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• Server side video mixing

  16 mars 2014, par Nicolas Goy

  I have a serie of video files encoded in mpeg2 (I can change this encoding), and I have to produce a movie in flash flv (this is a requirement, I can't change that encoding).

  One destination movie is a compilation of different source video files.

  I have a playlist defining the destination movie. For example :

  Video file        Position        Offset       Length 
  
  little_gnomes     0               0            8.5
  
  fairies           5.23            0.12         12.234
  
  pixies            14              0            9.2

  Video file is the name of the file, position is when the file should be started (in the master timeline), offset is the offset within the video file, and length is the length of the video to play. The numbers are seconds (in double).

  This would result in something like that (final movie timeline) :

                 0--5.23|--8.5|--14|--17.464|--23.2|
  
  little_nomes   **************
  
  fairies               *********************
  
  pixies                           *****************

  Where video overlaps, the last video to be added override the last one, the audio should be mixed.

  The resulting video track would be :

                 0--5.23|--8.5|--14|--17.464|--23.2|
  
  little_nomes   *******
  
  fairies               ***********
  
  pixies                           *****************

  While the resulting audio would be :

                 0--5.23|--8.5|--14|--17.464|--23.2|
  
  little_nomes   11111112222222
  
  fairies               222222211112222222222
  
  pixies                           22222222221111111

  Where 1 or 2 is the number of mixed audio tracks.

  There can be a maximum of 3 audio tracks.

  I need to write a program which takes the playlist as input and produce the flv file. I'm open to any solution (must be free/open source).

  An existing tool that can do that would be the simplest, but I found none. As for making my own solution, I found only ffmpeg, I was able to do basic things with it, but the documentation is terribly lacking.

  It can be any language, it doesn't have to be super fast (if it takes 30 minutes to build a 1h movie it's fine).

  The solution will run on opensolaris based x64 servers. If I have to use linux, this would work too. But windows is out of the question.

• Merge commit ’ed06e5d92b4c67b49068d538461fbbe0a53a8c5e’

  9 mars 2014, par Michael Niedermayer

  Merge commit ’ed06e5d92b4c67b49068d538461fbbe0a53a8c5e’
  * commit ’ed06e5d92b4c67b49068d538461fbbe0a53a8c5e’ :
  
    hevc : Do not turn 32bit timebases into negative numbers
  Conflicts :
  
  	libavcodec/hevc.c
  See : bf2ce19e51fde57727a4ee33a4b3c2622f37a5c4
  
  Merged-by : Michael Niedermayer <michaelni@gmx.at>
  

  • [DH] libavcodec/hevc.c

• Playing With Emscripten and ASM.js

  1er mars 2014, par Multimedia Mike — General

  The last 5 years or so have provided a tremendous amount of hype about the capabilities of JavaScript. I think it really kicked off when Google announced their Chrome web browser in September, 2008 along with its V8 JS engine. This seemed to spark an arms race in JS engine performance along with much hyperbole that eventually all software could, would, and/or should be written in straight JavaScript for maximum portability and future-proofing, perhaps aided by Emscripten, a tool which magically transforms C and C++ code into JS. The latest round of rhetoric comes courtesy of something called asm.js which purports to narrow the gap between JS and native code performance.

  I haven’t been a believer, to express it charitably. But I wanted to be certain, so I set out to devise my own experiment to test modern JS performance.

  Up Front Summary
  I was extremely surprised that my experiment demonstrated JS performance FAR beyond my expectations. There might be something to these claims of magnficent JS speed in numerical applications. Basically, here were my thoughts during the process :

