Recherche avancée

Sur d’autres sites (6859)

• How does mp4 block matching work

  19 juin 2019, par YAHsaves

  I’ve been working on a video encoder that uses block matching to find similar blocks on previous frames.

  For the sake of simplicity I’ll leave out most of the details, but I’m wondering if I got the block matching algorithm right.

  In order to find a block on a previous frame my encoder uses the mean squared algorithm for the Y channel in YUV color space.

  This works by comparing each pixel of the block we want to match, with the block on the previous frame. It takes the difference of each pixel and squares it.

  After all the pixels are compared the block that has the least average difference is chosen as the desired block.

  Now this is where I need help. My encoder looks at every possible block in a 256x256 area and uses half pixel searches as well. The smallest block size it can use is 4x4.

  From what I’ve read online this is the same things mp4 uses.

  However I can’t find nearly as many blocks as mp4 appears to be able to find.

  For example here are 2 frames I want to compress. The first will be the I frame and the second is the P frame :

  

  Now after my encoder has run it is able to reduce the second frame by 80% and, what it can’t match close enough it saves as "difference" blocks. Which are grey blocks only recording the difference. They look like this :

  

  Now what I don’t get is to save these "difference" blocks as a jpg takes roughly 90kb to be accurate enough.

  Multiply that by 24 (24 frames per second) you get 2070kb per second. That’s not including how much space the actual motion vectors take up or anything else.

  However somehow mp4 is able to compress the video of the images above into a mere 700kb per second and still look better than my encoder at much larger data amounts.

  Why is this ? Is there something I’m doing wrong when looking for blocks ? Any help would be much appreciated.

• Revision 53ff43adc3 : Prunes out full-rd computation based on modeled rd Adds a speed feature to elim

  9 juillet 2013, par Deb Mukherjee

  Changed Paths :
   Modify /vp9/common/vp9_blockd.h

  
   Modify /vp9/encoder/vp9_onyx_if.c

  
   Modify /vp9/encoder/vp9_onyx_int.h

  
   Modify /vp9/encoder/vp9_rdopt.c

  
  
  Prunes out full-rd computation based on modeled rd

  Adds a speed feature to eliminate full-rd computation if the modeled
  rd or rd based on a different parameter in the same mode is already
  a lot larger than the best rd yet.

  Specifically, only search the sharp and smooth filters if the modeled
  rd cost based on the regular filter is within a certain factor of the
  best rd cost so far. Also, skip full-rd computation of non splitmv
  inter modes if the modeled rd cost based on pred error is within the
  same factor of the best rd cost so far.

  Also adds some enhancements in the rd search for splitmv mode to
  speed things up by early breakouts. Negligible impact on performance.

  Resuts on derfraw300 :
  psnr : -0.013% with the splitmv enhancements, -0.24% with the rd
  breakout feature on.
  speedup : 6% with splitmv enhancements, 20% with also residual breakout
  (tested on football sequence at 600 Kbps)

  Change-Id : I37abc308ea9f110c1679ce649b6a7e73ab1ad5fc

• How to replace the text in the filename and move the file to different directory

  2 octobre 2019, par Hugo Shih

  I’m a newbie to this bash scripting and I’m writing a script with ffmpeg to help my production more efficient. I think I put two questions at once and hope it won’t be confused :

  Here is my script :

  #!/bin/bash
  
  
  
  for file in *.mov
  
  do
  
    ffmpeg  -probesize 50M -analyzeduration 100M -ss 00:00:10.00 -i "$file" -map 0:0 -map 0:1 - 
  
    c:a aac_at -ab 256k -ar 48000 -ac 2 -strict -2 -async 1 -c:v libx264 -crf 20 -r 24000/1001 
  
    -s 1920x1080 -aspect 16:9 -pix_fmt yuv420p -preset fast -partitions 
  
    partb8x8+partp4x4+partp8x8+parti8x8 -b-pyramid 1 -weightb 1 -8x8dct 1 -fast-pskip 1 - 
  
    direct-pred 1 -coder ac -trellis 1 -motion-est hex -subq 6 -me_range 16 -bf 3 -b_strategy 1 
  
    -sc_threshold 40 -keyint_min 24 -g 48 -qmin 3 -qmax 51 -qdiff 4 -metadata creation_time=now 
  
    -sn -t 00:01:00.02 -y "${file%.*}_H264_1080".mov
  
  done

  Basically what I’m tyring to do is taking a "Movie_ProRes_1080.mov" file and make it as a H264 and rename it as "Movie_H264_1080.mov". The way I work around is Export the file as "Movie" and let it added behind which is really not what I want because I need to export another file to fit this purpose. The main goal is I can use the "Movie_ProRes_1080.mov" convert it to "Movie_H264_1080".

  Also, from the script, it rendered out to the same location. Would it be possible to render to a different directory ? Like source at /Users/editor/source, but render out at /Users/editor/output.