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• How to use subtitles filter to a video VFR ( variable frame rate ) video ? [migrated]

  14 février 2019, par iaaflaafc

  I have created a VFR (variable frame rate) video from a list of images by using the following ffmpeg command.

  ffmpeg -f concat -i concat.txt -vsync vfr -pix_fmt yuv420p output.mp4

  where the concat.txt is a text file which contains the list of the image locations and durations in the below format.

  file image01.png
  
  duration 3
  
  file image02.png
  
  duration 5
  
  file image02.png

  I have a subtitle file (.srt file) that I would like to add to the generated video by using the subtitles filter.

  I tried to use the subtitles filter by running the following command

  ffmpeg -i output.mp4 -codec:a copy -vf subtitles=subtitles.srt -max_muxing_queue_size 4096 outputWithSubtitles.mp4

  I find that the resultant output file does not show any subtitles.

  I find that if instead of VFR video if constant frame rate video is generated, then this issue is not observed i.e., during the generation of the first video from images if -vsync vfr option is not used, then a constant frame rate video is generated and using subtitles filter works.

  Is there a way to use the subtitles filter on a VFR video ?

• Convert RGB Video to Gray Scale video for file size reduction

  28 janvier 2020, par flamelite

  I am creating Color Video(RGB) using OpenCV in my application and generated video file needs to be uploaded to server. Color video file size is large enough to create bottleneck while uploading to server in the current bandwidth available. So, i tried to reduce the file size by converting it to grayscale video in the opencv.
  Please find below the OpenCV implementation of my current work :

  cap = cv2.VideoCapture(RGB_video_filepath)
  
      fps = cap.get(cv2.CAP_PROP_FPS)
  
      print("Input Video FPS: ".format(fps))
  
      outputfilepath = "gray_video_output.avi"
  
  
  
      mjpg_forcc = cv2.VideoWriter_fourcc('M', 'J', 'P', 'G')
  
      divx_forcc = cv2.VideoWriter_fourcc(*'DIVX')
  
      xvid_forcc = cv2.VideoWriter_fourcc(*'XVID')
  
      fmpp4_codec = cv2.VideoWriter_fourcc('F','M','P','4')
  
      mp4v_codec = cv2.VideoWriter_fourcc(*'MP4V')
  
      vid_writer = cv2.VideoWriter(outputfilepath, mjpg_codec, 2, (640, 480), 0)
  
  
  
      while cv2.waitKey(1) < 0:
  
          # get frame from the video
  
          hasFrame, frame = cap.read()
  
  
  
  
  
          # Stop the program if reached end of video
  
          if not hasFrame:
  
              print("Done processing !!!")
  
              print("Output file is stored as ", outputfilepath)
  
              break
  
  
  
          gray_frame = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
  
          vid_writer.write(gray_frame)
  
          print("Frame shape: {} {}".format(frame_count, frame.shape))
  
          cv2.imshow("Camera frame", frame)
  
          cv2.waitKey(1)
  
  
  
      print("Total frames: {}".format(frame_count))
  
      vid_writer.release()
  
      cap.release()

  Using above workflow, i created the GRAY scale video, but i found that video file sizes are almost same (RGB video file size : 25 MB, Gray scale video size : 23 MB).

  After digging into OpenCV, i found that OpenCV copies the grayscale(single channel) frame 3 times and writes into video as 3 channel although OpenCV uses FFMPEG for video file writing on Linux based OS.

  I tried to convert the same RGB video file to Grayscale video file using FFMPEG as below :

  ffmpeg -i inputvideofile -vf hue=s=0 outputvideofile

  Here, i kept the Hue and saturation channel to be empty and surprisingly RGB video file(25 MB) gets converted to gray scale with file size reduced to 6 MB.

  **I am curious to know if we can achieve the video file size reduction by converting RGB to Gray scale using OpenCV on the fly ? **

  Any help/update is appreciated.
  Thanks !!

• How to merge webm video with mp4 video with transperency ?

  30 novembre 2017, par kostya572

  I have 1080p webm video and 500x300 mp4 video. How could I place muted mp4 video on top-center position of webm video with transparency ? The output file format needed ".webm". Here what similar code I found, but it uses two mp4 videos and second video scales full width on front of first one :

  ffmpeg \
       -i in1.mp4 -i in2.mp4 \
       -filter_complex " \
           [0:v]setpts=PTS-STARTPTS, scale=480x360[top]; \
           [1:v]setpts=PTS-STARTPTS, scale=480x360, \
                format=yuva420p,colorchannelmixer=aa=0.5[bottom]; \
           [top][bottom]overlay=shortest=1" \
       -vcodec libx264 out.mp4

  Output log :