1. Battle of the Codecs
In Digital Film and TV
James Uren
Independent Training Consultant,
Senior Lecturer Broadcasting,
Creative Director 3D Stories
Optimisation and Compression
Ravensbourne College of Design & Communication
24 November 2011
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3. Demistifying Codecs
The technologies of digital film
So many codecs...
The technology behind compression
The future...
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4. Digital Images Defined in Pixels
Standard-Definition (SDTV)
Europe: 720 x 576 pixels/frame. US/Japan: 720 x 480 pixels/frame
High-Definition (HDTV)
1920 x 1080 pixels/frame (Also called CIF (Common Image Format)). 1280 x 720 pixels/frame
Digital Film and D-cinema
Full-aperture 2K is 2048 x 1536 (gives a 1.33:1 (4:3) image)
Academy 2K is 1828 x 1080 (For a 1.85:1 image)
Full-aperture 4K is 4096 x 3072 pixels/frame
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5. Digital Image Pixel Resolutions
720 x 576
1280 x 720
1920 x 1080
2048 x 1536
4096 x 3072
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11. Film?
• So resolution unknown - is 4K enough?
• So in film we have:
• Digital Intermediate (DI) for post-production
• Digital cinema projection
• Digital Capture - the last piece of the puzzle.
12. HD Capture
CCD (Charge Coupled Device) or CMOS (Complementary
Metal-Oxide Silicon) sensor options:
CMOS now lower noise than CCDs
CMOS allows A/D & processing on-chip
CMOS can have >100 million transistors...
ARRIFLEX D21
Examples:
The ARRIFLEX D21 uses a 35mm 6 megapixel (2.9K) CMOS
imager
The SI 2K has a 2/3” 2.5 megapixel (2K) CMOS imager
SI-2K
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17. Inter-Frame Compression
• MPEG is an inter-frame
codec and forms the
backbone of many codecs
(e.g. H.264)
• Exploits commonality
between frames
• Sends one full ‘I Frame’
then predicts through the
Group of Pictures (GOP)
18. Intra-Frame Compression
• Compressing each frame independently
• Also known as ‘I Frame only’
• Examples include ProRes, DNxHD
• Best for offline editing as no GOP reconstruction required
22. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
Codec
23. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
Colour bandwidth ratios
24. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
High Quality
- the bit rate is higher - 220 Mbit/s
25. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
Pixel resolution
(HD Anamorphic)
26. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
Frame rate and
scan type
27. Example
• Apple ProRes 422 (HQ) 1440x1080 25p 48kHz
Audio sampling
frequency
28. Interlaced or Progressive?
Interlace
Each image (frame) comprises two interlaced fields
Any movement between the fields causes artefacts
Introduced for TV because it halves transmission bandwidth
Used in 50i (Europe), 59.94i (USA/Japan) SDTV/HDTV
Progressive
Each image is complete & contains all the pixels
Can produce judder motion artefact at 24p or 25p
50p gives best motion portrayal
Higher frame rates would be better still (See BBC research into 150 fps)
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29. Interlaced or Progressive?
Not straightforward: Imagers do not scan the image, neither do flat panel displays
Interlace introduces artefacts and reduces vertical resolution
What the experts say:
“All data reduction systems (such as MPEG) give better results at lower bitrates with progressive sources”
“Progressive scanning gives improved motion portrayal”
“1080i offers better static resolution, 720p better motion portrayal”
(EBU The Review No.301)
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30. Picture Impairments
Too much compression can introduce impairments:
Blocking
Mosquito Noise / Wings
Contouring
Smearing
Freezes
Breathing
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33. But...
• ...this is only if you compress to much.
• ProRes 4444 at 250Mbit/s is indistinguishable from
uncompressed...
• ...and much less data
34. D-Cinema
The DCI (Digital Cinema Initiatives) was set up in 2002
Aim was to set standards for a viable D-Cinema system
Members include Disney, Fox, MGM, Paramount, Sony Pictures Entertainment
(Columbia), Universal & Warner Bros.
Adopted JPEG2000 compression
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35. D-Cinema
JPEG2000 uses wavelet compression
JPEG2000 does not divide the picture into discrete blocks
D-cinema uses a mild level of compression to avoid artefacts (~250Mbit/s)
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37. The Future
• Requirements on compression reduces with Moore’s Law
• But this is countered by increasing resolutions and frame
rates
• Compression is here to stay, but offline and online continue
to blur
38. Round-Up
• A complete digital pipeline from capture to display
• Don’t fear compression
• Great pictures mean a high bitrate end-to-end, but too high and
your pipeline starts to get expensive... But not for long.
39. Battle of the Codecs
In Digital Film and TV
James Uren
Independent Training Consultant,
Senior Lecturer Broadcasting,
Creative Director 3D Stories
Ravensbourne College of Design & Communication