Recherche avancée

Sur d’autres sites (8655)

• Cut a video in between key frames without re-encoding the full video using ffpmeg ? [closed]

  13 juillet 2024, par bguiz

  I would like to cut a video at the beginning at any particular timestamp, and it need to be precise, so the nearest key frame is not good enough.

  


  Also, these videos are rather long - an hour or longer - so I would like to avoid re-encoding this altogether if possible, or otherwise only re-encode a minimal fraction of the total duration. Thus, would like to maximise the use of -vcodec copy.

  


  How can I accomplish this using ffmpeg ?

  


  NOTE : See scenario, and my own rough idea for a possible solution below.

  


  ---

  


  Scenario :

  


  

  • Original video

    

    • Length of 1:00:00

    


    • Has a key frame every 10s

    


    


  


  • Desired cut :

    

    • From 0:01:35 through till the end

    


    


  


  • Attempt #1 :

    

    • Using -ss 0:01:35 -i blah.mp4 -vcodec copy, what results is a file where :

    


    • audio starts at 0:01:30

    


    • video also starts at 0:01:30

    


    • this starts both the audio and the video too early

    


    


  


  • using -i blah.mp4 -ss 0:01:35 -vcodec copy, what results is a file where :

    

    • audio starts at 0:01:35,

    


    • but the video is blank/ black for the first 5 seconds,

      

      • until 0:01:40, when the video starts

      


      


    


    • this starts the audio on time,
but the video starts too late

    


    


  


  


  ---

  


  Rough idea

  


  

  • (1) cut 0:01:30 to 0:01:40

    

    • re-encode this to have new key frames,
including one at the target time of 0:01:35

    


    • then cut this to get the 5 seconds from 0:01:35 through 0:01:40

    


    


  


  • (2) cut 0:01:40 through till the end

    

    • without re-encoding, using -vcodec copy

    


    


  


  • (3) ffmpeg concat the first short clip (the 5 second one)
with the second long clip

  


  


  I know/ can work out the commands for (2) and (3), but am unsure about what commands are needed for (1).

  


• Speedup matplotlib animation to video file

  9 juillet 2015, par gaggio

  On Raspbian (Raspberry Pi 2), the following minimal example stripped from my script correctly produces an mp4 file :

  import numpy as np
  
  import matplotlib.pyplot as plt
  
  from matplotlib import animation
  
  
  
  def anim_lift(x, y):
  
  
  
      #set up the figure
  
      fig = plt.figure(figsize=(15, 9))
  
  
  
      def animate(i):
  
          # update plot
  
          pointplot.set_data(x[i], y[i])
  
  
  
          return  pointplot
  
  
  
      # First frame
  
      ax0 = plt.plot(x,y)
  
      pointplot, = ax0.plot(x[0], y[0], 'or')
  
  
  
      anim = animation.FuncAnimation(fig, animate, repeat = False,
  
                                     frames=range(1,len(x)), 
  
                                     interval=200,
  
                                     blit=True, repeat_delay=1000)
  
  
  
      anim.save('out.mp4')
  
      plt.close(fig)
  
  
  
  # Number of frames
  
  nframes = 200
  
  
  
  # Generate data
  
  x = np.linspace(0, 100, num=nframes)
  
  y = np.random.random_sample(np.size(x))
  
  
  
  anim_lift(x, y)

  Now, the file is produced with good quality and pretty small file size, but it takes 15 minutes to produce a 170 frames movie, which is not acceptable for my application. i’m looking for a significant speedup, video file size increase is not a problem.

  I believe the bottleneck in the video production is in the temporary saving of the frames in png format. During processing I can see the png files apprearing in my working directory, with the CPU load at 25% only.

  Please suggest a solution, that might also be based on a different package rather than simply matplotlib.animation, like OpenCV (which is anyway already imported in my project) or moviepy.

  Versions in use :

  • python 2.7.3
  • matplotlib 1.1.1rc2
  • ffmpeg 0.8.17-6:0.8.17-1+rpi1

• Speedup matplotlib animation to video file

  20 mai 2022, par gaggio

  On Raspbian (Raspberry Pi 2), the following minimal example stripped from my script correctly produces an mp4 file :

  



  import numpy as np
import matplotlib.pyplot as plt
from matplotlib import animation

def anim_lift(x, y):

    #set up the figure
    fig = plt.figure(figsize=(15, 9))

    def animate(i):
        # update plot
        pointplot.set_data(x[i], y[i])

        return  pointplot

    # First frame
    ax0 = plt.plot(x,y)
    pointplot, = ax0.plot(x[0], y[0], 'or')

    anim = animation.FuncAnimation(fig, animate, repeat = False,
                                   frames=range(1,len(x)), 
                                   interval=200,
                                   blit=True, repeat_delay=1000)

    anim.save('out.mp4')
    plt.close(fig)

# Number of frames
nframes = 200

# Generate data
x = np.linspace(0, 100, num=nframes)
y = np.random.random_sample(np.size(x))

anim_lift(x, y)


  



  Now, the file is produced with good quality and pretty small file size, but it takes 15 minutes to produce a 170 frames movie, which is not acceptable for my application. i'm looking for a significant speedup, video file size increase is not a problem.

  



  I believe the bottleneck in the video production is in the temporary saving of the frames in png format. During processing I can see the png files apprearing in my working directory, with the CPU load at 25% only.

  



  Please suggest a solution, that might also be based on a different package rather than simply matplotlib.animation, like OpenCV (which is anyway already imported in my project) or moviepy.

  



  Versions in use :

  



  

  • python 2.7.3

  


  • matplotlib 1.1.1rc2

  


  • ffmpeg 0.8.17-6:0.8.17-1+rpi1