Gray Point (A Plea to Forget About White Point)

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1 HPA Technology Retreat Indian Wells, California Gray Point (A Plea to Forget About White Point) George Joblove 2016 HPA Technology Retreat Indian Wells, California George Joblove rev. g 1

High dynamic range (HDR) Dynamic-range limitations of traditional imaging systems Imaging systems are not able to reproduce the full dynamic range of the real world The dynamic range of cameras is

2 High dynamic range (HDR) Dynamic-range limitations of traditional imaging systems Imaging systems are not able to reproduce the full dynamic range of the real world The dynamic range of cameras is limited The dynamic range of displays is typically even more limited High dynamic range allows the preservation of contrast (black levels and white levels) and the simultaneous preservation of shadow detail and highlight detail PHOTO: GEORGE JOBLOVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 2

3 Issues surrounding high dynamic range Multiple masters Metadata White point PHOTO: GEORGE JOBLOVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 3

The problem We are now facing a world with a variety of displays and projectors with different dynamic ranges The widespread assumption is that it is

4 The problem We are now facing a world with a variety of displays and projectors with different dynamic ranges The widespread assumption is that it is necessary to create a separate color grade for each case PHOTO: GEORGE JOBLOVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 4

5 The problem The root of the problem is the fixation on white points and black points This approach is based on the perspective of video engineering, where the image is represented by a signal with a fixed range (e.g., 0 to 1) With HDR, this assumption yields 1 complications, complexity, and confusion HPA Technology Retreat Indian Wells, California George Joblove rev. g 5

6 The problem Focusing on white point worked when all displays had the same dynamic-range limitations HDR displays have higher, but differing, peak levels (4000 cd/m 2, 10,000 cd/m 2, etc.) (and also differing black levels) How to properly render an image on displays with differing white points (and black points)? Communication of information about mastering conditions (metadata) to the display device? HPA Technology Retreat Indian Wells, California George Joblove rev. g 6

7 Gray point The solution is to focus on gray point Cinematographic approach Use mid-gray as the datum rather than black and white Mid-gray is at the center of the tone scale Each display then scales the of the image to place this reference gray at that display s gray point, and rolls off highlights and shadows to the display s limits log image camera peak white display peak white display black camera black log scene 0 = mid-gray exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 7

8 Gray point Notes on this schematic: Log-log scale The use of linear scales has been part of the problem In log terms, no such thing as absolute black Scale of values here is arbitrary (unspecified) Curves may be assumed to include rendering and grading log image camera peak white display peak white display black camera black log scene 0 = mid-gray exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 8

9 Precedent and experience There is ample precedent for this approach A century of photographic and cinematographic experience Camera film and sensors compress (and clip) dynamic range of scene Film and digital prints further compress captured dynamic range log image camera peak white display peak white display black camera black log scene 0 = mid-gray exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 9

Mid-gray Motion-picture imaging has always been based on the concept

in stops (a logarithmic measure of and exposure) Mid-gray, 18% gray,

10 Mid-gray Motion-picture imaging has always been based on the concept of a mid-gray reference, on a log scale Equidistant steps, measured in stops (a logarithmic measure of and exposure) Mid-gray, 18% gray, LAD gray (Half-stop increments) HPA Technology Retreat Indian Wells, California George Joblove rev. g 10

11 Mid-gray Motion-picture imaging has always been based on the concept of a mid-gray reference, on a log scale The cinematographer employs lighting and controls exposure to capture the scene within the limits of the film s or sensor s dynamic range Film and sensor sensitivity measurement is keyed to 18% gray, to place the exposure of the mean scene at the mean of the film s or sensor s dynamic range log image camera peak white display peak white display black camera black log scene 0 = mid-gray exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 11

Mid-gray Motion-picture imaging has always been based on the concept of a mid-gray reference, on a log scale Exposure based on metering of 18% reflectance gray card Camera negative film reproduces

12 Mid-gray Motion-picture imaging has always been based on the concept of a mid-gray reference, on a log scale Exposure based on metering of 18% reflectance gray card Camera negative film reproduces gray-card exposure at a standardized density (LAD gray) LAD gray is reproduced at a standard density on the film print (about 1.0, or 10% of peak white) log image camera peak white display peak white display black camera black log scene 0 = mid-gray exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 12

13 Mid-gray The system continues to work in the digital domain For normal exposure, a properly-exposed gray card results in digital values that are displayed at about 10% of peak white 5 cd/m 2 for cinema projection 10 cd/m 2 for video display camera peak white display peak white log image log scene 0 = mid-gray display black camera black exact reproduction as captured by camera as reproduced by display HPA Technology Retreat Indian Wells, California George Joblove rev. g 13

14 Ansel Adams s Zone System This approach has an analogue in still photography The limited dynamic range of displays and projectors compared to cameras is analogous to that of photographic prints compared to film negatives ANSEL ADAMS, THE NEGATIVE ANSEL ADAMS, THE NEGATIVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 14

15 Ansel Adams s Zone System The photographer Ansel Adams formulated the Zone System An approach to dealing with limited dynamic range of film compared to scene, and limited dynamic range of photographic print compared to original negative ANSEL ADAMS, THE NEGATIVE ANSEL ADAMS, THE NEGATIVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 15

corresponds to a one-stop variation in (or exposure) ANSEL ADAMS, THE NEGATIVE ANSEL ADAMS,

16 Ansel Adams s Zone System Luminance range of original scene considered as eleven zones 0 through X, where V represents to yield a mid-gray in the print, and each zone step corresponds to a one-stop variation in (or exposure) ANSEL ADAMS, THE NEGATIVE ANSEL ADAMS, THE NEGATIVE HPA Technology Retreat Indian Wells, California George Joblove rev. g 16

17 Gray point Proposal: The only metadata that needs to be provided is mid-gray Each display device should then scale the input to place mid-gray at that device s appropriate gray point, and roll off highlights and shadows to fit within the device s dynamic range log image display A peak white display B peak white display B black display A black log mastered display 0 = mid-gray graded value as reproduced by display A as reproduced by display B HPA Technology Retreat Indian Wells, California George Joblove rev. g 17

18 Thank you George Joblove Prima Lumina Consulting george (at) joblove.com HPA Technology Retreat Indian Wells, California George Joblove rev. g 18

High Dynamic Range Imaging

High Dynamic Range Imaging 1 2 Lecture Topic Discuss the limits of the dynamic range in current imaging and display technology Solutions 1. High Dynamic Range (HDR) Imaging Able to image a larger dynamic