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Moving Pictures

Moving Pictures. Implementing Video on Flickr Cal Henderson. Hello. Flickr. Large scale kitten sharing website Started 2003, launched 2004 5 years old this december Almost 3 billion photos. Enter: Video. Video was added this year Launched April 2008

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Moving Pictures

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  1. Moving Pictures Implementing Video on Flickr Cal Henderson

  2. Hello

  3. Flickr • Large scale kitten sharing website • Started 2003, launched 2004 • 5 years old this december • Almost 3 billion photos

  4. Enter: Video • Video was added this year • Launched April 2008 • Many hundreds of thousands of videos uploaded • Many millions of playbacks

  5. “Video? That’s just like photos!” -Me, before Flickr Video

  6. 12 4 Steps • 4 main tasks • Uploading • Transcoding • Storage • Serving & Playback

  7. 1. Upload

  8. Simple upload • Web forms • Just like any other file <form action="/uploadify/" method="post" enctype="multipart/form-data"> <input type="file" name="fred" /> <input type="submit" value="Go!" /> </form> • But files are large / huge

  9. Issues • Two components for uploading: • Sending from the client • Receiving on the server • Both of these present problems

  10. Sending from the client • Multiple options • Simple form • Flash • Desktop app

  11. Simple Forms • Pros • Very easy to implement • Works on every browser out of the box • Cons • Upload progress is harder • ‘Slow’ • Select a single file at once

  12. Flash • Pros • Upload progress is easy • ‘Fast’ • Multi select of files • Cons • Harder to implement • Flash isn’t quite ubiquitous

  13. Desktop App • Pros • Upload progress is easy • ‘Very fast’ • Multi select of files • Drag and drop • Cons • Hard to develop • Hard to deploy (relative to the web)

  14. Making Progress • Upload progress • Not impossible with plain forms • Just need to be able to query the upload progress via AJAX • Multiple machines • The VIP issue • Enter Perlbal

  15. Making Progress 1. Browser starts upload Web 1 Browser 2. Web server broadcasts progress via UDP Load balancer Web 2 3. Browser queries progress via AJX call

  16. Receiving on the server • File uploads are slow • Much slower than serving pages • Apache processes are heavy • Waste of resources • Use a poll based server (like jetty)

  17. Receiving on the server • Or, use a buffering layer • Perlbal is great for this • Or a lightweight Apache • E.g. w/ mod_proxy Slow Fast Browser Buffer Server

  18. But wait • It’s not just the first step that’s slow • Moving files around between servers is slow • Do it out of band • Asynchronous jobs are in order anyway • Do it!

  19. 2. Transcode

  20. Transcode? • Why transcode at all? • Input comes from many sources • Point & shoots • DV Cams • Mobile devices • Video editing software • All in different formats

  21. So many formats • But very few of these formats can be played back cross platform • Without special software or hardware • Formats are designed to do one thing well • They don’t always manage even that • Transcoding puts all videos on an equal footing

  22. Video file basics • Most file types are really just containers • MOV, FLV, AVI • The data inside can be in multiple formats • We call these codecs (encoder + decoder) • Files contains multiple ‘streams’ • Both audio and video

  23. Interleave • Audio and video are often interleaved • Hence AVI • A video file looks like this: • Headers • Video chunk • Audio chunk • Video chunk • Audio chunk • Etc

  24. Compress • Raw video files are huge • A bitmap for every frame • Rarely used, even in post production • At 30 fps, that gets crazy pretty quickly • We don’t need to store every frame • Static backgrounds don’t change (much) between frames

  25. Compresssss • Intraframe • Treat each frame as a picture • Compress it (just like JPEG) • DCT (Discrete Cosine Transform) • Interframe • Store the differences between frames • Treat the pixels as a 3D array to be compressed

  26. The IPB • Three frame types: I, P & B • Intra coded pictures • A full raw frame • Predicted pictures • Based on a single reference frame • Bi-predictive pictures • Based on two or more reference frames

  27. IPBIPBIPBIPB • Reference frames may be I, P or B • P & B frames may contain a mix of image data and motion vector displacements • I frames require the most bits • Then P frames • Then B frames

  28. Bad terminology • We should really say picture • (Not frame) • Because of interlacing • Really, we encode fields not frames • Picture is the general term • And H.264 contains ‘slices’ • Sub-regions of the field • ‘Macroblocks’ & ‘Artifacts’

  29. I-Frames • Also called Key Frames • Allow easy random seeking • Twice a second for Digital TV & DVDs • More widely spaced online

  30. Seekable

  31. Seekable

  32. Oh, and audio too • We can worry less about this • Older problem, well solved • MP3 is pretty good • Who cares how it works? • Syncing is the only issue • Presentation Time Stamps (PTS) and Decode Time Stamps (DTS) in MPEG-2

  33. Flash! Woah-oh! The big question: Flash?

  34. Non-flash sites • QuickTime • Windows Media • This is gradually disappearing • Flash player is ubiquitous • Compression is good enough • Interactive too • But no 3D/VR as with QuickTime :(

  35. The Flash Player • Flash Player 6 • March 2002 • Video: Sorenson Spark (H.263) • Audio: MP3 • Or ADPCM / Uncompressed • Or Nellymoser Asao

  36. Second Generation • Flash Player 7 • August 2005 • Video: On2 TrueMotion VP6 • Audio: MP3

  37. The hot shit • Flash Player 9 (update 3) • December 2007 • Video: H.264 (MPEG-4 Part 10) • w/ container formats from MPEG-4 Part 14 • Audio: AAC (MPEG-4 Part 3) • Plus 3GPP Timed Text (MPEG-4 Part 17)

  38. TrueMotion VP6 • Proprietary • Reasonable compression • Created by On2 • Patented • Probably illegal for GPL code • YouTube uses it for lower quality and old streams

  39. H.264 • Not proprietary • Good compression • MPEG Standard • Open, but patented • Patent licenses from the MPEG LA • Unclear how this applies to (L)GPL code • But probably badly • YouTube using it for higher quality streams • iPhones and AppleTV

  40. Software • Open source transcode tools • FFmepg • libavcodec for VP6 • Probably illegal – dubious • Also pretty shoddy • Can only decode H.264

  41. More software • MEncoder • libmpcodecs uses libavcodec • VLC • libvlc uses libavcodec • So basically the same • Different muxing, same codecs

  42. Free H.264? • Unfortunately, not really • x264 is the only usable one • It’s pretty good • MEncoder can use it • Still limited in options at this point • Again, dubiously legal

  43. Non-free tools • Flash encoder • Not automatable • On2 FlixEngine • Creators of VP6 • Windows or Linux • Some support for H.264 • Rhozet Carbon Coder • The new hot shit • Good H.264 support • Windows

  44. Choices • Video codec • Resolution • Bitrate (VBR, CBR) • Keyframes • Audio codec • Channels • Bit depth • Sampling rate

  45. Doing it at scale • Not really a problem • Very easily parallelizable • Amazon EC2 is awesome here • Exactly what it was design for • Grow/shrink as needed • But, per-CPU software licensing

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