Recherche avancée

Sur d’autres sites (5139)

• 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” ?

• FFMPEG concat demuxer - how to make file formats compatible ?

  10 avril 2015, par user206481

  I need to automate mp4 concatenation server-side and I’m using FFMPEG. I will get uploads of mp4 files and I want to attach a Title.mp4 and End.mp4 to each one. I am also overlaying a soundtrack (the input videos do not have sound) There is a potential high server load so I’d like to do it as efficiently as possible using ffmpeg’s concat demuxer to avoid re-encoding the video.

  After receiving samples of each file, I am not successful and I believe it is due to mismatched file formats. My result has good Title.mp4 and audio, then when the sample uploaded mp4 is supposed to play there is garbled green/pink/red pixels on the top half of the video, then the End.mp4 plays fine. Here is my ffmpeg command and output :

  $ ffmpeg -f concat -i <(printf "file '%s'\n" Title.mp4 Sample.mp4 End.mp4) -i SoundTrack.wav -c:v copy -strict -2 -y Out.mp4
  
  
  
  ffmpeg version 2.6.1 Copyright (c) 2000-2015 the FFmpeg developers
  
  built with gcc 4.1.2 (GCC) 20080704 (Red Hat 4.1.2-55)
  
  configuration: --prefix=/home/dpmsmobi/ffmpeg_build --extra-cflags=-I/home/dpmsmobi/ffmpeg_build/include --extra-ldflags=-L/home/dpmsmobi/ffmpeg_build/lib --bindir=/home/dpmsmobi/bin --enable-gpl --enable-nonfree --enable-libmp3lame --enable-libvorbis --enable-libvpx --enable-libx264
  
    libavutil      54. 20.100 / 54. 20.100
  
    libavcodec     56. 26.100 / 56. 26.100
  
    libavformat    56. 25.101 / 56. 25.101
  
    libavdevice    56.  4.100 / 56.  4.100
  
    libavfilter     5. 11.102 /  5. 11.102
  
    libswscale      3.  1.101 /  3.  1.101
  
    libswresample   1.  1.100 /  1.  1.100
  
    libpostproc    53.  3.100 / 53.  3.100
  
  Input #0, concat, from '/dev/fd/63':
  
    Duration: N/A, start: 0.000000, bitrate: 1810 kb/s
  
      Stream #0:0: Video: h264 (Main) (avc1 / 0x31637661), yuv420p(tv), 768x512 [SAR 1:1 DAR 3:2], 1810 kb/s, 30 fps, 30 tbr, 30k tbn, 60 tbc
  
  Guessed Channel Layout for  Input Stream #1.0 : stereo
  
  Input #1, wav, from 'SoundTrack.wav':
  
    Metadata:
  
      encoded_by      : Adobe Premiere Pro CC 2014 (Maci
  
      encoder         : Adobe Premiere Pro CC 2014 (Macintosh)
  
      date            : 2015-04-07
  
      creation_time   : 11:12:10
  
      time_reference  : 0
  
    Duration: 00:00:15.06, bitrate: 1551 kb/s
  
      Stream #1:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 48000 Hz, 2 channels, s16, 1536 kb/s
  
  Output #0, mp4, to 'Out.mp4':
  
    Metadata:
  
      encoder         : Lavf56.25.101
  
      Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 768x512 [SAR 1:1 DAR 3:2], q=2-31, 1810 kb/s, 30 fps, 30 tbr, 30k tbn, 30k tbc
  
      Stream #0:1: Audio: aac ([64][0][0][0] / 0x0040), 48000 Hz, stereo, fltp, 128 kb/s
  
      Metadata:
  
        encoder         : Lavc56.26.100 aac
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (copy)
  
    Stream #1:0 -> #0:1 (pcm_s16le (native) -> aac (native))
  
  Press [q] to stop, [?] for help
  
  [concat @ 0x1dedc20] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
  
  [concat @ 0x1dedc20] DTS 69750 < 91000 out of order
  
  [mp4 @ 0x1f75060] Non-monotonous DTS in output stream 0:0; previous: 91000, current: 69750; changing to 91001. This may result in incorrect timestamps in the output file.
  
