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• Introducing WebM, an open web media project

  20 mai 2010, par noreply@blogger.com (christosap)

  A key factor in the web’s success is that its core technologies such as HTML, HTTP, TCP/IP, etc. are open and freely implementable. Though video is also now core to the web experience, there is unfortunately no open and free video format that is on par with the leading commercial choices. To that end, we are excited to introduce WebM, a broadly-backed community effort to develop a world-class media format for the open web.

  WebM includes :
  

  • VP8, a high-quality video codec we are releasing today under a BSD-style, royalty-free license
  • Vorbis, an already open source and broadly implemented audio codec
  • a container format based on a subset of the Matroska media container

  The team that created VP8 have been pioneers in video codec development for over a decade. VP8 delivers high quality video while efficiently adapting to the varying processing and bandwidth conditions found on today’s broad range of web-connected devices. VP8’s efficient bandwidth usage will mean lower serving costs for content publishers and high quality video for end-users. The codec’s relative simplicity makes it easy to integrate into existing environments and requires less manual tuning to produce high quality results. These existing attributes and the rapid innovation we expect through the open-development process make VP8 well suited for the unique requirements of video on the web.

  A developer preview of WebM and VP8, including source code, specs, and encoding tools is available today at www.webmproject.org.

  We want to thank the many industry leaders and web community members who are collaborating on the development of WebM and integrating it into their products. Check out what Mozilla, Opera, Google Chrome, Adobe, and many others below have to say about the importance of WebM to the future of web video.

  
  

  AMD

  ARM

  Brightcove

  Broadcom

  Collabora

  Digital Rapids

  Encoding.com

  Grab Networks

  iLinc

  INLET

  Kaltura

  Logitech

  MIPS

  Nvidia

  Ooyala

  Qualcomm

  Skype

  Sorenson

  Telestream
  

  Texas Instruments

  Verisilicon

  ViewCast

  Wildform

  
  

  Posted by Jeremy Doig, Engineering Director of video and Mike Jazayeri, Group Product Manager, Google

  

• Of ctors and dtors

  18 février 2011, par Multimedia Mike — Programming, Sega Dreamcast

  I haven’t given up on the Sega Dreamcast programming. I was able to compile a bunch of homebrew code for the DC many years ago and I can’t make it work anymore. Again, I was working with a purpose-built, open source RTOS named KallistiOS (or KOS). I can make the programs compile but not run. I had ELF files left over from years ago which still executed. But when I tried to build new ELF files, no luck— the programs crashed before even reaching my main() function.

  I found the problem : ELF files are comprised of a number of sections and 2 of these sections are named ’.ctors’ and ’.dtors’ which stand for constructors and destructors. The KOS RTOS performs a manual traversal of .ctors section during program initialization and this is where things go bad. The traversal code doesn’t seem to account for a .ctors section that only contains a single entry. I commented out the function that does the traversal and programs started to work, at least until it was time to exit the program and return control to the program loader. That’s when the counterpart .dtors section traversal code ran and demonstrated the same problem. I’ll exhibit the problematic code at the end of this post.

  So I’m finally tinkering with Sega Dreamcast programming once again and with a slightly better grasp of software engineering than the first time I did this.

  Portable and Compatible C ?
  If nothing else, this low-level embedded stuff exposes you to some serious toolchain arcana, the likes of which you will likely never see working strictly in the desktop arena.

  Still, this exercise makes me wonder why C code from a decade ago doesn’t compile reliably now. Part of it is because gcc has gotten stricter about the syntax it will accept. In the case of this specific crashing problem, I suspect it comes down to a difference in the way the linker generates the final ELF file. I’ve written a list of items I have had to modify in the KOS codebase in order to get it to compile on more recent gcc versions. I wonder if it would be worth publishing the specifics, or if anyone would ever find the information useful ? Oh, who am I kidding ? Of course I’ll write it up, perhaps publish a new version of the code, if only because that’s the best chance I have of finding my own work again some years down the road.

