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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.

• How to batch insert same clip scene at the beginning of multiple videos ?

  13 juin 2013, par FLYqtFLY

  I have been for a few days researching on a suitable auto or semi-automatic way of mass merging files this way. Basically, what I want to achieve is inserting the same clip, before a bunch of other clips.

  In this case I´ve been trying with .flv, .mp4, .avi files (always merging same codecs), and I didn´t succeed after testing all the programs available in both Windows and OSx. Believe me, I´ve tried them all. The only ones which are supposed to handle quite well this quite of tasks are from Videocharge : Watermark Master and Videocharge Studio. None of them could perform correctly the task. Maybe there was some way I could have achieved this by scripting with After Effects or Sony Vegas, I just didn´t find any.

  Now I´m trying to find some way to achieve the same thing either by shell, unix scripting... I don´t care. I just don´t want to mount Ubuntu only for these tasks. I´m not a programmer, but I´m quite stuborn (which sometimes leads me to neverending nights) so I would appreciate some help or guidance from anyone keen and good enough on batch video processing or scripting.

  Right now, the only useful paths I´ve found drive me into either using mencode or ffmpeg through commands, but I am not able to perform the merge on batch. I don´t care about the way to sort the videos out. But taking into consideration that the operation would be performed on hundreds or thousands of videos, it wouldn´t be suitable to have them all in separate folders each accompanied by the "unique" intro clip. I guess that the most logic way would be storing the "bunch" videos in a folder, and the intro clip on the same folder as the encoder or renderer.

  Thanks in advance for any help or guidance,

• avformat/apetag : bump micro version

  11 février 2017, par James Almer

  avformat/apetag : bump micro version
  In case parsers care about the version that started writing
  
  correct flags.
  Signed-off-by : James Almer <jamrial@gmail.com>
  

  • [DH] libavformat/version.h