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• 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.

• ffmpegthumbnailer issue : dyld : Library not loaded : /usr/local/lib/libavutil.52.18.100.dylib

  18 octobre 2013, par scientiffic

  When I try to run ffmpegthumbnailer, I get the following error :

  dyld: Library not loaded: /usr/local/lib/libavutil.52.18.100.dylib
  
    Referenced from: /usr/local/bin/ffmpegthumbnailer
  
    Reason: image not found
  
    Trace/BPT trap: 5

  I installed ffmpeg using the directions here :

  http://ffmpeg.org/trac/ffmpeg/wiki/MacOSXCompilationGuide

  In my /usr/local/lib folder, I have the file "libavutil.a", but not the one specified in the error mesage.

  How can I solve this error ?

  This is what I was using to try to generate a thumbnail :

  ffmpegthumbnailer -i /public/uploads/tmp/1382121359-37490-7826/thumb_Untitled.mov -o /public/uploads/tmp/1382121359-37490-7826/tmpfile.png -c png -q 10 -s 158

• ReSample RTP Audio data over Socket as PCM data using FFMPEG

  5 février 2020, par GJ.

  I want to receive a RTP Stream and send the raw data received in it over TCP / UDP socket. For this i am trying following commands.

  1. I am sending the RTP stream using following command.

     ffmpeg -re -i hello.wav -ar 8000 -f mulaw -f rtp rtp ://127.0.0.1:1234

  2. Receiver to re transmit the stream over TCP / UDP socket.

     ffmpeg -i rtp ://127.0.0.1:1234 -f mulaw tcp ://127.0.0.1:5555 -hide_banner

  3. A Player to play this Socket stream to verify.

     ffplay tcp ://127.0.0.1:5555 ?listen

  My second command shows that its receiving data and transmitting the data to tcp over 5555 port.

  Input #0, rtp, from 'rtp://127.0.0.1:1234':
  
    Duration: N/A, start: 0.000000, bitrate: 64 kb/s
  
      Stream #0:0: Audio: pcm_mulaw, 8000 Hz, mono, s16, 64 kb/s
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (pcm_mulaw (native) -> pcm_mulaw (native))
  
  Press [q] to stop, [?] for help
  
  Output #0, mulaw, to 'tcp://127.0.0.1:5555':
  
    Metadata:
  
      encoder         : Lavf58.29.100
  
      Stream #0:0: Audio: pcm_mulaw, 8000 Hz, mono, s16, 64 kb/s
  
      Metadata:
  
        encoder         : Lavc58.54.100 pcm_mulaw

  But the third command does not receive anything, even i tried dumping the response into a file using FFMPEG in command three but same results.

  What am i doing wrong, Please suggest the correct commands using FFMPEG.