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• Stretch Subtitles Timing by Factor

  3 août 2022, par Matt McManis

  I'm muxing an mkv video, but the Subtitle .ssa file is out of sync by around 1 second by the end of the video.

  



  Delaying the subtitles by 1 second in the beginning is not enough, the timing needs to be stretched to fix an exponentially growing delay. Sync starts out fine in the beginning, but a few milliseconds delay multiplies for each minute that passes.

  



  I fixed them using MKVToolNix with Stretch 0.999.

  



  ---

  



  How can I stretch Subtitles using FFmpeg ?

  



  I saw here how to stretch audio by a factor the same way. ffmpeg, stretch audio to x seconds

  



  I'm using something like this :

  



  ffmpeg -y 

-fflags +genpts 

-i "video.m4v"  
-i "audio.ac3" 
-i "subs.ssa" 

-c:v copy 
-c:a copy 
-c:s copy 

-map 0:v? 
-map 1:a? 
-map 2:s?

"video.mkv"


  



  I used -itsoffset -00:00:01.000 to remove a gap in the beginning, but it does not stretch.

  


• dvbsubdec : Fixed segfault when decoding subtitles

  21 octobre 2016, par Lorenz Brun

  dvbsubdec : Fixed segfault when decoding subtitles
  This fixes a segfault (originally found in Movian, but traced to libav)
  
  when decoding subtitles because only an array of rects is allocated,
  
  but not the actual structs it contains. The issue was probably
  
  introduced in commit 2383323 where the loop to allocate the rects in
  
  the array was thrown away.
  Signed-off-by : Vittorio Giovara <vittorio.giovara@gmail.com>
  

  • [DBH] libavcodec/dvbsubdec.c

• FFMPEG with NVENC and DVB Subtitles and Overlay option

  3 novembre 2019, par Sambir

  Hi I have some questions regarding NVENC, DVB subs and overlay feature.

  I managed to burn dvb subtitles to the input video but I noticed that this cannot be performed fully on the GPU because I am using the Overlay feature of FFMPEG which is only possible with software.

  Current code Scales the subtitles to 1920x1080 before applying the overlay. I noticed that this uses more CPU than just overlaying the subtitles directly over the video. The problem is that I want to overlay the subtitles in the center bottom. Current code centers the subtitles in the center of the screen (kinda annoying)

  Eventually I would like a full hw transcode option (guess it’s not possible ? is it)

  I would like the subtitles unscaled in the center bottom (cpu load 50% less)
  I would like the subtitles scaled 50% in the center bottom (cpu load 30% less) this makes the subtitles better readable.

  Current code for subtitles scaled to video size (how to full hw transcode ?) :

  Code subtitle unscaled but centered. How to change to center bottom :

  -hwaccel_device 4 -hwaccel cuvid -vcodec h264_cuvid -resize 1920x1080 -deint 2 -drop_second_field 1 -i 'http://stream' -filter_complex "[i:0x1389]hwdownload,format=nv12[base];[base][i:0x13ee]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2[v];[v]hwupload_cuda[k]" -map "[k]" -map i:0x1393 -acodec libfdk_aac -b:a 96k -c:v h264_nvenc -gpu 4 -preset llhq -vprofile high -rc:v vbr -qmin:v 26 -qmax:v 32 -b:v 4M -maxrate 4M -bufsize 8M -threads 0 -r 25 -g 100 -f flv 'rtmp://127.0.0.1:8001/input/nvenctest.stream'

  ffprobe output :

  [mpegts @ 0x3786200] sub2video: using 1920x1080 canvas
  
  [Parsed_showinfo_0 @ 0x3785340] config in time_base: 1/90000, frame_rate: 0/1
  
  [Parsed_showinfo_0 @ 0x3785340] config out time_base: 0/0, frame_rate: 0/0
  
  No information about the input framerate is available. Falling back to a default value of 25fps for output stream #0:0. Use the -r option if you want a different framerate.
  
  Output #0, null, to 'pipe:':
  
    Metadata:
  
      encoder         : Lavf58.3.100
  
      Stream #0:0: Video: wrapped_avframe, bgra, 1920x1080, q=2-31, 200 kb/s, 25 fps, 25 tbn, 25 tbc
  
      Metadata:
  
        encoder         : Lavc58.9.100 wrapped_avframe
  
  [Parsed_showinfo_0 @ 0x3785340] n:   0 pts:      0 pts_time:0       pos:       -1 fmt:bgra sar:0/1 s:1920x1080 i:P iskey:1 type:? checksum:00000000 plane_checksum:[00000000] mean:[0] stdev:[0.0]
  
  [Parsed_showinfo_0 @ 0x3785340] n:   1 pts:      0 pts_time:0       pos:       -1 fmt:bgra sar:0/1 s:1920x1080 i:P iskey:1 type:? checksum:00000000 plane_checksum:[00000000] mean:[0] stdev:[0.0]
  
  [Parsed_showinfo_0 @ 0x3785340] n:   2 pts:  39599 pts_time:0.439989 pos:       -1 fmt:bgra sar:0/1 s:1920x1080 i:P iskey:1 type:? checksum:00000000 plane_checksum:[00000000] mean:[0] stdev:[0.0]
  
  [graph 0 input from stream 0:2 @ 0x37c9340] Changing frame properties on the fly is not supported by all filters.
  
  Segmentation fault (core dumped)