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• ffmpeg C API (libswscale) : Scale a frame into an output frame with given width/height preserving aspect ratio, fill out the rest with transparency

  12 juin 2015, par nik4emniy

  Note : I need to use ffmpeg’s C API in my project.
  Input : video(let’s say,first frame)/image, output_width, output_height
  Output : PNG image with output_width/output_height.

  What I’ve done so far :

  a) decoded a frame from input into frame1

  b) sws_scale frame1 with needed context (output_width, output_height) into frame2

  c) initialized AVCodec CODEC_ID_PNG

  d) encoded frame2 into initialized AVPacket

  e) wrote AVPacket’s data into file

  So I have a working cycle that produces a PNG image, but it doesn’t save the aspect ratio (obviously).
  What I want to achieve is do the same thing preserving aspect ratio (which would change the frame2’s width||height), but then "centring" that image and filling out the "empty" parts with tranparent layer in the final PNG.

  Does anyone have an idea on how this can be achieved ?
  Again, I need to use C API, not command line.

• Modifying incorrect h.264 dimension in existing video file

  11 juin 2015, par RichyJ

  After searching a lot, I’m more confused than ever !! To summarise :

  I recorded a video using my HTC One M8, using 1920x1088 resolution, and it came out fine. The next day, for some reason, in the settings I changed to 1920x1080 and the next video was weird - green bar across the top, diagonal green lines throughout and odd colour stripes. The underlying image was fine, although there seem to be some ’frame jumps’ at times. Unfortunately, this second video contained a section I would like to keep, so I’m trying to fix it...

  I’ve learned a bit about AVC/H.264, but it’s pretty confusing. Essentially, I wonder whether I can just change the ’1080’ in the file info to ’1088’ and salvage the footage - there’s no audio to worry about. I read that since 1080 is not directly divisible by 16, most encoders actually do 1088 then the player discards the remaining 8 lines at playback time. I wonder whether this is the root of the problem ? I tried to get into NALs, SPS/PPS etc, but couldn’t really fathom whether this was even relevant to my problem. A hex search didn’t even find anything that looked like the NALs given as examples elsewhere :

  Finding SPS/PPS data strings

  What does this NAL header data mean ?

  Fetching dimensions of a video

  I’ve loaded both files into a Hex editor and compared as best I can (around the moov and avcC parts), but haven’t fixed it yet. One of the single byte changes I made and saved to a new ’test’ file brought up additional info in the mediainfo program, showing that the original recording was at 1088 - this hadn’t been there before, but it still played wrongly. I found info regarding the encoding of height and width (units-1 * 16) but couldn’t work out how to use this info in practice.

  I tried ffmpeg and dumping to raw video, but couldn’t make this play at all as a yuv file.

  So, my question is, will I be able to change just one byte (or a few) in the file, to make it read as 1088 to the player, or am I looking in totally the wrong direction ?!? Is this even possible ? As I say, the actual images look intact throughout, just the colours are wrong and the lines are there, so I believe it’s something to do with YCrCb problems, but at this point, I’m lost...

  I know this isn’t specifically about programming, but the above links were all from this site, so thought it might be OK to ask here. Any help would be much appreciated !!

  I’ve recreated the conditions and done 2 short clips at 1080 and 1088 for you to see the problem but as I’m new, I can’t post them here yet. They’re on my Photobucket page if you are willing to look at them (hope this isn’t breaking the rules !!). The blueish line at the bottom is the windowsill...

  1088 still

  1080 still

• What is ffmpeg, avcodec, x264 ? [closed]

  11 septembre 2020, par onmyway133

  From wiki, I read that

  



  


  FFmpeg is a free software project that produces libraries and programs
 for handling multimedia data. The most notable parts of FFmpeg are
 libavcodec, an audio/video codec library used by several other
 projects, libavformat, an audio/video container mux and demux library,
 and the ffmpeg command line program for transcoding multimedia files.

  


  



  So ffmpeg is a wrapper of avcodec ? And I often hear that people encode video with x264 using ffmpeg. So ffmpeg is also a wrapper of x264 ?

  



  How are they related ?