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Flat Panel Display - October 2005

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Improving LCD TV Color using Multi-Primary

Technology

Moshe Ben-ChorinGenoa Color Technologies


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Film and HDTV (“Rec. 709”)Gamuts

Deep Yellow

CrimsonViolet

Turquoise

The Need for Better Color


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Agenda

• Wide color gamut displays

• The multi-primary concept

• Color Gamut and luminance

• Gamut structure and and optimization

• Different LCD implementations

• Image quality and perception

• Video data production and transfer

• Color Processing


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Increased RGB Gamut Effect

RGB displays (CCFL backlight)

Color gamut increase

72% NTSC ~100% NTSC

Decrease of luminance by 40%


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The RGB triangle can be expanded

However, it is still a triangle

Therefore, it cannot contain both yellow and turquoise regions

RGB Gamut is Triangular

Coverage of cyanspoor yellows

Coverage of yellows poor cyans


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The Multi-Primary Concept

• Adding more primaries to RGB (usually yellow and cyan) allows:

• Better utilization of the backlight spectrum• Boost of luminance using spectral overlap of color

filters• Enhancing color gamut in perception-sensitive area• Flexibility in the chromaticities and relative

luminances of primaries, due to the additional degrees of freedom


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Gamut and LuminanceSpectral Overlap

wavelength

Tran

smis

sion 100%

33 %

wavelength

33 %40 % 40 %

60 %

B/3 G/3 R/3

2B/5 3G/5 2R/5


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Multi-Primary vs. RGB

Adding yellow (and cyan) allows flexibility in the design of the color gamut, thus it may fit better the color gamut of film


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Gamut Structure

0.21

0.72

0.07

0.10

0.50

0.20

0.13

0.07

The red and the green primaries in the multi-primary display have a lower relative luminance with respect to that of the red and the green in the RGB gamut (Rec. 709)


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Red and Green Luminance (1)

Green 0.72

Red 0. 21

• The optimal object color solid describes all possible colors of objects having all possible reflectance spectra under a certain illumination

• Rec. 709 points have relative luminances, which correspond to ~100% reflectance of the relevant spectraBlue 0.07


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Red and Green Luminance (2)• In practice

• Therefore, green and blue may have much lower luminance than the Rec. 709 requirement

• Red reflectance near 100% in the pass band implies that required luminance may be close to that of Rec. 709

The reflectance of green and blue is much lower than 100%

Only the red reflectance is close to 100%


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• The green luminance in the RGB display is high because it is needed in order to produce bright yellow

• In practice, the green itself should not be so bright

• In the multi-primary display there is a yellow primary, and the green need not to be so bright

• The green primary can be made more saturated (and less bright) increasing the color gamut

• The red can also be changed to increase the gamut, but there is a compromise between the shift of the red and its relative luminance

Red and Green Luminance (3)


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LCD Example

• Five – primary LCD (RGBYC)

• CCFL backlight• Five color filters

• Color gamut – 95% NTSC• RGB reference 72%

• Luminance• Same as RGB

reference


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LED Backlight Example

• Five–primary LCD (RGBYC)

• Five color filters • RGB LED backlight

• Color gamut –extended in yellow, cyan and purples

• Luminance• 25% more than

RGB LED reference

The increased efficiency translate to reduced LED cost


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Multi-Primary Gamut Optimization

• Multi-primary display allows flexibility in the choice of color gamut

• What are the design considerations ?• What color gamut we would like to have ?• What can we achieve with the technology ?• How to set the compromise ?

• What are the tools for the design ?


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Requirements &

perceptual performance

Design considerations

Display parameters

backlightpossible color filtersLCD transmissionsub-pixel arrangementaperture ratio

Technology (LCD, projection)

Reference color gamuts

What color gamut we would like to have ?

Color gamut data

RGB displaysfilm“natural” objectsother multi-primary displays

Perceptual requirements

luminance & white point color gamut vs. luminanceRelative intensities of primaries

What can we achieve with the technology ?How to set the compromise ?


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Design Tools

• Genoa Designer

• Accepts optical parameters of the display and calculate primaries• Easy control of parameters• Allows calculation of tolerances • For given tolerances, calculate yields based on pass/fail criteria

• 3D Gamut viewer

• Compare 3D gamut of different display and references (e.g. film)

• Appearance Simulator

• Wide gamut multi-primary rear projection • Display images and video as they would appear on a display with

given primaries


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Multi-Primary Displays

• Multi-primary displays have

• increased color gamut• no loss of luminance• Tailored appearance according to

requirements and performance

• This results in better image quality


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Perception Test

Erno. H.A. LangendijkStefan SwinkelsPhilips Research Laboratories, Eindhoven

Dan EliavItay BaruchiGenoa Color Technologies


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Goals

• Compare image quality of multi-primary display and wide gamut RGB display

• Test image quality and preference of various primary configurations

• Examine the perceptual effect of red chromaticity and luminance

• Examine different choices of primaries in the cyan- blue region


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• Test preference of image on different TV sets

• Four displays, directed towards the viewer• 3 Genoa multi-primary sets (with different primaries)• 1 commercial RGB set (Philips Cineos)

• White point for all sets is adjusted to 7500K, 450 cd/m2.

