nvenc_hevc encoding "B-adapt disabled" message - how to rectify?

[hydra3333]

Hello.

I am attempting to use ffmpeg to transcode h.264/avc .mp4 files into hevc .mp4 using nvenc with a RTX 2060 SUPER.

I note at the bottom of the log below the message "B-adapt disabled". I did not intentionally do that

So, I wonder if someone could please clarify:

1. what does "B-adapt disabled" mean and what is its effect ? Is it just that b-frames are "off" ?

2. should I somehow turn b-frames on - and if, so then how to do that with ffmpeg when using hevc_nvenc ? (I can't seem to find the option)

Thanks.

"ffmpeg.exe" -v verbose -nostats -init_hw_device opencl=ocl:0.0 -filter_hw_device ocl -i "D:\TEMP\SCRATCH\test.mp4" -map_metadata -1 -vsync 0 -sws_flags lanczos+accurate_rnd+full_chroma_int+full_chroma_inp -filter_complex "[0:v]yadif=0:0:0,format=pix_fmts=yuv420p,setdar=dar=16/9" -strict experimental -c:v hevc_nvenc -pix_fmt nv12 -preset slow -rc:v vbr_hq -2pass 1 -rc-lookahead:v 32 -cq 16 -qmin 14 -qmax 32 -spatial_aq 1 -temporal_aq 1 -strict experimental -profile:v main -level 6 -movflags +faststart+write_colr -af loudnorm=I=-16:TP=0.0:LRA=11:measured_I=-24.63:measured_LRA=14.00:measured_TP=-6.33:measured_thresh=-35.74:offset=0.10:linear=true:print_format=summary -c:a libfdk_aac -cutoff 18000 -ab 384k -ar 48000 -y "T:\HDTV\Converted\test.hevc.m

p4"

