c++ - 为什么 Nvenc Encoder 可以提高解码速度？

Question

我正在尝试使用 C++ 中的 Ffmpeg 提高将 h264 视频解码为 mp4 视频的速度。我找到了一个可以做到这一点的 cli 命令，但我仍然不完全理解用 c++ 重现它的命令，所以我希望你们能给我一个解释。

我找到的命令使用 h264_nvenc 编码器来提高使用 GPU 的速度。但是，我无法理解为什么 h264 编码器会提高解码 h264 视频(或者可能编码为 mp4 帧)的速度。任何人都可以给我一个详细的解释吗？

这是我找到的 cli 命令:

ffmpeg -hwaccel cuvid -r 120 -i <input_file> -c:v h264_nvenc -preset fast -b:v 8M -vf scale=960:1080 -pix_fmt yuv420p <output_file> -y

谢谢

编辑:带有选项“-c:v h264_nvenc”的命令日志

C:\Users\Shared\GolfTec\test>ffmpeg.exe -hwaccel cuvid -r 120 -i C:\\Users\\Shared\\GolfTec\\test\\raw0.h264 -c:v h264_nvenc -preset medium -b:v 8M -vf scale=960:1080 -pix_fmt yuv420p C:\\Users\\Shared\\GolfTec\\test\\video0.mp4 -y
ffmpeg version 3.4.1 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.2.0 (GCC)
  configuration: --disable-static --enable-shared --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray 
--enable-libfreetype --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy 
--enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 
--enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis 
--enable-cuda --enable-cuvid --enable-d3d11va --enable-nvenc 
--enable-dxva2 --enable-avisynth --enable-libmfx
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
[h264 @ 0000027ecb2a6700] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, h264, from 'C:\\Users\\Shared\\GolfTec\\test\\raw0.h264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (CAVLC 4:4:4), yuv444p(progressive), 960x1080, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (h264_nvenc))
Press [q] to stop, [?] for help
Output #0, mp4, to 'C:\\Users\\Shared\\GolfTec\\test\\video0.mp4':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0: Video: h264 (h264_nvenc) (Main) (avc1 / 0x31637661), yuv420p, 960x1080, q=-1--1, 8000 kb/s, 120 fps, 15360 tbn, 120 tbc
    Metadata:
      encoder         : Lavc57.107.100 h264_nvenc
    Side data:
      cpb: bitrate max/min/avg: 0/0/8000000 buffer size: 16000000 vbv_delay: -1
frame=  269 fps=261 q=25.0 Lsize=    2551kB time=00:00:02.23 bitrate=9356.5kbits/s speed=2.17x
video:2549kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.074325%

