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• What Every Programmer Should Know

  24 décembre 2012, par Multimedia Mike — General

  During my recent effort to force myself to understand Unicode and modern text encoding/processing, I was reminded that this is something that “every programmer should just know”, an idea that comes up every so often, usually in relation to a subject in which the speaker is already an expert. One of the most absurd examples I ever witnessed was a blog post along the lines of “What every working programmer ought to know about [some very specific niche of enterprise-level Java programming]“. I remember reading through the article and recognizing that I had almost no knowledge of the material. Disturbing, since I am demonstrably a “working programmer”.

  For fun, I queried the googles on the matter of what ever programmer ought to know.

  Specific Topics
  Here is what every programmer should know about : Unicode, time, memory (simple), memory (extremely in-depth), regular expressions, search engine optimization, floating point, security, basic number theory, race conditions, managed C++, VIM commands, distributed systems, object-oriented design, latency numbers, rate monotonic algorithm, merging branches in Mercurial, classes of algorithms, and human names.

  Broader Topics
  20 subjects every programmer should know, 97 things every programmer should know, 12 things every programmer should know, things every programmer should know (27 items), 10 papers every programmer should read at least twice, 10 things every programmer should know for their first job.

  Meanwhile, I remain fond of this xkcd comic whose mouseover text describes all that a person genuinely needs to know. Still, the new year is upon us, a time when people often make commitments to bettering themselves, and it couldn’t hurt (much) to at least skim some of the lists and find out what you never knew that you never knew.

  What About Multimedia ?
  Reading the foregoing (or the titles of the foregoing pieces), I naturally wonder if I should write something about what every programmer should know about multimedia. I think it would look something like a multimedia programming FAQ. These are some items that I can think of :

  1. YUV : The other colorspace (since most programmers are only familiar with RGB and have no idea what to make of the YUV that comes out of most video decoding APIs)
  2. Why you can’t easily seek randomly to any specific frame in a video file (keyframe/interframe discussion and their implications)
  3. Understand your platform before endeavoring to implement multimedia software (modern platforms, particularly mobile platforms, probably provide everything you need in the native APIs and there is likely little reason to compile libavcodec for the platform)
  4. Difference between containers and codecs (longstanding item, but I would argue it’s less relevant these days due to standardization on the MPEG — MP4/H.264/AAC — stack)
  5. What counts as a multimedia standard in this day and age (comparing the foregoing MPEG stack with the WebM/VP8/Vorbis stack)
  6. Trade-offs to consider when engineering a multimedia solution
  7. Optimization doesn’t always work the way you think it does (not everything touted as a massive speed-up in the world of computing — whether it be multithreaded CPUs, GPGPUs, new SIMD instruction sets — will necessarily be applicable to multimedia processing)
  8. A practical guide to legal issues would not be amiss
  9.  ???

  What other items count as “something multimedia-related that every programmer should know” ?

• Call ffmpeg in c++ with system() function fails

  2 septembre 2014, par zhen lee

  I write a c++ program which needs to convert some(say:10) mp4 videos to flv videos.
  I use ffmpeg in my program for each video like this :

  system("ffmpeg -i video -filter:v yadif -ar 44100 -sameq -y -f flv temp.flv")

  however,it turns out :only first video will be converted successfully,the others will fail.
  it means :
  when i change the input order of which video to convert and re-execution the program,it behave the same:only the first video(will be different each time as i changed the input video order) will be converted successfully.

  The error message like :

  [h264 @ 0xaee0740] concealing 45 DC, 45 AC, 45 MV errors
  [h264 @ 0xaee0ce0] AVC : nal size 305665
  Last message repeated 1 times
  [h264 @ 0xaee0ce0] no frame !
  [h264 @ 0xaee1280] AVC : nal size 572993
  Last message repeated 1 times
  [h264 @ 0xaee1280] no frame !
  [aac @ 0xad9ccc0] channel element 0.13 is not allocated
  Error while decoding stream #0:1
  [aac @ 0xad9ccc0] channel element 0.13 is not allocated
  Error while decoding stream #0:1
  [aac @ 0xad9ccc0] channel element 0.13 is not allocated
  Error while decoding stream #0:1
  ......
  The most strange thing is :when i run ffmpeg command in bash shell,all video will be converted successfully .

  After google it,I have try these(certainly failed) :

  1. remove -sameq option,the result is same ;

  2. write ffmpeg commod in a shell script ConvertToFlv.sh like :

     /usr/local/bin/ffmpeg -i "$dir/$1" -filter:v yadif -ar 44100 -sameq -y -f flv "$dir/temp.fiv"

     then call this script in program like

     system("ConvertToFlv.sh"+video)

     or

     system("sh ConvertToFlv.sh"+video)

     The result is same.

  The ffmpeg configure is :

    ffmpeg version 0.9.1.git Copyright (c) 2000-2012 the FFmpeg developers
  
    built on Dec 17 2012 16:17:30 with gcc 4.1.2 20080704 (Red Hat 4.1.2-48)
  
    configuration: --enable-gpl --enable-postproc --enable-nonfree --enable-postproc --enable-swscale --enable-avfilter --enable-pthreads --enable-libxvid --enable-libx264 --enable-libmp3lame --enable-libfaac --disable-ffserver --disable-ffplay
  
    libavutil      51. 41.100 / 51. 41.100
  
    libavcodec     54.  4.100 / 54.  4.100
  
    libavformat    54.  1.100 / 54.  1.100
  
    libavdevice    53.  4.100 / 53.  4.100
  
    libavfilter     2. 62.101 /  2. 62.101
  
    libswscale      2.  1.100 /  2.  1.100
  
    libswresample   0.  7.100 /  0.  7.100
  
    libpostproc    52.  0.100 / 52.  0.100

  and my machine envirment is :

  Linux master 2.6.18-194.el5 #1 SMP Fri Apr 2 14:58:14 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux

  I’m irritable now,I hope someone can give me some advice,really appreciate it.

• I want use ffmpeg record mp3 in command line but get Unknown input format : 'oss' error

  28 décembre 2012, par user504909

  my ffmpeg version is :

  ffmpeg version N-48238-g10a3fa8 Copyright (c) 2000-2012 the FFmpeg developers
  
  built on Dec 27 2012 16:14:07 with gcc 4.7.2 (GCC)

  use cmd in windows :

  ffmpeg.exe -f oss -i dsp.mp3

  output :

  libavutil      52. 12.100 / 52. 12.100
  
  libavcodec     54. 81.100 / 54. 81.100
  
  libavformat    54. 50.102 / 54. 50.102
  
  libavdevice    54.  3.102 / 54.  3.102
  
  libavfilter     3. 30.101 /  3. 30.101
  
  libswscale      2.  1.103 /  2.  1.103
  
  libswresample   0. 17.102 /  0. 17.102
  
  libpostproc    52.  2.100 / 52.  2.100
  
  Unknown input format: 'oss'

  list all format for "output" :

  ffmpeg.exe -formats | grep "output"

  (I installed the cygwin, so can use grep in window command)
  output :

   E caca            caca (color ASCII art) output device
  
  DE rtp             RTP output
  
  DE rtsp            RTSP output
  
  DE sap             SAP output
  
   E sdl             SDL output device

  I just want record the audio from my mic, how to use command line to do it ?