libx264 preset's rc_lookahead value is overwritten

[Andrew Wason]

Specifying -preset slow with libx264 should result in rc_lookahead of 50, instead it is 40.

In X264_init() x4->params.rc.i_lookahead is initialized to 50 by x264_param_default_preset() for the "slow" preset. But then later it is overwritten by avctx->rc_lookahead which is 40

This problem was introduced in commit d4b967131842e7c225e5f83cb82337d4474a0bb7 which added the rc_lookahead private option.

From a gdb session:

257         if (x4->preset || x4->tune)
(gdb) 
258             if (x264_param_default_preset(&x4->params, x4->preset, x4->tune) < 0) {
(gdb) p x4->preset
$7 = 0x14790c0 "slow"
(gdb) n
263         if (avctx->level > 0)
(gdb) p x4->params.rc.i_lookahead
$8 = 50
[...]
335         if (avctx->rc_lookahead >= 0)
(gdb) 
336             x4->params.rc.i_lookahead             = avctx->rc_lookahead;
(gdb) 
337         if (avctx->weighted_p_pred >= 0)
(gdb) p x4->params.rc.i_lookahead
$9 = 40

Full ffmpeg output:

ffmpeg -i /tmp/apollo11.mov -an -codec:v libx264 -preset:v slow -y /tmp/foo.mp4

ffmpeg version N-32420-g88fddd0, Copyright (c) 2000-2011 the FFmpeg developers
  built on Sep  9 2011 20:33:39 with gcc 4.4.3
  configuration: --extra-cflags=-I/opt/motionbox/foundation/6.4.0/include --extra-ldflags=-L/opt/motionbox/foundation/6.4.0/lib --enable-libx264 --enable-gpl --enable-debug=gdb3 --disable-optimizations
  libavutil    51. 16. 0 / 51. 16. 0
  libavcodec   53. 13. 0 / 53. 13. 0
  libavformat  53. 12. 0 / 53. 12. 0
  libavdevice  53.  3. 0 / 53.  3. 0
  libavfilter   2. 39. 0 /  2. 39. 0
  libswscale    2.  1. 0 /  2.  1. 0
  libpostproc  51.  2. 0 / 51.  2. 0
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x25d6780] max_analyze_duration 5000000 reached at 5000000

Seems stream 0 codec frame rate differs from container frame rate: 600.00 (600/1) -> 3.00 (3/1)
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/tmp/apollo11.mov':
  Metadata:
    creation_time   : 1998-01-05 21:59:31
  Duration: 00:00:06.66, start: 0.000000, bitrate: 22 kb/s
    Stream #0.0(eng): Video: svq1 (SVQ1 / 0x31515653), yuv410p, 80x60, 13 kb/s, SAR 1:1 DAR 4:3, 3 fps, 3 tbr, 600 tbn, 600 tbc
    Metadata:
      creation_time   : 1998-01-05 21:59:31
    Stream #0.1(eng): Audio: qcelp (Qclp / 0x706C6351), 8000 Hz, 1 channels, flt
    Metadata:
      creation_time   : 1998-01-05 21:59:31
Incompatible pixel format 'yuv410p' for codec 'libx264', auto-selecting format 'yuv420p'
[buffer @ 0x25d1200] w:80 h:60 pixfmt:yuv410p tb:1/1000000 sar:1/1 sws_param:
[buffersink @ 0x25d7ec0] auto-inserting filter 'auto-inserted scaler 0' between the filter 'src' and the filter 'out'
[scale @ 0x25d8520] w:80 h:60 fmt:yuv410p -> w:80 h:60 fmt:yuv420p flags:0x4
[libx264 @ 0x25de5c0] using SAR=1/1
[libx264 @ 0x25de5c0] using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2
[libx264 @ 0x25de5c0] profile Main, level 1.0
[libx264 @ 0x25de5c0] 264 - core 116 mbox/4.3.1 - H.264/MPEG-4 AVC codec - Copyleft 2003-2011 - http://www.videolan.org/x264.html - options: cabac=1 ref=5 deblock=1:0:0 analyse=0x1:0x111 me=umh subme=8 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=1 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=0 b_adapt=2 b_bias=0 direct=3 weightb=0 open_gop=0 weightp=2 keyint=250 keyint_min=3 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to '/tmp/foo.mp4':
  Metadata:
    creation_time   : 1998-01-05 21:59:31
    encoder         : Lavf53.12.0
    Stream #0.0(eng): Video: h264 (![0][0][0] / 0x0021), yuv420p, 80x60 [SAR 1:1 DAR 4:3], q=-1--1, 3 tbn, 3 tbc
    Metadata:
      creation_time   : 1998-01-05 21:59:31
Stream mapping:
  Stream #0.0 -> #0.0 (svq1 -> libx264)
Press [q] to stop, [?] for help
frame=   20 fps=  0 q=21.0 Lsize=      16kB time=00:00:06.33 bitrate=  20.8kbits/s    
video:15kB audio:0kB global headers:0kB muxing overhead 7.083741%
frame I:1     Avg QP:20.35  size:   867
[libx264 @ 0x25de5c0] frame P:9     Avg QP:22.59  size:   852
[libx264 @ 0x25de5c0] frame B:10    Avg QP:23.73  size:   611
[libx264 @ 0x25de5c0] consecutive B-frames: 15.0% 50.0% 15.0% 20.0%
[libx264 @ 0x25de5c0] mb I  I16..4: 15.0%  0.0% 85.0%
[libx264 @ 0x25de5c0] mb P  I16..4:  4.4%  0.0% 57.8%  P16..4: 10.6% 11.7% 15.6%  0.0%  0.0%    skip: 0.0%
[libx264 @ 0x25de5c0] mb B  I16..4:  0.0%  0.0% 37.5%  B16..8: 18.0% 18.0% 19.6%  direct: 6.5%  skip: 0.5%  L0:49.4% L1:27.2% BI:23.4%
[libx264 @ 0x25de5c0] direct mvs  spatial:0.0% temporal:100.0%
[libx264 @ 0x25de5c0] coded y,uvDC,uvAC intra: 96.1% 94.7% 63.3% inter: 75.4% 92.2% 28.5%
[libx264 @ 0x25de5c0] i16 v,h,dc,p:  0% 64% 18% 18%
[libx264 @ 0x25de5c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 32% 13% 11%  5%  6%  9%  6%  8% 10%
[libx264 @ 0x25de5c0] i8c dc,h,v,p: 35% 18% 29% 17%
[libx264 @ 0x25de5c0] Weighted P-Frames: Y:33.3% UV:22.2%
[libx264 @ 0x25de5c0] ref P L0: 76.5%  8.8%  5.9%  5.9%  1.5%  1.5%
[libx264 @ 0x25de5c0] ref B L0: 96.8%  2.4%  0.9%
[libx264 @ 0x25de5c0] kb/s:17.57

[Andrew Wason]

It looks like other x264 preset options have the same issue, these options have their "slow" preset defaults changed due to the default value of avctx->flags2:

param	preset value	value used
x4->params.i_bframe_pyramid	X264_B_PYRAMID_NORMAL	X264_B_PYRAMID_NONE
x4->params.analyse.b_transform_8x8	1	0
x4->params.analyse.b_weighted_bipred	1	0
x4->params.analyse.b_mixed_references	1	0

[Carl Eugen Hoyos]

I suspect this has just been fixed (by you), please reopen if I am wrong.