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• Anomalie #2713 : Gestion des auteurs - Bugs

  18 mai 2012, par Julien -

  ben non ! (re-testé en 19418) En fait, voir capture d’écran : auteur6.png : J’ai une ligne vide initiale : celle-ci ne pose pas de pb en effet. J’ai une autre ligne vide entre rubrique1 et rubrique2 : c’est celle-ci qui pose pb. Elle ajoute un élément vide, 1ere chose, et à l’enregistrement, (...)

• Making Sure The PNG Gets There

  14 juin 2013, par Multimedia Mike — General

  Rewind to 1999. I was developing an HTTP-based remote management interface for an embedded device. The device sat on an ethernet LAN and you could point a web browser at it. The pitch was to transmit an image of the device’s touch screen and the user could click on the picture to interact with the device. So we needed an image format. If you were computing at the time, you know that the web was insufferably limited back then. Our choice basically came down to GIF and JPEG. Being the office’s annoying free software zealot, I was championing a little known up and coming format named PNG.

  So the challenge was to create our own PNG encoder (incorporating a library like libpng wasn’t an option for this platform). I seem to remember being annoyed at having to implement an integrity check (CRC) for the PNG encoder. It’s part of the PNG spec, after all. It just seemed so redundant. At the time, I reasoned that there were 5 layers of integrity validation in play.

  I don’t know why, but I was reflecting on this episode recently and decided to revisit it. Here are all the encapsulation layers of a PNG file when flung over an ethernet network :

  
  
  

  So there are up to 5 encapsulations for the data in this situation. At the innermost level is the image data which is compressed with the zlib DEFLATE method. At first, I thought that this also had a CRC or checksum. However, in researching this post, I couldn’t find any evidence of such an integrity check. Further, I don’t think we bothered to compress the PNG data in this project long ago. It was a small image, monochrome, and transferring via LAN, so the encoder could get away with signaling uncompressed data.

  The graphical data gets wrapped up in a PNG chunk and all PNG chunks have a CRC. To transmit via the network, it goes into a TCP frame, which also has a checksum. That goes into an IP packet. I previously believed that this represented another integrity check. While an IP frame does have a checksum, the checksum only covers the IP header and not the payload. So that doesn’t really count towards this goal.

  Finally, the data gets encapsulated into an ethernet frame which has — you guessed it — a CRC.

  I see that other link layer protocols like PPP and wireless ethernet (802.11) also feature frame CRCs. So I guess what I’m saying is that, if you transfer a PNG file over the network, you can be confident that the data will be free of any errors.

• How can I skip an image error when trimming an mp3 with ffmpeg

  29 septembre 2016, par John Pollard

  I am running the following command to get cut a 30 second section of the file

  ffmpeg -i 'river.mp3' -ss 30 -t 30 -acodec libmp3lame -b:a 128k 'audio_trimmed.mp3'

  but I am getting a "Invalid PNG signature" error. Is there a way to skip the image since I don’t need it and have the command run successfully ?

  ffmpeg version 2.8.5 Copyright (c) 2000-2016 the FFmpeg developers
  
    built with Apple LLVM version 7.3.0 (clang-703.0.31)
  
    configuration: --prefix=/usr/local/Cellar/ffmpeg/2.8.5 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-opencl --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libvorbis --enable-vda
  
    libavutil      54. 31.100 / 54. 31.100
  
    libavcodec     56. 60.100 / 56. 60.100
  
    libavformat    56. 40.101 / 56. 40.101
  
    libavdevice    56.  4.100 / 56.  4.100
  
    libavfilter     5. 40.101 /  5. 40.101
  
    libavresample   2.  1.  0 /  2.  1.  0
  
    libswscale      3.  1.101 /  3.  1.101
  
    libswresample   1.  2.101 /  1.  2.101
  
    libpostproc    53.  3.100 / 53.  3.100
  
  [mp3 @ 0x7f8419010600] Skipping 0 bytes of junk at 73436.
  
  [png @ 0x7f8419812600] Invalid PNG signature 0xFFD8FFE000104A46.
  
  [mp3 @ 0x7f8419010600] decoding for stream 1 failed
  
  [mp3 @ 0x7f8419010600] Could not find codec parameters for stream 1 (Video: png, none(pc)): unspecified size
  
  Consider increasing the value for the 'analyzeduration' and 'probesize' options
  
  Input #0, mp3, from 'river.mp3':
  
    Metadata:
  
      genre           : Podcast
  
      album           : River City Church's Podcast
  
      artist          : Antley Fowler
  
      title           : Jubilee Brings Justice
  
      date            : 2016-08-15 10:23
  
    Duration: 00:28:28.07, start: 0.025056, bitrate: 64 kb/s
  
      Stream #0:0: Audio: mp3, 44100 Hz, mono, s16p, 64 kb/s
  
      Stream #0:1: Video: png, none(pc), 90k tbr, 90k tbn, 90k tbc
  
      Metadata:
  
        title           : Episode Artwork
  
        comment         : Cover (front)
  
  [buffer @ 0x7f8418c18de0] Unable to parse option value "0x0" as image size
  
  [buffer @ 0x7f8418c18de0] Unable to parse option value "-1" as pixel format
  
  [buffer @ 0x7f8418c18de0] Unable to parse option value "0x0" as image size
  
  [buffer @ 0x7f8418c18de0] Error setting option video_size to value 0x0.
  
  [graph 0 input from stream 0:1 @ 0x7f8418c18ee0] Error applying options to the filter.
  
  Error opening filters!