  • There’s no way that JavaScript can come anywhere close to C performance for a numerically intensive operation ; a simple experiment should demonstrate this.
  • Here’s a straightforward C program to perform a simple yet numerically intensive operation.
  • Let’s compile the C program on gcc and get some baseline performance numbers.
  • Let’s use Emscripten to convert the C program to JavaScript and run it under Chrome.
  • Ha ! Pitiful JS performance, just as I expected !
  • Try the same program under Firefox, since Firefox is supposed to have some crazy optimization for asm.js code, allegedly emitted by Emscripten.
  • LOL ! Firefox performs even worse than Chrome !
  • Wait a minute… the Emscripten documentation mentioned using optimization levels for generating higher performance JS, so try ‘-O1′.
  • Umm… wow : Chrome’s performance increased dramatically ! What about Firefox ? Not only is Firefox faster than Chrome, it’s faster than the gcc-generated code !
  • As my faith in C is suddenly shaken to its core, I remembered to compile the gcc version with an explicit optimization level. The native C version pulled ahead of Firefox again, but the Firefox code is still close.
  • Aha ! This is just desktop– but what about mobile ? One of the leading arguments for converting everything to pure JavaScript is that such programs will magically run perfectly in mobile browsers. So I wager that this is where the experiment will fall over.
  • I proceed to try the same converted program on a variety of mobile platforms.
  • The mobile platforms perform rather admirably as well.
  • I am surprised.

  The Experiment
  I wanted to run a simple yet numerically-intensive and relevant benchmark, and something I am familiar with. I settled on JPEG image decoding. Again, I wanted to keep this simple, ideally in a single file because I didn’t know how hard it might be to deal with Emscripten. I found NanoJPEG, which is a straightforward JPEG decoder contained in a single C file.
  

  I altered nanojpeg.c (to a new file called nanojpeg-static.c) such that the main() program would always load a 1920×1080 (a.k.a. 1080p) JPEG file (“bbb-1080p-title.jpg”, the Big Buck Bunny title), rather than requiring a command line argument. Then I used gettimeofday() to profile the core decoding function (njDecode()).

  Compiling with gcc and profiling execution :

  gcc -Wall nanojpeg-static.c -o nanojpeg-static
  ./nanojpeg-static
  

  Optimization levels such as -O0, -O3, or -Os can be applied to the compilation command.

  For JavaScript conversion, I installed Emscripten and converted using :

  /path/to/emscripten/emcc nanojpeg-static.c -o nanojpeg.html \
    —preload-file bbb-1080p-title.jpg -s TOTAL_MEMORY=32000000
  

  The ‘–preload-file’ option makes the file available to the program via standard C-style file I/O functions. The ‘-s TOTAL_MEMORY’ was necessary because the default of 16 MB wasn’t enough. Again, the -O optimization levels can be sent in.

  For running, the .html file is loaded (via webserver) in a web browser.

  Want To Try It Yourself ?
  I put the files here : http://multimedia.cx/emscripten/. The .c file, the JPEG file, and the Emscripten-converted files using -O0, -O1, -O2, -O3, -Os, and no optimization switch.

  Results and Charts
  Here is the spreadsheet with the raw results.

  I ran this experiment using Ubuntu Linux 12.04 on an Intel Atom N450-based netbook. For this part, I was able to compare the Chrome and Firefox browser results against the C results :

  
  
  

  These are the results for a 2nd generation Android Nexus 7 using both Chrome and Firefox :

  
  
  

  Here is the result for an iPad 2 running iOS 7 and Safari– there is no Firefox for iOS and while there is a version of Chrome for iOS, it apparently isn’t able to leverage an optimized JS engine. Chrome takes so long to complete this experiment that there’s no reason to muddy the graph with the results :
  

  
  
  

  Interesting that -O1 tends to provide better optimization than levels 2 or 3, and that -Os (optimize for size) seems to be a good all-around choice.

  Don’t Get Too Smug
  JavaScript can indeed get amazing performance in this day and age. Please be advised, however, that this isn’t the best that a C decoder implementation can possibly do. This version doesn’t leverage any SIMD extensions. According to profiling (using gprof against the C code), sample saturation in color conversion dominates followed by inverse DCT functions, common cases for SIMD ASM or intrinsics. Allegedly, there will be some support for JS SIMD optimizations some day. We’ll see.

  Implications For Development
  I’m still not especially motivated to try porting the entire Native Client game music player codebase to JavaScript. I’m still wondering about the recommended development flow. How are you supposed to develop for Emscripten and asm.js ? From what I can tell, Emscripten is not designed as a simple aide for porting C/C++ code to JS. No, it reduces the code into JS code you can’t possibly maintain. This seems to imply that the C/C++ code needs to be developed and debugged in its entirety and then converted to JS, which seems arduous.