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• ffmpeg : Generate empty audio and video (working for video)

  17 septembre 2021, par David Ferris

  I'm trying to generate a black video with FFMPEG. I have accomplished this with the following :

  


  ffmpeg -t 5 -f lavfi -i color=c=black:s=1920x1080 -c:v libx264 -tune stillimage -pix_fmt yuv420p out.mp4


  


  Unfortunately this video doesn't have any audio tracks. Following this, I have tried to insert -i anullsrc=channel_layout=stereo:sample_rate=44100 :

  


  ffmpeg -t 5 -i anullsrc=channel_layout=stereo:sample_rate=44100 -f lavfi -i color=c=black:s=1920x1080 -c:v libx264 -tune stillimage -pix_fmt yuv420p out.mp4


  


  Unfortunately this gives the error :

  


  


  anullsrc=channel_layout=stereo:sample_rate=44100 : No such file or
directory

  


  


  How can I modify my initial script to generate a video with empty audio ?

  


• imagemagick gradient mask file creation

  6 avril 2016, par lang2

  I’m playing with this creative script here : http://www.fmwconcepts.com/imagemagick/transitions/. The plan is to mimic what happens with the script with ffmpeg and generate video with transition effects between pictures. My current understanding is this :

  1. I have two pictures A and B.
  2. I need in between a couple of pictures (say 15) that are partially A and partially B.
  3. To do that I use the composite -compose src-over A.jpg B.jpg mask-n.jpg out.jpg command.
  4. During the process, the mask-n.jpg gets generated automatically that gradually change from all black to all white.
  5. Depends on the mathematically equations, the way the transition effect looks is different.

  In one of the example, Fred the author gave this :

  convert -size 128x128 gradient: maskfile.jpg

  This will generate a image like this :
  

  This is partially black and partially white. For the transition to work, I’ll need an all white one and an all black one and a couple of others in between. What’s the magical command to do that ?

• avutil/aes : Don't use misaligned pointers

  21 octobre 2022, par Andreas Rheinhardt

  avutil/aes : Don't use misaligned pointers
  The AES code uses av_aes_block, a union consisting of
  
  uint64_t[2], uint32_t[4], uint8_t[4][4] and uint8_t[16].
  
  subshift() performs byte-wise manipulations of two av_aes_blocks,
  
  but when encrypting, it does so with a shift of two bytes ;
  
  more precisely, it uses
  
  "av_aes_block *s1 = (av_aes_block *) (s0[0].u8 - s)"
  
  and lateron uses the uint8_t[16] member to access s0.
  
  Yet av_aes_block requires to be suitably aligned for
  
  the uint64_t[2] member, which s0[0].u8 - 2 is certainly
  
  not. This is in violation of 6.3.2.3 (7) of C11. UBSan
  
  reports this in the aes_ctr, mov-3elist-encrypted,
  
  mov-frag-encrypted, mov-tenc-only-encrypted and srtp
  
  tests.
  
  Furthermore, there is another issue here : The pointer points
  
  outside of s0 ; this works, because all the accesses lateron
  
  use an index >= 3. (Clang-)UBSan reports this as
  
  "runtime error : index -2 out of bounds for type 'uint8_t[16]'".
  This commit fixes both of these issues : The latter issue
  
  is fixed by applying an offset of "+ 3" during the cast
  
  and subtracting this from the indices used lateron.
  
  The former issue is solved by not casting to av_aes_block*
  
  at all ; instead simply cast to unsigned char*.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavutil/aes.c