Recherche avancée

Sur d’autres sites (12874)

• How to extract time-accurate video segments with ffmpeg ?

  25 mai 2016, par Jim Miller

  This is not a particularly new question area around here, but I’ve tried what’s been suggested there without much luck. So, my story :

  I’ve got a hunk of 15 seconds of straight-from-the-camera.mov video out of which I want to extract a specific chunk, which I can identify by start time and stop time, in seconds. I started by trying to do what I’ll call a "copy extraction" : to get seconds 9 to 12,

  ffmpeg -i test.mov -vcodec copy -acodec copy -ss 9 -to 12 test-copy.mov

  This was a not-bad start, but there are some black frames at the beginning and end of the clip, which I can’t have — it has to be a clean edit from the original. So, I tried recoding the original into a new, trimmed clip :

  ffmpeg -i test.mov -ss 00:00:09 -t 00:00:03 test-out.mov

  This is better, but not quite : There are no longer any black frames at the beginning of the clip, but they’re still there at the end.

  After some more browsing and reading, I then suspected that the problem is that ffmpeg is having trouble finding the proper points because of a lack of keyframes in the original video. So I recoded the original video to (presumably) add keyframes, in a couple of different ways. Since I want to be able to pick video at boundaries of a second ("from 9 seconds to 12 seconds"), I tried, copying various suggestions around the web,

  ffmpeg -i test.mov -force_key_frames "expr:gte(t, n_forced)" test-forced.mp4

  and

  ffmpeg -i test.mov -g 1 test-g-inserted.mp4

  (I built these as mp4’s based on some comments about an mp4 container being needed to support the keyframe search, but I’m honestly just hacking here.) I then tried the extraction as before, but on these new videos that presumably now have keyframes in them. No luck — both seem to be about the same ; the start is OK but there are still black frames at the end. (FWIW, both test-forced.mp4 and test-g-inserted.mp4 also have trailing black frames.)

  So : I’m still stuck, and would like to not be. Any insights out there as to what I’m doing wrong ? I feel like I’m close, but I really need to get rid of those trailing black frames....

• RGB-frame encoding - FFmpeg/libav

  4 février 2014, par learner

  I am learning video encoding & decoding in FFmpeg. I tried the code sample on this page (only the video encoding & decoding part). Here the dummy image being created is in YCbCr format. How do I achieve similar encoding by creating RGB frames ? I am stuck at :

  Firstly, how to create this RGB dummy frame ?

  Secondly, how to encode it ? Which codec to use ? Most of them work with YUV420p only...

  EDIT : I have a YCbCr encoder and decoder as given on the this page. The thing is, I have RGB frame-sequence in my database and I need to encode it. But the encoder is for YCbCr. So, I am wondering to convert RGB frames to YCbCr (or YUV420P) somehow and then encode them.
  At decoding end, I get decoded YCbCr frames and I convert them back to RGB. How to go ahead with it ?

  I did try the swscontext thing, but the converted frames lose color information and also scaling errors. I thought of doing it manually using two for loops and colorspace conversion formulae
  but I am not able to access individual pixel of a frame using FFmpeg/libav library ! Like in OpenCV we can easily access it with something like : Mat img(x,y) but no such thing here ! I am totally a newcomer to this area...

  Someone can help me ?

  Many Thanks !

• Revision 360c80cfb9 : Use fast HEX search in real time mode In good quality mode motion search, the b

  15 février 2014, par Yunqing Wang

  Changed Paths :
   Modify /vp9/encoder/vp9_mcomp.c

  
   Modify /vp9/encoder/vp9_mcomp.h

  
   Modify /vp9/encoder/vp9_onyx_if.c

  
   Modify /vp9/encoder/vp9_onyx_int.h

  
   Modify /vp9/encoder/vp9_rdopt.c

  
  
  Use fast HEX search in real time mode

  In good quality mode motion search, the best matches are normally
  found after searching in a large area. In real time mode, to make
  encoding fast, a center-biased fast HEX search is used, which
  converges quickly most of the time. A 4-point diamond search is
  also carried out as the following refining search, which gives more
  precise results, and maintains good motion search quality.

  At speed 5, the borg test on rtc set showed an overall PSNR loss of
  0.936%. The encoding speed gain is 4% - 5%.

  Change-Id : I42cd68bb56a09ca1b86293c99d5f7312225ca7ae