Displays Setup


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Displays Gamut

Rec. 709 Wide Gamut RGB Genoa Multi Primary


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• Subjects: • 22-27 per experiment

• Task: • Rank the displays in order of preference and write down

comments

• Image material:• 5-17 different scenes per experiment

• Native HD (720p, 1280x720, 60Hz) video streams• SD (from DVD) converted to 720p• Still images/video frames (1280x720)

• Ranking• Results analyzed statistically per scene, and over all

scenes• Ranking converted via paired comparison to quality scale

Experimental Design


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Test Scenes


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• Least preferred is set to “0” (most left position)

• More preferred is to the right

• Distance between two scale values reflects how many subjects (%) prefer one display above the other

• Difference of…Corresponds to…0.5 –> 60 – 70%1 –> 80 – 90%2 –> 95 – 100%

• Green underline indicates statistically significant difference

• Red underline indicates statistically non-significant difference

How to read the quality scale

A

BC


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Red (1)• Test preference of red primary • Three multi-primary sets with different red

primaries and one RGB set• Red primaries differ in hue and relative luminance

6.80.3020.674MP3

7.80.3170.668MP2

11.50.3350.646MP1

190.3290.638RGB

Y/YW

(%)yx

ChromaticityMP1MP2

MP3

RGB


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Non significant

Quality Scale

Red (2)

Ranking results:• MP2 is most preferred • The color of MP1 and RGB although bright is

not red enough, MP3 is too dark

RGB MP2MP3

1 20Quality Scale

All Subjects/All Scenes

1 20

Significant

MP1RGB MP2MP3MP1 All Subjects/

Red Scene


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Blue-Cyan (1)

Test design:• Test preference of Blue-Cyan primary combinations

MP1 saturated blue cyan

MP2 more coverage on purpleshigh cyanless saturated blues

MP3 more coverage on purpleslow cyansaturated blue


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Non significant

• Ranking results:• All three multi-primary sets are preferred over RGB• Clear difference in blue and cyan colors

• MP3 is most preferred set, but its advantage over MP1 and MP2 is rather small

Significant

Blue-Cyan (2)

RGB

MP2

MP3MP1

1 20Quality Scale

All Subjects/Scenes


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Perception Test –Conclusions

• Multi-primary displays are preferred over wide gamut RGB

• The red chromaticity-luminance relationship has been investigated

• Deeper red with lower luminance is preferred over a Rec. 709 “red”

• Extension of color gamut in the cyan region has major impact on image quality


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Video Data


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Video Data for Wide Color Gamut

• Current production and data transfer process is RGB tuned

• The RGB is defined with respect to a CRT gamut (Rec. 709)

• What are the implications for wide gamut displays ?

• Is there a way to produce wide gamut data ?

• Is there a way to transfer wide gamut data ?

• Is there available wide gamut data (outside Rec. 709 gamut) in DVD and broadcast material ?


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Is there a way to produce wide gamut data?


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RGB Camera and Color Data

• Do cameras have “restricted gamut” ?

• Cameras usually have three color sensors: red, green and blue

• Most digital cameras are designed to show their output on a Rec. 709 color display

• All colors may be captured using three color sensor device

• humans use three-sensor device (the eye) to view all colors

• Camera would capture monochromatic light (lasers) the color of which is outside the Rec. 709 gamut

• Three sensor photometers provide XYZ data covering the whole gamut of visible colors, using physical filters


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The Opponent Colors Model

L

Red - Green signal

M

SBlue - Yellow signal

Luminance signal


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Fidelity

• For color fidelity• Camera sensitivity curves

should be linear combinations of x(λ), y(λ), z(λ) color matching functions

• Those sensitivity curves would have negative lobes

• Matrixing• Negative lobes may be

produced by multiplying the RGB sensors output by a suitable matrix


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Is There a Way to Produce Wide Gamut Data?

• Cameras capture most of the visible spectrum

• Perfect fidelity implies one-to-one correspondence between HVS “signals” and camera output

• This may be achieved by suitable spectral responsivityof camera sensors and matrixing

• In practice perfect fidelity is difficult, but reasonable accuracy may be obtained on Rec. 709 gamut and its vicinity

• After matrixing negative RGB signals exist. If clipped to zero, data representing colors outside Rec. 709 gamut is lost

• Data transfer and processing must support negative RGB values


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Is there a way to transfer wide gamut data?

Is there any color data in DVD and broadcast material outside Rec 709 triangle?