ffmpeg version N-95812-gd73f062706-hydra3333/v02_courtesy_DeadSix27 Copyright (c) 2000-2019 the FFmpeg developers
  built with gcc 9.2.0 (GCC)
  configuration: --arch=x86_64 --target-os=mingw64 --cross-prefix=x86_64-w64-mingw32- --pkg-config=pkg-config --disable-w32threads --enable-pthreads --enable-cross-compile --enable-pic --disable-shared --enable-static --enable-libsoxr --enable-libass --enable-iconv --enable-libtwolame --enable-libzvbi --enable-libcaca --enable-libmodplug --enable-libmp3lame --enable-version3 --enable-zlib --enable-librtmp --enable-libvorbis --enable-libtheora --enable-libspeex --enable-libgsm --enable-libopus --enable-bzlib --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libvo-amrwbenc --enable-libvpx --enable-libilbc --enable-libwavpack --enable-libwebp --enable-dxva2 --disable-avisynth --enable-vapoursynth --enable-gray --enable-libmysofa --enable-libflite --enable-lzma --enable-libsnappy --enable-libzimg --enable-libx264 --enable-libx265 --enable-libaom --enable-libdav1d --enable-frei0r --enable-filter=frei0r --enable-librubberband --enable-libvidstab --enable-libxvid --enable-libgme --enable-runtime-cpudetect --enable-libfribidi --enable-gnutls --enable-gmp --enable-fontconfig --enable-libfontconfig --enable-libfreetype --enable-libbluray --enable-libcdio --disable-schannel --enable-ladspa --enable-libxml2 --enable-libdavs2 --enable-libopenmpt --enable-libxavs --enable-libxavs2 --enable-libsrt --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-cuda-nvcc --enable-opengl --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-gpl --extra-version=hydra3333/v02_courtesy_DeadSix27 --pkg-config-flags=--static --extra-libs='-lpsapi -lintl -liconv -lssp' --extra-cflags=-DLIBTWOLAME_STATIC --extra-cflags=-DMODPLUG_STATIC --extra-cflags=-DLIBXML_STATIC --extra-cflags=-DGLIB_STATIC_COMPILATION --enable-nonfree --enable-libfdk-aac --enable-decklink --prefix=/home/u/Desktop/workdir/x86_64_products/ffmpeg_static_non_free_opencl.installed --enable-opencl
  libavutil      56. 36.100 / 56. 36.100
  libavcodec     58. 62.100 / 58. 62.100
  libavformat    58. 35.100 / 58. 35.100
  libavdevice    58.  9.101 / 58.  9.101
  libavfilter     7. 66.100 /  7. 66.100
  libswscale      5.  6.100 /  5.  6.100
  libswresample   3.  6.100 /  3.  6.100
  libpostproc    55.  6.100 / 55.  6.100
Routing option strict to both codec and muxer layer
    Last message repeated 1 times
[AVHWDeviceContext @ 000002ac417bf000] 0.0: NVIDIA CUDA / GeForce RTX 2060 SUPER
[AVHWDeviceContext @ 000002ac417bf000] DXVA2 to OpenCL mapping function found (clCreateFromDX9MediaSurfaceKHR).
[AVHWDeviceContext @ 000002ac417bf000] DXVA2 in OpenCL acquire function found (clEnqueueAcquireDX9MediaSurfacesKHR).
[AVHWDeviceContext @ 000002ac417bf000] DXVA2 in OpenCL release function found (clEnqueueReleaseDX9MediaSurfacesKHR).
[AVHWDeviceContext @ 000002ac417bf000] The cl_khr_d3d11_sharing extension is required for D3D11 to OpenCL mapping.
[AVHWDeviceContext @ 000002ac417bf000] D3D11 to OpenCL mapping not usable.
[h264 @ 000002ac417d4700] Reinit context to 1920x1088, pix_fmt: yuv420p
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'D:\TEMP\SCRATCH\test.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : VideoReDo (Lavf57.71.100)
  Duration: 01:14:50.92, start: 0.200000, bitrate: 2824 kb/s
    Stream #0:0(und): Video: h264 (High), 1 reference frame (avc1 / 0x31637661), yuv420p(tv, left), 1920x1080 (1920x1088) [SAR 1:1 DAR 16:9], 2562 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: mp3 (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 256 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Stream mapping:
  Stream #0:0 (h264) -> yadif (graph 0)
  setdar (graph 0) -> Stream #0:0 (hevc_nvenc)
  Stream #0:1 -> #0:1 (mp3 (mp3float) -> aac (libfdk_aac))
Press [q] to stop, [?] for help
[h264 @ 000002ac417c9dc0] Reinit context to 1920x1088, pix_fmt: yuv420p
[graph_1_in_0_1 @ 000002ac4183a780] tb:1/48000 samplefmt:fltp samplerate:48000 chlayout:0x3
[Parsed_loudnorm_0 @ 000002ac4183b580] auto-inserting filter 'auto_resampler_0' between the filter 'graph_1_in_0_1' and the filter 'Parsed_loudnorm_0'
[format_out_0_1 @ 000002ac4183a580] auto-inserting filter 'auto_resampler_1' between the filter 'Parsed_loudnorm_0' and the filter 'format_out_0_1'
[auto_resampler_0 @ 000002ac4183ad80] ch:2 chl:stereo fmt:fltp r:48000Hz -> ch:2 chl:stereo fmt:dbl r:192000Hz
[auto_resampler_1 @ 000002ac4183b980] ch:2 chl:stereo fmt:dbl r:192000Hz -> ch:2 chl:stereo fmt:s16 r:48000Hz
[graph 0 input from stream 0:0 @ 000002ac4183c380] w:1920 h:1080 pixfmt:yuv420p tb:1/90000 fr:25/1 sar:1/1 sws_param:flags=2
[auto_scaler_0 @ 000002ac4183b480] w:iw h:ih flags:'bilinear' interl:0
[format @ 000002ac4183b080] auto-inserting filter 'auto_scaler_0' between the filter 'Parsed_setdar_2' and the filter 'format'
[Parsed_setdar_2 @ 000002ac4183ba80] w:1920 h:1080 dar:16/9 sar:1/1 -> dar:16/9 sar:1/1
[auto_scaler_0 @ 000002ac4183b480] w:1920 h:1080 fmt:yuv420p sar:1/1 -> w:1920 h:1080 fmt:nv12 sar:1/1 flags:0x2
[hevc_nvenc @ 000002ac54e90540] Loaded Nvenc version 9.1
[hevc_nvenc @ 000002ac54e90540] Nvenc initialized successfully
[hevc_nvenc @ 000002ac54e90540] 1 CUDA capable devices found
[hevc_nvenc @ 000002ac54e90540] [ GPU #0 - < GeForce RTX 2060 SUPER > has Compute SM 7.5 ]
[hevc_nvenc @ 000002ac54e90540] supports NVENC
[hevc_nvenc @ 000002ac54e90540] AQ enabled.
[hevc_nvenc @ 000002ac54e90540] Temporal AQ enabled.
[hevc_nvenc @ 000002ac54e90540] Lookahead enabled: depth 32, scenecut enabled, B-adapt disabled.

[hydra3333]

Ah, had a look at the code and "-rc-lookahead" forces b-adapt to become off.
https://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/nvenc.c;h=111048d043263fc85...7760da58188d82

Not sure what the real-world effect of removing that and having b-adapt is vs leaving "-rc-lookahead" in ?