x264 settings for zero-delay encoding?

[genesys]

I'm trying to encode a video with zero delay (I'm doing this in my own application using ffmpeg) using x264 codec. By this I mean i want instant results after each frame i feed into the encoder. I already set b-frames to 0 and threads to 1 but i still get a delay that is gop_size+1. Can someone tell me what settings I need in order to get zero-delay encoding? (i can also provide my c++ code if that helps answering the question, but won't post it for the time being as most people here probably use x264 through the commandline tool)

[nm]

What rc-lookahead value do you use? The code might help too.

Have you read this blog post (and comments) about low latency encoding with x264: http://x264dev.multimedia.cx/archives/249

[genesys]

hey nm, thank you for your response

yes I saw that post already and a bit further down someone asks a similar question regarding low latency and the author recommends editing the ffmpeg sourcecode - but as I'm on msvc++ I use a precompiled build of ffmpeg and I also don't need this ultra-low-latency between encoder and decoder, I just need the encoding result right after the first frame was fed into the encoder.

I was already looking for the rc_lookahead setting as i suspect it to be relevant to my problem, but the rc_lookahead setting was removed from AVCodecContext sometime last year and replaced by a "private option" which I do not know how to read or set (http://ffmpeg.org/pipermail/ffmpeg-cvslog/2011-August/039836.html)


this is my code for the setup of the codeccontext:

static AVStream* add_video_stream(AVFormatContext *oc, enum CodecID codec_id, int w, int h, int fps)
{
AVCodecContext *c;
AVStream *st;
AVCodec *codec;

/* find the video encoder */
codec = avcodec_find_encoder(codec_id);
if (!codec) {
fprintf(stderr, "codec not found\n");
exit(1);
}

st = avformat_new_stream(oc, codec);
if (!st) {
fprintf(stderr, "Could not alloc stream\n");
exit(1);
}

c = st->codec;

/* Put sample parameters. */
c->bit_rate = 400000;
/* Resolution must be a multiple of two. */
c->width = w;
c->height = h;
/* timebase: This is the fundamental unit of time (in seconds) in terms
* of which frame timestamps are represented. For fixed-fps content,
* timebase should be 1/framerate and timestamp increments should be
* identical to 1. */
c->time_base.den = fps;
c->time_base.num = 1;
c->gop_size = 12; /* emit one intra frame every twelve frames at most */

c->codec = codec;
c->codec_type = AVMEDIA_TYPE_VIDEO;
c->coder_type = FF_CODER_TYPE_VLC;
c->me_method = 7; //motion estimation algorithm
c->me_subpel_quality = 4;
c->delay = 0;
c->max_b_frames = 0;
c->thread_count = 1; // more than one threads seem to increase delay
c->refs = 3;

c->pix_fmt = PIX_FMT_YUV420P;

/* Some formats want stream headers to be separate. */
if (oc->oformat->flags & AVFMT_GLOBALHEADER)
c->flags |= CODEC_FLAG_GLOBAL_HEADER;

return st;
}

[genesys]

okay - i figured out how to set coder private options and set rc-lookahead to 0, which broght my framedelay down to 1 independant of gop_size. Still one frame too much.
ffmpeg dumps me this info about the used settings:

[libx264 @ 000000000057DAA0] profile High, level 3.0
[libx264 @ 000000000057DAA0] 264 - core 120 r2146 bcd41db - H.264/MPEG-4 AVC cod
ec - Copyleft 2003-2011 - http://www.videolan.org/x264.html - options: cabac=0 r
ef=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=4 psy=1 psy_rd=1.00:0.00 mixed
_ref=1 me_range=16 chroma_me=1 trellis=0 8x8dct=1 cqm=0 deadzone=21,11 fast_pski
p=1 chroma_qp_offset=0 threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 b
luray_compat=0 constrained_intra=0 bframes=0 weightp=2 keyint=12 keyint_min=1 sc
enecut=40 intra_refresh=0 rc=abr mbtree=0 bitrate=400 ratetol=1.0 qcomp=0.60 qpm
in=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'out2.mp4':
Stream #0:0: Video: h264, yuv420p, 640x480, q=-1--1, 400 kb/s, 90k tbn, 25 t
bc


Any idea what else i need to change to get 0 frames delay?

[nm]

See what --tune zerolatency does:


- zerolatency:
--bframes 0 --force-cfr --no-mbtree
--sync-lookahead 0 --sliced-threads
--rc-lookahead 0


I guess you need to set sync-lookahead to 0 (it's 1 by default with zero b-frames) and perhaps switch off mbtree.

Note that you can use sliced threads to speed up encoding.

[genesys]

this works! thank you very much :)