Black, White and in between

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April 28, 2016
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May 3, 2016
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Black, White and in between

• Gamma 2.6

• 12 bit Luminance Coding

• Black level coding

• Post Production Implications

Measurement Processes

Gamma

Gamma describes luminance coding function

Gamma is a form of perceptual compression – Eye sees smaller steps in dark than in light – Weights bits perceptually

Why Gamma 2.6?

The Human Visual System is more sensitive to absolute luminance changes in dark images than bright images.

Requires finer luminance steps in dark than in bright

Power law function with exponent of 2.6 is a good fit to human visual system

Gamma in Numbers

SMPTE (TV) specifies encoding at gamma 0.45 (=inverse of gamma 2.2)

CRT displays naturally provide gamma= 2.5 (if black is correctly set)

Most efficient value where gamma coding is inverse of perception

Luminance Coding

DCI identifies 12 bits per color per pixel

Result of luminance coding experiment – Experiment objective: To determine the threshold of distinction for human subjects – “how many bits do you need so that a human can’t see any stepping or contours.

How many bits?

Barten HVS prediction

Experiment

Conditions:

Test for theatrical luminance levels

32 cd/m^2, 3.0 cd/m^2, 0.27 cd/m^2 – “worst case luminance pattern

Vertical and horizontal repeating lines

Test ~ 50 subjects – Forced choice – Statistical analysis

Results:

Many of subjects could distinguish 2 counts in 12 bits (=11 bits)

Almost all subjects could distinguish 4 counts in 12 bits (=10 bits) – Note that the patterns were noise free – a small amount of noise will reduce discrimination

Conclusion

12 bits per color per pixel should be “safe” into the future.

Encoding Black

Question:

What does code value for black mean?

1. Absolute black?

Luminance

code value corresponds to a required luminance. Absolutely, after considering projector contrast.

Benefits

Universal coding value – Absolute meaning for all projectors and all time

Difficulties with Absolute Black

Discontinuity formed in transfer function – Causes color shifts in dark areas

Details lost on lower C/R projectors

No visual advantage from higher C/R theatre projector

Mastering Projector Black

Black code value represents darkest possible on mastering projector

In exhibition, black code value represents darkest possible in theatre

Tone Mapping

Can fix these issues – Complex perceptual considerations necessary in developing algorithm

Lots of ways to do it wrong – Potential to cause unwanted distortions in look of image

Would need to be standardized

Projector Black = “Nature’s Tone Mapping”

Code values map to projector’s modulator and provide incremental luminance

Added to the light is a constant of the leakage from contrast of projector and screening room.

Provides very graceful degradation to both higher and lower contrast projectors.

Conclusions:

  • Power Law with Gamma 2.6 matches the eye response time more closely than log or linear
  • 12 bits per color (gamma encoded) is necessary to mask transitions induced by quantization in all potential situations
  • Natural tone mapping gives a better visual result than attempting to reproduce exact luminance values near black

Post Production Implications

Colour and Dynamic Range Correction – Objective – emulate the theatre environment

WYSIWYG – Reference projector – Controlled environment

Post Dimensions

Brightness – 14 fL peak white – Equivalent to 16 fL open gate

Resolution – 2048 x 1080

Contrast ratio – Min 2000:1 on to off

Gamma function – Pure exponential gamma = 2.6

Color calibration – 3 primary, DCI color space

Stray light – Minimized

Francisco Hernandez
Francisco Hernandez is the CEO & Founder for Multimedia Tech, an audio, video, automation and IT business specializing in home theater installations, TV installations, home automation and IT systems. Francisco directs business strategies, relations and operations. With nearly 20 years experience, Francisco started as an entrepreneur at the early age of 16 owning and expanding from basic start-ups to small business enterprises.
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