Presentation at IASA 2017 conference, Berlin, September 2017.

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Compressed Video Quality
IASA 2017
Iain Richardson
@iainvcodex

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Agenda
1. Compressed video
2. Video quality – what is it?
3. Video quality – how can we measure it?
4. Walk-throughs
5. What to remember
6. Resources

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Compressed video

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Video Source
Decompress
(Decode)
Compress
(Encode)
Video Display
Coded video
Encoder + Decoder = CODEC

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Container
Coded
video
Coded
audio
Video format
.264, .265,
VP9…
Container format
MP4, MOV, WebM, MXF…
Audio format
.aac, .ogg, .mp3…

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1990
2000
2010
Early
concepts MPEG1 MPEG2 H.264 HEVC
VC-1 VP8 VP9
MPEG4
AO
Media
FVC
2017

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Video quality

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We know it when we see it…

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The spectrum of visual quality
Perfect
(Lossless)
Awful
Visually
Lossless?
Good
enough?

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What affects visual quality? (1)
Compression ratio
Computation
Delay
Codec type
Codec version
Transmission errors
SYSTEM
ISSUES

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What affects visual quality? (2)
Detail
Motion
Masking
CONTENT
ISSUES

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Spatial masking

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What affects visual quality? (3)
Environment
Attention
Experience
HUMAN
ISSUES
Task

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Attention

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Saccades: Yarbus, 1967

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Change blindness
The “door” experiment:
https://youtu.be/VkrrVozZR2c?t=1m47s

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Source: “Directed visual attention and the dynamic
control of information flow”, Encyclopedia of Visual
Attention, 2005
The visual
world
Unlimited
Retina
>100Bn
bits/sec
Optic nerve
~3M bits/sec
Attention
<10k bits/sec
Long term
memory
<10 bits/sec
How much do we really see?

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Honoré Daumier
“Advice to a young artist”

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Measuring video quality

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Video quality
measurement
Subjective Objective

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Video quality
measurement
Subjective Objective
Goldeneye
Multiple
viewers
Absolute Comparison

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Absolute Category Rating
Bad
Poor
Fair
Good
Excellent

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Pair-wise comparison
A B

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Mean Opinion
or
Goldeneye?

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Video quality
measurement
Subjective Objective
Full
reference
Reduced
reference
No
reference
Distortion
based
Model
based
Hybrid

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Objective metrics
• Peak Signal to Noise Ratio (PSNR)
• Structural Similarity Index (SSIM)
• Just Noticeable Difference (JND)
• … and many more.

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Walk-throughs
• Encoding at multiple quality / bitrate levels
• Visual comparison
• Objective comparison
• Building a rate-distortion curve

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Rate-Distortion curves
Increasing
Quality
Increasing
Bitrate
Video A,
Codec A,
Bitrate A

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Increasing
Quality
Increasing
Bitrate
Video A,
Codec A

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Increasing
Quality
Increasing
Bitrate
Video A,
Codec A

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Increasing
Quality
Increasing
Bitrate
Video A,
Codec A
Video A,
Codec B

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Increasing
Quality
Increasing
Bitrate
Video A,
Codec A
Video A,
Codec B
Video B,
Codec A
Video B,
Codec B
VIDEO A
VIDEO B

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The trade-offs
New codecs (HEVC, VP9)
Higher bitrate
Larger file sizes
Slower encoding
Better quality originals
Old codecs (MP2, H264)
Lower bitrate
Smaller file sizes
Faster encoding
Worse quality originals

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What to remember
• ‘Perfect’ video quality doesn’t really exist
• Visually lossless may be good enough
• Video quality measurement is NOT an exact science
• There is always a trade-off
• Every vendor says that their product has the ‘best’ quality.

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Do:
Assess multiple
different videos
Assess multiple
bitrates or file sizes
Don’t:
Rely on a vendor’s
video or test
Rely on a single
quality measurement

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Resources
• twitter.com/iainvcodex : Link to these slides
• vcodex.com : Tutorials and resources
• beamshare.com : Web-based video ingest and sharing
• media.xiph.org/video/derf/ : Raw video files
• Handbrake : User-friendly video coding and decoding
• FFMpeg : Not-so-user-friendly video coding and decoding
• VLC : Multi-standard video player (among other things)
• Vooya : .Y4M comparison tool
• Command lines for testing

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FFMPEG command lines
• Encoding y4m with H.264 video codec, MP4 container, 2Mbps
bitrate, PSNR measurement:
ffmpeg -i source .y4m -codec h264 -b 2000000 destination.mp4 –psnr
(Other codecs: -codec mpeg2video , -codec hevc, -codec vp9, …)
• Decoding from coded media file (any) to y4m:
ffmpeg -i source.mp4 destination.y4m