  
  
  <----- many more Non-monotonous DTS messages omitted here ---->
  
  
  
  frame=  427 fps=0.0 q=-1.0 Lsize=    4123kB time=00:00:15.06 bitrate=2242.5kbits/s    
  
  video:3873kB audio:236kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.344173%

  I can successfully concatenate the Title.mp4 to the End.mp4, and I can successfully concatenate two Sample.mp4 files, so I know I’ve got the ffmpeg command right. I can also successfully concat the files using the following ffmpeg command with filter_complex instead of concat demuxer (this takes considerably longer due to re-encoding) :

  ffmpeg -i Title.mp4 -i Sample.mp4 -i End.mp4 -i SoundTrack.wav -filter_complex '[0:0] [1:0] [2:0] concat=n=3:v=1 [v]' -map '[v]' -map 3:0 -crf 20 -strict -2 -y Out2.mp4 

  Here is the MediaInfo output for each type of mp4 file :

  $ mediainfo Title.mp4
  
  General
  
  Complete name                            : Title.mp4
  
  Format                                   : MPEG-4
  
  Format profile                           : Base Media / Version 2
  
  Codec ID                                 : mp42
  
  File size                                : 693 KiB
  
  Duration                                 : 3s 100ms
  
  Overall bit rate mode                    : Variable
  
  Overall bit rate                         : 1 831 Kbps
  
  Encoded date                             : UTC 2015-04-07 19:15:03
  
  Tagged date                              : UTC 2015-04-07 19:15:03
  
  ©TIM                                     : 00:00:00:00
  
  ©TSC                                     : 30
  
  ©TSZ                                     : 1
  
  
  
  Video
  
  ID                                       : 1
  
  Format                                   : AVC
  
  Format/Info                              : Advanced Video Codec
  
  Format profile                           : Main@L3.1
  
  Format settings, CABAC                   : Yes
  
  Format settings, ReFrames                : 3 frames
  
  Codec ID                                 : avc1
  
  Codec ID/Info                            : Advanced Video Coding
  
  Duration                                 : 3s 100ms
  
  Bit rate mode                            : Variable
  
  Bit rate                                 : 1 811 Kbps
  
  Maximum bit rate                         : 3 000 Kbps
  
  Width                                    : 768 pixels
  
  Height                                   : 512 pixels
  
  Display aspect ratio                     : 3:2
  
  Frame rate mode                          : Constant
  
  Frame rate                               : 30.000 fps
  
  Standard                                 : NTSC
  
  Color space                              : YUV
  
  Chroma subsampling                       : 4:2:0
  
  Bit depth                                : 8 bits
  
  Scan type                                : Progressive
  
  Bits/(Pixel*Frame)                       : 0.154
  
  Stream size                              : 685 KiB (99%)
  
  Language                                 : English
  
  Encoded date                             : UTC 2015-04-07 19:15:03
  
  Tagged date                              : UTC 2015-04-07 19:15:03
  
  Color range                              : Limited
  
  
  
  $ mediainfo Sample.mp4
  
  General
  
  Complete name                            : Sample.mp4
  
  Format                                   : MPEG-4
  
  Format profile                           : Base Media
  
  Codec ID                                 : isom
  
  File size                                : 2.93 MiB
  
  Duration                                 : 7s 9ms
  
  Overall bit rate                         : 3 505 Kbps
  
  Encoded date                             : UTC 1970-01-01 00:00:00
  
  Tagged date                              : UTC 1970-01-01 00:00:00
  
  Writing application                      : Lavf52.64.2
  
  
  
  Video
  
  ID                                       : 1
  
  Format                                   : AVC
  
  Format/Info                              : Advanced Video Codec
  
  Format profile                           : Baseline@L3.1
  
  Format settings, CABAC                   : No
  
  Format settings, ReFrames                : 1 frame
  
  Format settings, GOP                     : M=1, N=30
  
  Codec ID                                 : avc1
  
  Codec ID/Info                            : Advanced Video Coding
  
  Duration                                 : 7s 9ms
  
  Bit rate                                 : 3 500 Kbps
  
  Width                                    : 768 pixels
  
  Height                                   : 512 pixels
  
  Display aspect ratio                     : 3:2
  
  Frame rate mode                          : Variable
  
  Frame rate                               : 30.250 fps
  
  Minimum frame rate                       : 23.462 fps
  
  Maximum frame rate                       : 296.053 fps
  
  Color space                              : YUV
  
  Chroma subsampling                       : 4:2:0
  
  Bit depth                                : 8 bits
  
  Scan type                                : Progressive
  
  Bits/(Pixel*Frame)                       : 0.294
  
  Stream size                              : 2.92 MiB (100%)
  
  Language                                 : English
  
  Encoded date                             : UTC 1970-01-01 00:00:00
  
  Tagged date                              : UTC 1970-01-01 00:00:00

  I’m pretty sure it’s the mp42 vs isom Codec ID’s, and potentially the constant vs variable frame rates. I can’t change the input mp4’s but I know their format will stay the same. How can I reformat the Title and End mp4’s to match the input mp4 files so I can use ffmpeg concat demux ?

• Anomalie #3418 : Les tables des plugins ne s’installent pas

  19 avril 2015, par Franck Dalot

  Je viens de faire un test en SPIP 3.0.17 [21515] et php 5.6.6 cela semble être bon, les tables du plugin "agenda" sont bien présente.
  Pour info, .ovhconfig contenait juste :
  app.engine=php
  app.engine.version=5.6

  http.firewall=none
  environment=production