MovieLabs Proposals: An Extended Dynamic Range EOTF Scott Miller, Dolby Labs

What is an EOTF and why is it so important? EOTF stands for Electro-Optical Transfer Function It describes how to turn digital code words into visible light The image on the reference display is what truly defines the signal An artist makes creative decisions on what the content will look like while viewing a reference display Could be sitting at a grading desk in a post production suite Could be sitting in front of camera control units at a live production

Why is a new EOTF needed? Ideally, the EOTF should be defined by the human visual response – but our current system is defined by this: Current “Gamma” nonlinearity based on CRT physics Finally standardized by ITU-R Rec. BT.1886 It is a sort of perceptual EOTF – but only at relatively low luminance levels and small dynamic ranges But higher dynamic ranges are just around the corner

Building a new EOTF Detailed user preference studies showed an absolute dynamic range of 0 to 10,000 cd/m2 satisfied most viewers’ desires Assume practical system will need to be a maximum of 12 bits Due to current infrastructure and silicon constraints Use human visual system to determine performance we need

Contrast Step Curves PQ: Most efficient use of bits throughout entire range PQ is based on Barten CSF model, but: Modeled with an invertible function Parameters optimized for 12bit extended color volume

Barten Contrast Sensitivity Function (CSF)

Perceptual Quantizer (PQ) EOTF

Doesn’t a 10,000 cd/m2 system “waste” a lot of code words that we can’t use yet The short answer is “no” The logarithmic shape of PQ at the top end allows substantial gains in peak brightness without costing huge numbers of code words Think of it as headroom for further expansion PQ headroom from 5000 to 10,000 cd/m2 = 7% of code space Headroom above white in traditional SMPTE range gamma = 8% of code space The midpoint of the code range for PQ represents about 93 cd/m2 If we were using a traditional gamma function, headroom waste would be a huge problem

Gamma 2.4 Code Words

Perceptual Quantizer (PQ) Code Words

Conclusions We need a standardized EOTF to define a new system for extended dynamic range PQ is the most efficient way to encode extended dynamic range Each code value corresponds to just under a perceptual step - less wasted codes A 10,000 cd/m2 system makes a lot of sense The extra dynamic range is appreciated, and preferred by viewers It gives the system headroom for display improvements in the future No more bits are required compared to ~1000 cd/m2 systems

Standardization This EOTF has many potential use cases for baseband and compressed video Similar to the many traditional uses of gamma MovieLabs proposal: 10,000 cd/m2 peak, XYZ color primaries, 12bit PQ But the EOTF could also be used with other color spaces SMPTE 10E Drafting Group working on perceptual EOTF MPEG looking at signaling for perceptual EOTF in HEVC VUI metadata SMPTE likely starting a High Dynamic Range / Wide Color Gamut study group in 10E as well