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

  30 octobre 2023, 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....

  


• FFMPEG : How do I maintain the aspect ratio of various images used in a slideshow ?

  7 mars 2019, par Kimberly W

  I’m just starting out with FFMPEG and trying to use it to make a slideshow. Ideally, I’d like to get it where I can input an arbitrary number of images, and each image is shown for (example) 2 seconds and them moves on to the next. Each image also maintains it’s original aspect ratio and is not stretched in any way (they can of course be scaled up/down to fit the resolution of the output video).

  I started off with a basic command like the following.

  ffmpeg -r 1/9 -pattern_type glob -i "*.jpg" -c:v libx264 -y -pix_fmt yuv420p -vf scale="720:trunc(ow/a/2)*2" out.mp4

  In this example, the first image is a wide (landscape) image and some of the others are tall (portrait) images. The portrait ones get squished in output video. Also the images aren’t displayed for equal amount of times.

  A couple of problems I’ve been running into :

  1. There’s an error for width not divisible by 2, because the images can be literally any random width. To resolve that, I’ve been trying various -vf options I’ve found through googling (like the one above). They take care of the error, but don’t solve my aspect ratio issue.
  2. All the images seem to be stretch or squished to fit the dimensions of the first input image. In reality, the images are of various different dimensions (like frames in a comic book). There’s no pattern to them.

  Is there a ffmpeg command for just taking images and creating as slideshow, while preserving their original aspect ratios ?

• Video modifications using ffmpeg and windows batch command

  1er mars 2019, par macenike

  I have a series of 8 videos which have different number of frames, some of them lose a couple of them at the beginning. I was trying to concatenate at the beginning of the videos with less frame a copy of their first frame. Unfortunately my code either crash while comparing the length of the videos or fail to accomplish the task required. Here I add the code that I’m using.

  @ECHO off
  
  SETLOCAL EnableDelayedExpansion
  
  MKDIR COMPRESSED\
  
  MKDIR RAW\
  
  SET  j=0
  
  SET  jj=1
  
  SET max=1
  
  FOR %%a IN (*.avi) DO (
  
      SET /A j=j+1
  
      FOR /F "delims=" %%V IN ('ffprobe.exe -v error -count_frames -select_streams v:0 -show_entries stream^=nb_read_frames -of       default^=noprint_wrappers^=1:nokey^=1 ".\%%~nxa" 2^>^&1') DO SET elem[!j!]=%%V
  
  SET elem[!j!]
  
  )
  
  
  
  FOR %%a IN (*.avi) DO (
  
      IF !elem[%jj%]! GTR !elem[%max%]!(
  
          SET max=!jj!
  
      )
  
  SET /A jj=jj+1  
  
  ) 
  
  SET max
  
  
  
  SET /A x=1
  
  FOR %%a IN (*.avi) DO ( 
  
  
  
      IF "!elem[!x!]!" LSS "!elem[!max!]!" (
  
              ffmpeg -i ".\%%~nxa" -vf "select=eq(n\,0)" -q:v 3 output_image.jpg
  
              ffmpeg -y -i ".\%%~nxa" -loop 1 -i output_image.jpg -t 0.01 -s 1936x1216 ".\%%~nxa"
  
              SET elem[!x!]=elem[!x!]+1
  
              SET elem[!x!]                   
  
      )
  
  SET /A x=x+1    
  
  ) 
  
  
  
  
  
  FOR %%a IN (*.avi) DO (
  
      ffprobe -v error -count_frames -select_streams v:0 -show_entries stream=nb_read_frames ".\%%~nxa"
  
      ffmpeg -i ".\%%~nxa" -c:v libxvid -vtag xvid -qscale:v 2 -force_key_frames 1 -s 968x400 -r 100 ".\COMPRESSED\%%a"
  
      SET mystr=%%~na
  
      SET mystr=!mystr:~-1!
  
      IF !mystr! == 3 (
  
          ffmpeg -i ".\%%~nxa" -c:v mpeg4 -b:v 4M -c:a libfdk_aac -b:a 192k ".\COMPRESSED\%%~na.mp4"
  
      )
  
      IF !mystr! == 5 (
  
          ffmpeg -i ".\%%~nxa" -c:v mpeg4 -b:v 4M -c:a libfdk_aac -b:a 192k ".\COMPRESSED\%%~na.mp4"
  
      )
  
  )