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• Make video frames from a livestream identifiable across multiple clients

  23 septembre 2016, par mschwaig

  I need to distribute a video stream from a live source to several clients with the additional requirement that each frame is identifiable across all clients.

  I have already done research into the topic, and I have arrived at a possible solution that I can share. My solution seems suboptimal and this is my first experience of working with video streams, so I want to see if somebody knows a better way.

  The reason why I need to be able to identify specific frames within the video stream is that the streaming clients need to be able to talk about the time differences between events each of them identifies in their video stream.

  A little clarifying example

  I want to enable the following interaction :

  • Two client applications Dewey and Stevie connect to the streaming server
  • Dewey displays the stream and Stevie saves it to disk
  • Dewey identifies a specific video frame that is of interest to Stevie, so he wants to tell Stevie about it
  • Dewey extracts some identifying information from the video frame and sends it to Stevie
  • Stevie uses the identifying information to extract the same frame from the copy of the livestream he is currently saving

  Dewey cannot send the frame to Stevie directly, because Malcolm and Reese also want to tell him about specific video frames and Stevie is interested in the time difference between their findings.

  Suggested solution

  The solution that I found was using ffserver to broadcast a RTP stream and use the timestamps from the RTCP packets to identify frames. These timestamps are normally used to synchronize audio and video, and not to provide a shared timeline across several clients, which is why I am skeptical this is the best way to solve my problem.

  It also seems beneficial to have frame numbers, like an increasing counter of frames instead of arbitrary timestamps which increase by some perhaps varying offset as for my application I also have to reference neighboring frames and it seems easier to compute time differences from frame numbers, than the other way around.

• Converting png images series to webm with transparent white background from Daz3d

  25 août 2015, par James

  I’m trying to make a webm video with a transparent background from a Daz3D model.
  My process is export png image series with transparent background from Daz3D, use ffmpeg to convert png series to webm video.
  This was working well in Daz3D 4.6.

  But in Daz3D 4.8 the exported background is black instead of white, so when converted to webm is ok on Chrome as has the transparency, but on Firefox is black and has a halo (as Firefox does not support transparency so displays background).

  So I’m looking for a solution with Daz3D, or tools like ImageMagik.
  I almost got it with ImageMagik,

  convert -alpha extract *.png mask.png
  
  mogrify -flatten talk*.png
  
  for /f %x in ('dir /s /b blink*.png') do @composite -compose CopyOpacity mask-0.png %x %x

  But for some reason the final webm has a white background not transparent ...
  Some more info here,

  http://www.daz3d.com/forums/discussion/61237/daz3d-4-8-png-background-is-black

  and here,

  http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=28214

  >
  Doh, okay I figured it out. My images were good, but I somehow have two different versions of ffmpeg on my computer and was using the wrong one that doesn’t seem to support transparency.

  Now it is working.

  My only issue is the last shell line,

  for /f %x in (’dir /s /b blink*.png’) do @composite -compose CopyOpacity mask-0.png %x %x

  This only uses mask-0.png, instead of mask-1 for blink01.png, mask-2 for blink02.png etc.

• WebM Cabal

  8 octobre 2010, par Multimedia Mike — On2/Duck, VP8

  I traveled to a secret clubhouse today to take part in a clandestine meeting to discuss exactly how WebM will rule over all that you see and hear on the web. I can’t really talk about it. But I can show you the cool hat I got :

  
  
  Yeah, you’re jealous.
  

  The back of the hat has an Easter egg for video codec nerds– the original Duck Corporation logo (On2′s original name) :

  
  
  

  Former employees of On2 (now Googlers) were well-represented. It was an emotional day of closure as I met the person — the only person to date — who contacted me with a legal threat so many years ago. He still remembered me too.

  I met a lot of people involved in creating various Duck and On2 codecs and learned a lot of history and lore behind then– history I hope to be able to document one day.

  I’m glad I got that first rough draft of a toy VP8 encoder done in time for the meeting. It was the subject of much mirth.