compressed video quality
TRANSCRIPT
@iainvcodex
CompressedVideoQuality
IASA2017
IainRichardson@iainvcodex
@iainvcodex
Agenda
1. Compressedvideo2. Videoquality– whatisit?3. Videoquality– howcanwemeasureit?4. Walk-throughs5. Whattoremember6. Resources
@iainvcodex
Compressedvideo
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Video Source
Decompress(Decode)
Compress(Encode)
Video Display
Codedvideo
Encoder + Decoder = CODEC
@iainvcodex
Container
Codedvideo
Codedaudio
Video format.264, .265, VP9…
Container formatMP4, MOV, WebM, MXF…
Audio format.aac, .ogg, .mp3…
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1990
2000
2010
Earlyconcepts MPEG1 MPEG2 H.264 HEVC
VC-1 VP8 VP9
MPEG4
AOMedia
FVC
2017
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Videoquality
@iainvcodex
Weknowitwhenweseeit…
@iainvcodex
Thespectrumofvisualquality
Perfect(Lossless)
Awful
VisuallyLossless?
Goodenough?
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Whataffectsvisualquality?(1)
Compressionratio
Computation
Delay
Codectype
Codecversion
Transmissionerrors
SYSTEMISSUES
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Whataffectsvisualquality?(2)
DetailMotion
Masking
CONTENTISSUES
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Spatialmasking
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Whataffectsvisualquality?(3)
EnvironmentAttention
Experience
HUMANISSUES
Task
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Attention
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Saccades:Yarbus,1967
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Changeblindness
The “door” experiment:
https://youtu.be/VkrrVozZR2c?t=1m47s
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@iainvcodex
@iainvcodex
Source: “Directed visual attention and the dynamic control of information flow”, Encyclopedia of Visual Attention, 2005
Thevisualworld
Unlimited
Retina
>100Bnbits/sec
Opticnerve
~3Mbits/sec
Attention
<10kbits/sec
Longtermmemory
<10bits/sec
Howmuchdowereallysee?
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Honoré Daumier“Advice to a young artist”
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Measuringvideoquality
@iainvcodex
Videoqualitymeasurement
Subjective Objective
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Videoqualitymeasurement
Subjective Objective
Goldeneye Multipleviewers
Absolute Comparison
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AbsoluteCategoryRating
Bad
Poor
Fair
Good
Excellent
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Pair-wisecomparison
A B
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MeanOpinionor
Goldeneye?
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Videoqualitymeasurement
Subjective Objective
Fullreference
Reducedreference
Noreference
Distortionbased
Modelbased Hybrid
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Objectivemetrics
• PeakSignaltoNoiseRatio(PSNR)• StructuralSimilarityIndex(SSIM)• JustNoticeableDifference(JND)• … andmanymore.
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Walk-throughs
• Encodingatmultiplequality/bitratelevels• Visualcomparison• Objectivecomparison• Buildingarate-distortioncurve
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Rate-DistortioncurvesIncreasing
Quality
IncreasingBitrate
Video A,Codec A,Bitrate A
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IncreasingQuality
IncreasingBitrate
Video A,Codec A
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IncreasingQuality
IncreasingBitrate
Video A,Codec A
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IncreasingQuality
IncreasingBitrate
Video A,Codec A
Video A,Codec B
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IncreasingQuality
IncreasingBitrate
Video A,Codec A
Video A,Codec B
Video B,Codec A
Video B,Codec B
VIDEO A
VIDEO B
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Thetrade-offs
Newcodecs(HEVC,VP9)HigherbitrateLargerfilesizesSlowerencodingBetterqualityoriginals
Oldcodecs(MP2,H264)LowerbitrateSmallerfilesizesFasterencodingWorsequalityoriginals
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Whattoremember
• ‘Perfect’videoqualitydoesn’treallyexist• Visuallylosslessmaybegoodenough• VideoqualitymeasurementisNOTanexactscience• Thereisalwaysatrade-off• Everyvendorsaysthattheirproducthasthe‘best’quality.
@iainvcodex
Do:
Assessmultipledifferentvideos
Assessmultiplebitratesorfilesizes
Don’t:
Relyonavendor’svideoortest
Relyonasinglequalitymeasurement
@iainvcodex
@iainvcodex
Resources
• twitter.com/iainvcodex :Linktotheseslides• vcodex.com :Tutorialsandresources• beamshare.com :Web-basedvideoingestandsharing• media.xiph.org/video/derf/:Rawvideofiles• Handbrake : User-friendlyvideocodinganddecoding• FFMpeg : Not-so-user-friendlyvideocodinganddecoding• VLC : Multi-standardvideoplayer(amongotherthings)• Vooya :.Y4Mcomparisontool• Commandlinesfortesting
@iainvcodex
FFMPEGcommandlines
• Encodingy4mwithH.264videocodec,MP4container,2Mbpsbitrate,PSNRmeasurement:
ffmpeg -i source.y4m-codech264 -b2000000destination.mp4 –psnr
(Othercodecs:-codecmpeg2video,-codechevc,-codecvp9,…)
• Decodingfromcodedmediafile(any)toy4m:ffmpeg -i source.mp4destination.y4m