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• webp : add a special case for a huffman table with only 1 symbol

  4 décembre 2013, par Justin Ruggles

  webp : add a special case for a huffman table with only 1 symbol
  The vlc reader cannot handle 0-bit huffman codes. For most
  
  situations WebP uses the "simple" huffman coding for this case,
  
  but that will only handle symbols up to 255. For the LZ77 distance
  
  codes, larger symbol values are needed, so it can happen in rare
  
  cases that a normal huffman table is used that only has a single
  
  symbol.
  

  • [DBH] libavcodec/webp.c

• FFmpeg seeking in Mpeg4 streams

  4 juillet 2012, par Chris Robinson

  I am currently attempting to develop a player that can perform accurate seeking based on an mpeg4 elementary video stream. I'm in the planning stage and trying to decide how to go about things and I'd like some advice before I start.
  Some things to note are :

  • I will have complete control over the encoding of the file.
  • The original content will be I-frame only
  • FFmpeg is the encoding/decoding library
  • Audio can be disregarded for now. I will only be dealing with the video stream.
  • Frame accurate seeking must be implemented

  So, when I'm encoding the content, can I query what type of frame (I, P, B) has been encoded so I can construct an additional index stream for the seeking operation. If not, I can query the GOP after it has been encoded to find the I-frame.

  As for playback, the user needs to be able to type in a specific time and go to that frame (the nearest I-frame will be suitable for now). We can assume that the GOP is closed and the length is fairly short (e.g. 15 frames). My thoughts are to query the index stream that I created during encode and determine the relevant distance into the stream for the requested time.

  I'm not sure how to seek using the FFMpeg library when playing back files.

  Has anyone done anything similar and if so, can you give a brief explanation of how you did it ?

• How to control key-frame generation of ffmpeg ?

  2 janvier 2013, par jAckOdE

  I'm making a segmenter that intervene ffmpeg's write_frame function and write output data to separate files. Each segmented file contains segment of about 3 seconds video.

  The code does following :

  1 - Get transcoded packet  
  
  2 - Check if it contains key frame data, if yes goto 3. 
  
  3 - Check the duration of current segment, if it exceed 3 seconds, goto 4 
  
  4 - Close file, and create new segment, write packet to segment file, goto-1

  General speaking, every segment contains at least 3 seconds video data, and it starts with a key frame.

  The problem is that the output video's duration are very different, some contain 3 seconds, some contain 5 or 6.

  I suspect that the problem due to how ffmpeg generate key frames during transcoding. If the "distance" between two adjacent keyframes are 6s, i got 6 seconds segment.

  Here is my questions :

  1. is that true that ffmpeg generate keyframes at irregular intervals (and interval time can be up to few second (eg. 6) ?

  2. How can we control the ffmpeg key frame generation ? (i guess there should be a ffmpeg command's argument for this, -force_key_frames maybe, but I'm not sure)