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fftools/ffmpeg_dec: decode the first frame synchronously

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In an ideal world, a filtergraph like `-i A -i B ... -filter_complex concat`
would only keep resources in memory related to the file currently being output
by the concat filter. So ideally, we'd open and fully decode A, then open and
fully decode B, and so on.

Practically, however, fftools wants to get one frame from each input file in
order to initialize the filter graph (buffersrc parameters). So what happens
currently is that fftools will request a single frame from each input A, B, etc
that is plugged into the filtergraph.

When using frame threading, the design of the decoder (ff_thread_receive_frame)
is that it will not output any frames until we have received enough packets to
saturate all threads. This, however, forces the decoder to buffer at least as
many frames for each input file as we have threads, before outputting anything.

By decoding the first frame synchronously, we avoid this issue and allow
configuring the filter graph more quickly and without wasting excess resources
on frames that will not (yet) be used.
This commit is contained in:
Niklas Haas 2025-09-22 15:38:06 +02:00
parent 5e56937b74
commit 929a2ced9b
1 changed files with 4 additions and 1 deletions
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5
fftools/ffmpeg_dec.c
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@ -744,11 +744,14 @@ static int packet_decode(DecoderPriv *dp, AVPacket *pkt, AVFrame *frame)
while (1) {
FrameData *fd;
unsigned outputs_mask = 1;
unsigned flags = 0;
if (!dp->dec.frames_decoded)
flags |= AV_CODEC_RECEIVE_FRAME_FLAG_SYNCHRONOUS;
av_frame_unref(frame);
update_benchmark(NULL);
ret = avcodec_receive_frame(dec, frame);
ret = avcodec_receive_frame2(dec, frame, flags);
update_benchmark("decode_%s %s", type_desc, dp->parent_name);
if (ret == AVERROR(EAGAIN)) {