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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.

• Images to video in PHP ? [duplicate]

  24 juin 2013, par jesper

  This question already has an answer here :

  • How to create videos from images with php ?
    

    3 answers

  I have tried to make a images to video converter, and this works.

  ffmpeg  -i /home/psafari/public_html/common/images/icon3.png -vcodec mpeg4 /home/psafari/public_html/" . time().".avi

  But how can i set the duration of the movie ? So it's maybe 7 minutes long ?

  Another question :

  I have 2 pictures, now the command only make the video from image "icon3.png", but i need it to run maybe picture2.png the first 5 seconds, and the rest (6.55 seconds) it has to view icon3.png ex.

• Revision 8d3d2b76f3 : Tx size selection enhancements (1) Refines the modeling function and uses that

  22 juin 2013, par Deb Mukherjee

  Changed Paths :
   Modify /vp9/common/vp9_blockd.h

  
   Modify /vp9/encoder/vp9_encodeframe.c

  
   Modify /vp9/encoder/vp9_onyx_if.c

  
   Modify /vp9/encoder/vp9_onyx_int.h

  
   Modify /vp9/encoder/vp9_rdopt.c

  
  
  Tx size selection enhancements

  (1) Refines the modeling function and uses that to add some speed
  features. Specifically, intead of using a flag use_largest_txfm as
  a speed feature, an enum tx_size_search_method is used, of which
  two of the types are USE_FULL_RD and USE_LARGESTALL. Two other
  new types are added :
  USE_LARGESTINTRA (use largest only for intra)
  USE_LARGESTINTRA_MODELINTER (use largest for intra, and model for
  inter)

  (2) Another change is that the framework for deciding transform type
  is simplified to use a heuristic count based method rather than
  an rd based method using txfm_cache. In practice the new method
  is found to work just as well - with derf only -0.01 down.
  The new method is more compatible with the new framework where
  certain rd costs are based on full rd and certain others are
  based on modeled rd or are not computed. In this patch the existing
  rd based method is still kept for use in the USE_FULL_RD mode.
  In the other modes, the count based method is used.
  However the recommendation is to remove it eventually since the
  benefit is limited, and will remove a lot of complications in
  the code

  (3) Finally a bug is fixed with the existing use_largest_txfm speed feature
  that causes mismatches when the lossless mode and 4x4 WH transform is
  forced.

  Results on derf :
  USE_FULL_RD : +0.03% (due to change in the tables), 0% encode time reduction
  USE_LARGESTINTRA : -0.21%, 15% encode time reduction (this one is a
  pretty good compromise)
  USE_LARGESTINTRA_MODELINTER : -0.98%, 22% encode time reduction
  (currently the benefit of modeling is limited for txfm size selection,
  but keeping this enum as a placeholder) .
  USE_LARGESTALL : -1.05%, 27% encode-time reduction (same as existing
  use_largest_txfm speed feature).

  Change-Id : I4d60a5f9ce78fbc90cddf2f97ed91d8bc0d4f936