninitialized inputs

Nicolas George nicolas.george at normalesup.org
Thu May 3 16:33:17 CEST 2012

Le quintidi 15 floréal, an CCXX, Matthieu Bouron a écrit :
> The complex filter system add vsrc_buffer and asrc_buffer (in next patch) at
> the end of the avfilter context array and query_formats is currently called
> sequentially it. It can lead to a filter initialization failure for filters
> with input dependencies (like amerge)
> The following example will fail on the amerge filter since the two abbufer
> are not initialized:
>   [ amerge ] [ abuffer ] [ abuffer ] [ abuffersink ]

Well, I had guessed amerge would be somewhere there.

The fact is that amerge is doing something bad: it selects its format using
its input's formats. That has all kinds of bad consequences. The one you
pointed out, but not only that. For example:

	[in (stereo)] -> [ resample ] -> [ amerge ] ->

will, unless I am mistaken, select mono as input and insert a stereo->mono
converter between resample and amerge.

(The exact problem is that amerge will select its input format as the first
supported by resample, which is mono, and this happens before the
intersection mechanism has restricted this list to only stereo.)

This is bad, but this is the best I could think of at the time, and still

If you have a better idea...

> I agree, this implementation is naive.
> Another solution whould be to order the avfilter context array when
> avfilter_link is called.

It would not work with loops. Loops are still unsupported, but Michael says
they should be eventually.

I can suggest the following algorithm:

while (1) {
    work_to_do = 0;
    success = 0;
    for (each filter f) {
	if (!all_inputs_and_outputs_configured(f)) {
	    r = f->query_format();
	    if (r < 0) {
		if (r == AVERROR(EAGAIN))
		    work_to_do = 1;
		    return r;
	    } else {
		success = 1;
    if (work_to_do) {
	if (!success) /* a full round and no progress */

And amerge should return EAGAIN rather than EINVAL if its input is not yet


  Nicolas George
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