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Wide Gamut Data• DVD and digital broadcast use YCbCr color space

• RGB color space occupies 25% from YCC color space

• Points in the YCC space outside the RGB box have at least one negative R, G or B value

Y

Cb

Cr

W

K B

Yl

G

M

C

R

YC

C c

olor

spac

e

100%

25%

67%

85%

inside D65 object color solid

non valid

RGB colors


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Material Encoding

• In the encoding process, the color information is usually compressed into the reference triangle (rarely clipped)

• YCC space (DVD & broadcast) may contain color information beyond Rec. 709

Data analysis of a video frame

• The extended color information is clipped out by the TV after the transformation to RGB


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Wide Gamut Standards• Color definition in the TV world

• YCC RGB transformation• chromaticity of RGB primaries and white point (or XYZ of

primaries)• Non-linear transfer function (gamma)

• Two types of wide gamut standards

• Only positive RGB values • Gamut limited to primaries triangle• Examples:

• sRGB, Rec. 709, EBU• Adobe98, SMPTE240 (wide gamut)

• Positive and negative RGB values (usually with respect to REC. 709 primaries)• Unlimited color gamut, number of bits important • Examples:

• e-sRGB (known also as sRGB64)• Rec. 1361


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Color Processing


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Processed image

Improving Content by Image Processing

• 1080P Analogy:

Real world object 1080P Display

Color processing improves image quality of limited gamut data if it is done carefully


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Color Processing Challenges• Compatibility with existing video data flow and content requires

conversion of YCC or RGB input to multi-primary output signals

• What are the challenges?

• 3 many primaries transformation has infinite solutions

• No relationship between device dependent RGB input and physical primaries of the multi-primary display

• RGB color gamut (input) and multi-primary display gamut (output) are different

• Use the wide gamut to improve image quality

• Allow different look and feel for different contents

• Support for different video standards

• Real time, cost-effective solution


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Device Dependent Color

• The common data flow is device dependent (RGB/YCC) • Very simple, does not require much processing• Assumes that all displays have similar primaries

• In this method, colors on each TV will look different:

25500

CRT WG DLPPlasma


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Device Dependent Color Mapping

In device dependent mapping, RGB color data is not processed

In displays with primaries different from Rec. 709, hue and saturation of colors will change

The resulting image appearance is not natural, especially in skin tones

Rec. 709LED backlight

Hi’

Hi

SiSi

’


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a

cd

b

Material Encoding

a1

c1d1

b1

• Cameras capture colors outside the gamut of Rec. 709 displays

• Wide gamut display may show the color differentiation without compression

• On compressed data, processing may be required to restore original appearance

• To keep color differentiation between points on Rec. 709 edge and points outside Rec. 709, color data may be compressed


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Genoa Color Mapping

• Color mapping is performed:

• According to Genoa know-how in color reproduction, based on extensive research and profound experience

• In a flexible way, allowing consideration of customer preference and adaptation to diversity of content

• In a device independent color space, thus: • placing device dependent RGB input on absolute

reference to which multi-primary color is aligned• creating an inherent match to advanced video

standards, which relate RGB to colorimetric space


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Principles of Gamut Mapping

• Utilize the full extent of the color space volume

• Protect integrity of neutral, natural and other memory colors

• Color data external to the device gamut is re-mapped inside, with the following considerations:• Good differentiation between colors• Smooth and coherent transitions• Good reproduction of out-of-gamut color points

• Create a smashing image!


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Genoa Compatibility with Color Standards

• The color processing converts the RGB/YCC input to color independent space

•

• This conversion has programmable YCC RGB transformation, primaries and gamma behavior

• The conversion supports negative RGB signals as required by some standards

• Input up to 10 bit per channel

• This is compatible with many advanced video standards


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RGB to Multi-primary conversion

• The KESHET™ ASIC from Genoa provides color conversion from RGB/YCC input to multi-primary signals

• The conversion is performed at real time for resolutions up to 1080p

• The conversion support different video standards

• The conversion includes advanced gamut mapping


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Conclusions

• Multi-primary gamut enables better color reproduction compared to RGB gamut

• Some extended color information is found in video signals. Advanced standards will support it in the future

• Color enhancement of compressed data improves image quality

• Genoa processing reproduces well all types of content, and in most cases creates a visible advantage over RGB

• Genoa ASIC and algorithm support many wide gamut video standards

• Using Genoa technology consumers can enjoy the benefits of improved viewing experience today


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Thanks

• Everyone in Genoa Color Technologies for their participation and enthusiasm in developing the multi-primary technology

• Special thanks to Dr. Shmuel Roth and Dan Eliav for their help in the preparation of this presentation

• And again thanks to Dr. Erno H.A. Langendijkand Stefan Swinkels from Philips Research Labs for perception test

Improving LCD TV Color using Multi-Primary Technologygenoacolor.com/Media/DBImages/fpd-05_PC2_Ben_Chorin.pdf · Improving LCD TV Color using Multi-Primary Technology ... prefer one

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