Moving PicturesImplementing Video on Flickr
Cal Henderson
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Hello
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Flickr
• Large scale kitten sharing website
• Started 2003, launched 2004– 5 years old this december
• Almost 3 billion photos
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Enter: Video
• Video was added this year– Launched April 2008
• Many hundreds of thousands of videos uploaded
• Many millions of playbacks
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“Video? That’s just like photos!”
-Me, before Flickr Video
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12 4 Steps
• 4 main tasks– Uploading– Transcoding– Storage– Serving & Playback
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1. Upload
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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
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Issues
• Two components for uploading:– Sending from the client– Receiving on the server
• Both of these present problems
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Sending from the client
• Multiple options– Simple form– Flash– Desktop app
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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
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Flash
• Pros– Upload progress is easy– ‘Fast’– Multi select of files
• Cons– Harder to implement– Flash isn’t quite ubiquitous
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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)
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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
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Making Progress
Browser
Web 1
Web 2
1. Browser starts upload
2. Web server broadcastsprogress via UDP
3. Browser queries progressvia AJX call
Loadbalancer
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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)
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Receiving on the server
• Or, use a buffering layer– Perlbal is great for this
• Or a lightweight Apache– E.g. w/ mod_proxy
Browser Buffer ServerSlow Fast
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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!
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2. Transcode
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Transcode?
• Why transcode at all?
• Input comes from many sources– Point & shoots– DV Cams– Mobile devices– Video editing software
• All in different formats
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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
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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
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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
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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
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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
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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
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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
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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’
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I-Frames
• Also called Key Frames
• Allow easy random seeking
• Twice a second for Digital TV & DVDs
• More widely spaced online
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Seekable
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Seekable
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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
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Flash! Woah-oh!
The big question:
Flash?
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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 :(
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The Flash Player
• Flash Player 6– March 2002
• Video: Sorenson Spark (H.263)• Audio: MP3
– Or ADPCM / Uncompressed– Or Nellymoser Asao
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Second Generation
• Flash Player 7– August 2005
• Video: On2 TrueMotion VP6• Audio: MP3
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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)
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TrueMotion VP6
• Proprietary• Reasonable compression• Created by On2
– Patented– Probably illegal for GPL code
• YouTube uses it for lower quality and old streams
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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
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Software
• Open source transcode tools
• FFmepg– libavcodec for VP6– Probably illegal – dubious– Also pretty shoddy– Can only decode H.264
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More software
• MEncoder– libmpcodecs uses libavcodec
• VLC– libvlc uses libavcodec
• So basically the same– Different muxing, same codecs
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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
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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
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Choices
• Video codec• Resolution• Bitrate (VBR, CBR)• Keyframes• Audio codec
– Channels– Bit depth– Sampling rate
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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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3.Store
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Easy!
• Really, just like photos– But with bigger files
• Same disk layout as any other serving
• But the serving part is slower
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But..
• Remember the files are huge
• Operations take time– More likely to fail halfway through– Checksums are your friend
• Do it all asynchronously
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4. Serve & Playback
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The choice
Streamingvs
Progressive
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Streaming
• Pros– Easily seekable– Live feeds
• Cons– Special server software– Slower to start– Firewall troubles
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Progressive download
• Pros– Just use a web server– Play offline– Firewall/proxy friendly
• Cons– Harder to seek ahead (but not impossible)
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Streaming tech
• Non flash stuff– We’ll ignore that– You’re using flash, right?
• RTMP– Real-time Messaging Protocol– Proprietary (thanks Adobe!)
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RTMP
• RTMP - Raw TCP socket stuffs• RTMPT – RTMP tunneled over HHTP
– For firewalls, etc
• Flash Media Server– previously Flash Communication Server
• Wowza Pro– $1000/server
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Open source
• It’s not all bad
• Red5– Java implementation of RTMP server– Mostly feature complete– Beta quality, but usable in production– Facebook
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Progressive
• Used by the majority of large sites
• Very simple!
• Seekable with server support
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Seeking
• Serve the FLV starting at a different point
• Just add a simple FLV preamble before seeking into the file
• Simple to do in PHP, Perl, etc• mod_flvx for Apache• mod_secdownload for lighttpd
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5. Other considerations
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Review
• Videos are slow• Expensive to review
• Review grids– Doesn’t cover audio
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Not enough?
• Social tools are useful here
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Summary
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Summing up
• Flash makes sense– For uploading too
• H.264 is probably your best bet today• Transcoding software still costs money
– Unless you’re willing to take on the risk• Progressive download is basically
awesome
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The end!
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Awesome!
These slides are available online:iamcal.com/talks/