没有选项“-c:v h264_nvenc”的命令日志

C:\Users\Shared\GolfTec\test>ffmpeg.exe -hwaccel cuvid -r 120 -i C:\\Users\\Shared\\GolfTec\\test\\raw0.h264 -preset medium -b:v 8M -vf scale=960:1080 -pix_fmt yuv420p C:\\Users\\Shared\\GolfTec\\test\\video0.mp4 -y
ffmpeg version 3.4.1 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 7.2.0 (GCC)
  configuration: --disable-static --enable-shared --enable-gpl --enable-version3 --enable-sdl2 --enable-bzlib --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-cuda --enable-cuvid --enable-d3d11va --enable-nvenc --enable-dxva2 --enable-avisynth --enable-libmfx
  libavutil      55. 78.100 / 55. 78.100
  libavcodec     57.107.100 / 57.107.100
  libavformat    57. 83.100 / 57. 83.100
  libavdevice    57. 10.100 / 57. 10.100
  libavfilter     6.107.100 /  6.107.100
  libswscale      4.  8.100 /  4.  8.100
  libswresample   2.  9.100 /  2.  9.100
  libpostproc    54.  7.100 / 54.  7.100
[h264 @ 0000021e33556640] Stream #0: not enough frames to estimate rate; consider increasing probesize
Input #0, h264, from 'C:\\Users\\Shared\\GolfTec\\test\\raw0.h264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (CAVLC 4:4:4), yuv444p(progressive), 960x1080, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
Press [q] to stop, [?] for help
[libx264 @ 0000021e335589a0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 BMI2 AVX2
[libx264 @ 0000021e335589a0] profile High, level 4.2
[libx264 @ 0000021e335589a0] 264 - core 152 r2851 ba24899 - H.264/MPEG-4 AVC codec - Copyleft 2003-2017 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=abr mbtree=1 bitrate=8000 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'C:\\Users\\Shared\\GolfTec\\test\\video0.mp4':
  Metadata:
    encoder         : Lavf57.83.100
    Stream #0:0: Video: h264 (libx264) (avc1 / 0x31637661), yuv420p, 960x1080, q=-1--1, 8000 kb/s, 120 fps, 15360 tbn, 120 tbc
    Metadata:
      encoder         : Lavc57.107.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/8000000 buffer size: 0 vbv_delay: -1
frame=  269 fps= 39 q=-1.0 Lsize=    2279kB time=00:00:02.21 bitrate=8423.8kbits/s speed=0.32x
video:2277kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.119200%
[libx264 @ 0000021e335589a0] frame I:2     Avg QP:25.09  size: 16508
[libx264 @ 0000021e335589a0] frame P:193   Avg QP:23.94  size:  9094
[libx264 @ 0000021e335589a0] frame B:74    Avg QP:23.92  size:  7332
[libx264 @ 0000021e335589a0] consecutive B-frames: 62.8%  1.5%  0.0% 35.7%
[libx264 @ 0000021e335589a0] mb I  I16..4: 38.2% 54.3%  7.6%
[libx264 @ 0000021e335589a0] mb P  I16..4:  9.2% 11.8%  0.8%  P16..4: 46.5%  3.4%  3.8%  0.0%  0.0%    skip:24.5%
[libx264 @ 0000021e335589a0] mb B  I16..4:  4.3%  3.7%  0.2%  B16..8: 35.0%  1.5%  0.2%  direct: 9.5%  skip:45.5%  L0:56.6% L1:40.9% BI: 2.5%
[libx264 @ 0000021e335589a0] final ratefactor: 19.31
[libx264 @ 0000021e335589a0] 8x8 transform intra:53.1% inter:88.1%
[libx264 @ 0000021e335589a0] coded y,uvDC,uvAC intra: 22.0% 41.2% 4.5% inter: 12.8% 39.4% 0.0%
[libx264 @ 0000021e335589a0] i16 v,h,dc,p: 18% 32% 19% 31%
[libx264 @ 0000021e335589a0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 14% 17% 54%  2%  3%  3%  4%  1%  2%
[libx264 @ 0000021e335589a0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 11%  9% 63%  3%  4%  3%  3%  2%  2%
[libx264 @ 0000021e335589a0] i8c dc,h,v,p: 63% 23% 13%  1%
[libx264 @ 0000021e335589a0] Weighted P-Frames: Y:86.0% UV:64.8%
[libx264 @ 0000021e335589a0] ref P L0: 53.6%  8.3% 17.1% 11.4%  9.6%
[libx264 @ 0000021e335589a0] ref B L0: 58.9% 14.4% 26.6%
[libx264 @ 0000021e335589a0] ref B L1: 84.8% 15.2%
[libx264 @ 0000021e335589a0] kb/s:8317.64

Answer 1

好的，解码是一个更快的过程，因此通常会受到进一步处理的速率限制，主要是编码器，但也会受到过滤器的限制。通过运行查看 s/w h264 解码器的全速

ffmpeg -r 120 -i <input_file> -an -f null -

添加过滤以估计由于过滤器导致的减速。

ffmpeg -r 120 -i <input_file> -vf scale=960:1080 -pix_fmt yuv420p -an -f null -

发生这种情况是因为 ffmpeg 以顺序方式执行转码:

解复用 -> 解码 -> 过滤 -> 编码 -> 复用 -> 回到步骤 1 ...