DCI Requirements Image - Dynamics Matt Cowan Entertainment Technology Consultants www.etconsult.com
Gamma 2.6 12 bit Luminance Coding Black level coding Post Production Implications
Measurement Processes How to tell if your ass is too small
Gamma Gamma describes luminance coding function Lum = CV Gamma is a form of perceptual compression λ Eye sees smaller steps in dark than in light Weights bits perceptually
2.6 Gamma Curve Output luminance (0-1) 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 20 40 60 80 100 Input code value (0-100) 20% of code values for 1.5% of luminance
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
Why Gamma 2.6? 10 bit gamma 2.6 Steps visible Barten HVS prediction 12 bit gamma 2.6 10 bit log (step.009) 12 bit log (step.0027) Steps not visible Cinema operation range
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? 10 bit gamma 2.6 Steps visible Barten HVS prediction 12 bit gamma 2.6 10 bit log (step.009) 12 bit log (step.0027) Steps not visible Cinema operation range
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? 2. Mastering projector black? 3. Some arbitrary dark luminance level?
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
Difference in Encoding Absolute vs Projector Black - Bottom End 1000 Luminance output 100 Projector Absolute 10 10 100 1000 Input CV
Transfer Function Absolute Black - Available Range Light Output 1000 100 10 Master 2000:1 1000:1 projector 4000:1 projector 10 100 1000 Input CV Blackest possible
Transfer Function Output Luminance 1000 100 10 Master 2000:1 1000:1 4000:1 Projector Black 10 100 1000 Input CV Information lost in absolute black Blacks lost in absolute black
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 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
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