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• How to extract motion vectors from h264 without a full decode on the CPU

  25 septembre 2020, par Adrian May

  I'm trying to use my nose as a pointing device. The plan is to encode the video stream from a webcam pointed at my face as h264 or the like, get the motion vectors, cook the numbers a bit and chuck them into /dev/uinput to make the mouse pointer move about. The uinput bit was easy.

  


  This has to work with zero discernable latency. This, for instance :

  


  #!/bin/bash
[ -p pipe.mkv ] || mkfifo pipe.mkv
ffmpeg -y -rtbufsize 1M -s 640x360 -vcodec mjpeg -i /dev/video0 -c h264_nvenc pipe.mkv &
ffplay -flags2 +export_mvs -vf codecview=mv=pf+bf+bb pipe.mkv


  


  shows that the vectors are there but with a latency of several seconds which is unusable in a mouse. I know that the first ffmpeg step is working very fast by using the GPU, so either the pipe or the h264 decode in the second step is introducing the latency.

  


  I tried MV Tractus (same as mpegflow I think) in a similar pipe arrangement and it was also very slow. They do a full h264 decode on the CPU and I think that's the problem cos I can see them imposing a lot of load on one CPU. If the pipe had caused the delay by buffering badly then the CPU wouldn't have been loaded. I guess ffplay also did the decoding on the CPU and I couldn't persuade it not to, but it only wants to draw arrows which are no use to me.

  


  I think there are several approaches, and I'd like advice on which would be best, or if there's something even better I don't know about. I could :

  


  

  • Decode in hardware and get the motion vectors. So far this has failed. I tried combining ffmpeg's extract_mvs.c and hw_decode.c samples but no motion vectors turn up. vdpau is the only decoder I got working on my linux box. I have a nvidia gpu.

  


  • Do a minimal parse of the h264 to fish out the motion vectors only, ignoring all the other data. I think this would mean putting some kind of "motion only" option in libav's parser, but I'm not at all familiar with that code.

  


  • Find some other h264 parsing library that has said option and also unpacks the container.

  


  • Forget about hardware accelerated encoding and use a stripped down encoder to make only the motion vectors on either CPU or GPU. I suspect this would be slow cos I think calculating the motion vectors is the hardest part of the algorithm.

  


  


  I'm tending towards the second option but I need some help figuring out where in the libav code to do it.

  


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  6 juillet 2020, par sarahlburke58

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  Title : My Hero Academia : Heroes Rising
Release : 2019-12-20
Runtime : 104 min
Genre : Animation, Action
Stars : Daiki Yamashita, Nobuhiko Okamoto, Kenta Miyake, Ayane Sakura, Aoi Yuki, Yuki Kaji

  


  Overview : Class 1-A visits Nabu Island where they finally get to do some real hero work. The place is so peaceful that it's more like a vacation … until they're attacked by a villain with an unfathomable Quirk ! His power is eerily familiar, and it looks like Shigaraki had a hand in the plan. But with All Might retired and citizens' lives on the line, there's no time for questions. Deku and his friends are the next generation of heroes, and they're the island's only hope.

  


• Run ffmpeg on windows - real-time buffer [USB Camera] [video input] too full or near too full frame dropped

  14 juillet 2020, par Nikola Lukic

  I got successed on macos and linux with :

  


  Linux :

  


  ffmpeg \
    -f v4l2 \
    -framerate 25 -video_size 640x480 -i /dev/video0 \
    -f mpegts \
    -codec:v mpeg1video -s 640x480 -b:v 1000k -bf 0 \
    http://localhost:8081/supersecret


  


  MacOS test :

  


    ffmpeg -f avfoundation -framerate 30 -video_size 640x480 \
  -i "0" -f mpegts -codec:v mpeg2video -s 640x480 -b:v 1000k \
  -bf 0 http://localhost:8081/supersecret


  


  I already installed ffmpeg on windows a just need correct command.

  


  My error log is :

  


  


  /dev/video0: No such file or directory

  


  


  Update :

  


  After ffmpeg -list_devices true -f dshow -i dummy i got :

  


  ...
[dshow @ 000001985949ce80] DirectShow video devices (some may be both video and audio devices)
[dshow @ 000001985949ce80]  "USB Camera"
[dshow @ 000001985949ce80]     Alternative name "@device_pnp_\\?\usb#vid_0c45&pid_6340&mi_00#6&313cdc8e&0&0000#{65e8773d-8f56-11d0-a3b9-00a0c9223196}\global"
...


  


  Than i try to run my server with :

  


  ffmpeg -f dshow -i video="USB Camera"  -framerate 30 -video_size 640x480 -f mpegts -codec:v mpeg2video -s 640x480 -b:v 1000k -bf 0 http://localhost:8081/supersecret


  


  And i catch this log :

  


  Input #0, dshow, from 'video=USB Camera':
  Duration: N/A, start: 27271.559000, bitrate: N/A
    Stream #0:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 640x480, 30 fps, 30 tbr, 10000k tbn, 10000k tbc
[dshow @ 0000022a4be9f0c0] real-time buffer [USB Camera] [video input] too full or near too full (101% of size: 3041280 [rtbufsize parameter])! frame dropped!
    Last message repeated 173 times
[tcp @ 0000022a4beae340] Connection to tcp://localhost:8081 failed: Error number -138 occurred
http://localhost:8081/supersecret: Unknown error
[dshow @ 0000022a4be9f0c0] real-time buffer [USB Camera] [video input] too full or near too full (101% of size: 3041280 [rtbufsize parameter])! frame dropped!