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• Dividing, processing and merging files with ffmpeg

  9 novembre 2015, par João Carlos Santos

  I am trying to build an application that will divide an input video file (usually mp4) into chunks so that I can apply some processing to them concurrently and then merge them back into a single file.

  To do this, I have outlined 4 steps :

  1. Forcing keyframes at specific intervals so to make sure that each
     chunk can be played on its own. For this I am using the following
     command :

     ffmpeg -i input.mp4 -force_key_frames
"expr:gte(t,n_forced*chunk_length)" keyframed.mp4

     where chunk_length is the duration of each chunk.

  2. Dividing keyframed.mp4 into multiple chunks.
     Here is where I have my problem. I am using the following command :

     `ffmpeg -i keyframed.mp4 -ss 00:00:00 -t chunk_length -vcodec copy -acodec copy test1.mp4`

     to get the first chunk from my keyframed file but it isn’t capturing
     the output correctly, since it appears to miss the first keyframe.

     On other chunks, the duration of the output is also sometimes
     slightly less than chunk_length, even though I am always using the
     same -t chunk_length option

  3. Processing each chunk For this task, I am using the following
     commands :

     ffmpeg -y -i INPUT_FILE -threads 1 -pass 1 -s 1280x720 -preset
medium -vprofile baseline -c:v libx264 -level 3.0 -vf
"format=yuv420p" -b:v 2000k -maxrate:v 2688k -bufsize:v 2688k -r 25
-g 25 -keyint_min 50 -x264opts "keyint=50:min-keyint=50:no-scenecut" -an -f mp4 -movflags faststart /dev/null
     ffmpeg -y -i INPUT_FILE -threads 1 -pass 2 -s 1280x720 -preset
medium -vprofile baseline -c:v libx264 -level 3.0 -vf
"format=yuv420p" -b:v 2000k -maxrate:v 2688k -bufsize:v 2688k -r 25
-g 25 -keyint_min 50 -x264opts "keyint=50:min-keyint=50:no-scenecut" -acodec libfaac -ac 2 -ar 48000 -ab 128k -f mp4 -movflags faststart OUTPUT_FILE.mp4

     This commands are not allowed to be modified, since my goal here is
     to parallelize this process.

  4. Finally, to merge the files I am using concat and a list of the
     outputs of the 2nd step, as follows :

     ffmpeg -f concat -i mylist.txt -c copy final.mp4

  In conclusion, I am trying to find out a way to solve the problem with step 2 and also get some opinions if there is a better way to do this.

• FFmpeg .avi to .mp4 video transcoding - Android

  17 avril 2021, par Tiago Ornelas

  Using OpenCV 4.5.2 + FFMPEG on an android app

  


  I'm trying to convert an .avi video file into a .mp4 file using x264, by running

  


  ffmpeg -i input.avi -c:v libx264 output.mp4


  


  The transcoding is processed correctly but, when I play the video, the colors are a bit... saturated ?

  


  This transcoding is part of the following flow :

  


  

  1. Grab a .mov video file

  


  2. Use OpenCV VideoCapture and VideoWriter to write text on the video frames (output is .avi)

  


  3. Then I need to convert .avi file into .mp4 so it's reproducible on exoplayer.

  


  


  In step 2. I'm looping all video frames and writing them to a new file, writing a text on them.

  


  val videoWriter = VideoWriter(
            outputFilePath,
            VideoWriter.fourcc('M', 'J', 'P', 'G'),
            15.0,
            Size(1920.0, 1088.0),
            true
        )

 val frame = Mat()
 videoCapture.read(frame)
 Imgproc.putText(
    frame,
    "This is a text",
    Point(200.0, 200.0),
    3,
    5.0,
    Scalar(
        255.0,
        124.0,
        124.0,
        255.0
    ), 
    1
)
videoWriter.write(frame)



  


  I know that step 2. is probably not corrupting the frames because in my sample app, I'm displaying all frames in an ImageView, and they all match the original .mov video. So, my guess is that the issue is occurring on 3.

  


  I'm using 'com.arthenica:mobile-ffmpeg-min-gpl:4.4' for android to execute the FFMPEG command as follows :

  


  FFmpeg.executeAsync("-i $outputFilePath -c:v libx264 -y ${mp4File.path}")


  


  where outputFilePath is the path for the .avi file and mp4File is an existing empty .mp4 file.

  


  So I guess what I'm looking for is a way to have a lossless video color transcoding between .avi and .mp4 files.

  


  Here's a screenshot of my sample app. The image on top is the last frame of the .avi video. The image on the bottom is the last frame played on a video player for the .mp4 transcoded video. This frame color difference is noticeable throughout the whole video.

  


  EDIT : After some digging, I found out that the issue is that the VideoWritter is messing with the RGB colors. I still don't know the reason why this is happeninng.

  


  

  


• How to create app like snapchat in react-native ?

  6 octobre 2016, par Takamichi Tsutsumi

  I’m trying to create an app like snapchat with React Native.

  currently

  1. use react-native-camera to take video
  2. upload it to webserver with text info
  3. attach the text to video with ffmpeg on webserver
  4. stream that video using react-native-video

  I want do step 2 and 3 in native app.
  Does anyone have nice solution ?