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• What ffmpeg command line will force the last frame to be a key frame ?

  21 décembre 2019, par ChrisJJ

  When converting .JPGs to .MP4 with the default encoder, What ffmpeg command line will force the last frame to be a key frame, regardless of the duration ?

  I’ve seen the ffmpg option for controlling key frames -force_key_frames (below) but cannot see how it can deliver this result.

  -force_key_frames[:stream_specifier] time[,time...] (output,per-stream)
  
  -force_key_frames[:stream_specifier] expr:expr (output,per-stream)
  
  Force key frames at the specified timestamps, more precisely at the first frames after each specified time.
  
  
  
  If the argument is prefixed with expr:, the string expr is interpreted like an expression and is evaluated for each frame. A key frame is forced in case the evaluation is non-zero.
  
  
  
  If one of the times is "chapters[delta]", it is expanded into the time of the beginning of all chapters in the file, shifted by delta, expressed as a time in seconds. This option can be useful to ensure that a seek point is present at a chapter mark or any other designated place in the output file.
  
  
  
  For example, to insert a key frame at 5 minutes, plus key frames 0.1 second before the beginning of every chapter:
  
  
  
  -force_key_frames 0:05:00,chapters-0.1
  
  The expression in expr can contain the following constants:
  
  
  
  n
  
  the number of current processed frame, starting from 0
  
  
  
  n_forced
  
  the number of forced frames
  
  
  
  prev_forced_n
  
  the number of the previous forced frame, it is NAN when no keyframe was forced yet
  
  
  
  prev_forced_t
  
  the time of the previous forced frame, it is NAN when no keyframe was forced yet
  
  
  
  t
  
  the time of the current processed frame
  
  
  
  For example to force a key frame every 5 seconds, you can specify:
  
  
  
  -force_key_frames expr:gte(t,n_forced*5)
  
  To force a key frame 5 seconds after the time of the last forced one, starting from second 13:
  
  
  
  -force_key_frames expr:if(isnan(prev_forced_t),gte(t,13),gte(t,prev_forced_t+5))
  
  Note that forcing too many keyframes is very harmful for the lookahead algorithms of certain encoders: using fixed-GOP options or similar would be more efficient.

• avcodec/hnm4video : allocate frame only when theres a coded frame

  23 novembre 2013, par Michael Niedermayer

  avcodec/hnm4video : allocate frame only when theres a coded frame
  Fixes memleak
  
  Fixes : asan_heap-oob_e76bdf_2224_MOTHOO.HNM
  This patch also removes the setting of palette_has_changed,
  
  which was set on a frame that was never returned
  Found-by : Mateusz "j00ru" Jurczyk and Gynvael Coldwind
  
  Signed-off-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/hnm4video.c

• FFMPEG API — How much do stream parameters change frame-to-frame ?

  8 septembre 2015, par kacey

  I’m trying to extract raw streams from devices and files using ffmpeg. I notice the crucial frame information (Video : width, height, pixel format, color space, Audio : sample format) is stored both in the AVCodecContext and in the AVFrame. This means I can access it prior to the stream playing and I can access it for every frame.

  How much do I need to account for these values changing frame-to-frame ? I found https://ffmpeg.org/doxygen/trunk/demuxing__decoding_8c_source.html#l00081 which indicates that at least width, height, and pixel format may change frame to frame.

  • Will the color space and sample format also change frame to frame ?
  • Will these changes be temporary (a single frame) or lasting (a significant block of frames) and is there any way to predict for this stream which behavior will occur ?
  • Is there a way to find the most descriptive attributes that this stream is possible of producing, such that I can scale all the lower-quality frames up, but not offer a result that is mindlessly higher-quality than the source, even if this is a device or a network stream where I cannot play all the frames in advance ?

  The fundamental question is : how do I resolve the flexibility of this API with the restriction that raw streams (my output) do not have any way of specifying a change of stream attributes mid-stream. I imagine I will need to either predict the most descriptive attributes to give the stream, or offer a new stream when the attributes change. Which choice to make depends on whether these values will change rapidly or stay relatively stable.