  Problematic C Code
  See if this code makes any sense to you. It somehow traverse a list of 32-bit function pointers (in different directions, depending on constructors or destructors), executing each in turn. However, it appears to fall over if the list of pointers consists of a single entry.
  

  PLAIN TEXT

  C :

  1. typedef void (*fptr)(void) ;
  2.  
  3. static fptr ctor_list[1] __attribute__((section(".ctors"))) = { (fptr) -1 } ;
  4. static fptr dtor_list[1] __attribute__((section(".dtors"))) = { (fptr) -1 } ;
  5.  
  6. /* Call this to execute all ctors */
  7. void arch_ctors() {
  8.     fptr *fpp ;
  9.  
  10.     /* Run up to the end of the list (defined by crtend) */
  11.     for (fpp=ctor_list + 1 ; *fpp != 0 ; ++fpp)
  12.          ;
  13.  
  14.     /* Now run the ctors backwards */
  15.     while (—fpp> ctor_list)
  16.         (**fpp)() ;
  17. }
  18.  
  19. /* Call this to execute all dtors */
  20. void arch_dtors() {
  21.     fptr *fpp ;
  22.  
  23.     /* Do the dtors forwards */
  24.     for (fpp=dtor_list + 1 ; *fpp != 0 ; ++fpp )
  25.         (**fpp)() ;
  26. }

• FFmpeg pan filter error when routing stereo audio to rear channels of 5.1 output

  13 avril, par MilkyTech

  I'm trying to mix a stereo commentary track into the rear surround channels of a 5.1 audio stream using FFmpeg on Windows 10. My goal is to lower the volume of the original 5.1 movie audio, then add the stereo commentary so it plays from the rear left and right speakers (SL and SR).

  


  I've already converted the commentary to EAC3 to match the main track's codec :

  


  ffmpeg -i "CastCommentary.m4a" -c:a eac3 -b:a 640k CastCommentary.eac3

  


  Then I tried mixing them like this (from within Command Prompt, not PowerShell or a batch file) :

  


  ffmpeg -i "Tropic.Thunder.2008.UNRATED.mkv" -i "CastCommentary.eac3" -filter_complex "[0:a:0]volume=0.4[aud1]; [1:a:0]pan=5.1:FL=0:FR=0:FC=0:LFE=0:SL=c0:SR=c1[cm_rear]; [aud1][cm_rear]amix=inputs=2[aout]" -map 0:v -map "[aout]" -map 0:s? -t 600 -c:v copy -c:s copy -c:a eac3 -b:a 640k "Tropic.Thunder.5.1.commentary.test.mkv"


  


  But I keep getting errors like :

  


  [fc#0 @ ...] Error applying option 'SL' to filter 'pan': Option not found
Error : Option not found


  


  Or :

  


  [Parsed_pan_1 @ ...] Expected in channel name, got ""


  


  Or even :

  


  Output channel layout 5.1 does not match the number of channels mapped 2.


  


  I’ve tried variations of the pan syntax :

  


  

  • pan=5.1:FL=0:FR=0:FC=0:LFE=0:SL=c0:SR=c1

  


  • pan=5.1|FL=0|FR=0|FC=0|LFE=0|SL=c0|SR=c1

  


  • Wrapping in single/double

  


  • quotes Escaping for CMD (no caret issues in current runs)

  


  


  Nothing seems to work.

  


  🎯 Goal :

  


  

  • Keep 5.1 audio from the original movie (volume lowered)

  


  • Add stereo commentary to SL and SR

  


  • Output a proper 5.1 EAC3 mix

  


  


  🔧 System :

  


  

  • Windows 10

  


  • FFmpeg version : [latest static build from ffmpeg.org]

  


  • Running in true Command Prompt (not PowerShell)

  


  • Source audio : 5.1 EAC3 from a .mkv, stereo .eac3 from .m4a

  


  


  What’s the correct filter_complex syntax to route a stereo track to the rear channels of a 5.1 layout using FFmpeg on Windows ? Am I missing something about pan, amix, or Windows quirks ?