• Gamma 2.6
• 12 bit Luminance Coding
• Black level coding
• Post Production Implications
Gamma describes luminance coding function
Gamma is a form of perceptual compression – Eye sees smaller steps in dark than in light – Weights bits perceptually
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
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
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.
Barten HVS prediction
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
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
12 bits per color per pixel should be “safe” into the future.
What does code value for black mean?
code value corresponds to a required luminance. Absolutely, after considering projector contrast.
Universal coding value – Absolute meaning for all projectors and all time
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
Black code value represents darkest possible on mastering projector
In exhibition, black code value represents darkest possible in theatre
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
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.
Colour and Dynamic Range Correction – Objective – emulate the theatre environment
WYSIWYG – Reference projector – Controlled environment
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