色彩管理 by insight ewpg

色彩管理

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Outline  人眼結構  色外貌（Color Appearance）  Visual Experiments  Color Appearance Model  色彩管理系統（Color Management System）  Cross Media Color Calibration  ICC Profile  Color Calibration Model  Gamut Mapping

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人眼結構

眼球接受外界光線刺激，並適當地轉換成神經訊號電位，再送往更高層的側膝核與皮質區做進一步的處理 

人眼結構

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人類的眼球基本上是個三層膜、兩個房的構造，外部有六條控制眼動的肌肉附著於上，並包附著厚厚的保護性脂肪層，使得眼球能安全地在眼窩裡轉動。六條控制眼球運動的動眼肌肉

人眼結構

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Eye & Camera

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非相關色與相關（Unrelated or Related）

Unrelated Colors and Related Colors

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Related & Unrelated Colors

相關色(related colour)：觀看時受到其他顏色影響的色彩，為相關色，在光照情況下所有的色彩均是。非相關色(unrelated colour)：觀看時不受任何其他顏色影響的色彩，為相關色，必定為黑暗中單獨的自發光色。

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Induction  Viewing two colors at the same time influences both of their appearances. The following is an example of induction, a variation of simultaneous contrast.  Instructions: In the top half of the diagram below are two identical dark gray patches on a like background. The bottom half of the diagram shows the same dark gray patches on different backgrounds. You will see that they appear different because of their surroundings.

亮度對比

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Induction  The same phenomenon occurs when colors are used. Note that identical yellow patches in the middle of different colors appear to take on the characteristics of their surroundings.

彩度對比

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Color Contrast  並置或重疊兩種或更多色彩，會產生同時對比現象，此現象由側向抑制（Lateral inhibition）所造成。  側向抑制（Lateral inhibition）色光強度訊息經由神經脈衝傳向大腦，第二個受器的活動抑制第一個受器的作用，一部份脈衝轉向水平方向的神經細胞，對鄰近視網膜上的細胞產生負效果，稱為「側向抑制」

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色彩對比

不同排列時，呈現不同色貌

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Borders and Framing Color ď ś Colors often appear brighter and more vibrant when they are bordered by frames. Black lines are commonly used to enhance colors in applications like stained glass. This tactic creates a certain effect, as shown below, and prevents color clashing. Notice that the drawing on the left colors appear significantly brighter and pure.

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面積對比  How objects and colors appear is highly dependent on their context. The structural and spatial variables of a scene can influence appearance and perception. The following optical illusion demonstrates how we are sometimes fooled by our eyes.  Instructions: The diagram below features two circles with different surroundings. Would you believe that the two circles are identical?

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Twinkle, twinkle - The Blinking Effect  Challenge yourself to try and count the dots in the diagram below. Despite a static image, your eyes will make it dynamic attempting to "fill-in" the white circle intersections with the black of the background. Quite an amazing effect!  Instructions: Simply stare at the white circles and notice the intermittent blinking effect.

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色相對比

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亮度吸收現象使得黃底在具紅線時顯得偏暖，而在藍線時顯得偏冷

Contrast of Value  In order to maximize color recogition and text legibility, the goal is to find the optimal combination for the colors used. By varying hue, saturation and value, we can ensure good visibility with "highly contrasting" colors. The following display is an example of how different backgrounds affects our ability to distinguish colors.  Instructions: The text string below is repeated on a series of backgrounds with varying lightness. Which background is most favorable to the yellow text? 

Memory Color

Delk and Fillenbaum (1965)

We tend to see the colors of familiar objects as we expect them to be.

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Opponent After-images ď ś The nature of our visual system allows us to sometimes see "after-images" which appear once the original stimuli are removed. In the following demonstration, you will see that the colors in after-images are usually the opposite (complementary) colors of the original. ď ś Instructions: Stare at the black spot in the center of the four colored squares for about 30 seconds. Then scroll down and move your gaze to the black spot in the uniform white area. Note the colors of the afterimages relative to the colors of the original stimuli. Did they appear different? ď §

殘像（after image）殘像：又稱後像殘像為回應特定視覺刺激的補償作用，眼睛為了形成白光光譜而製造失蹤的波長。色彩並置時大腦會將先看到的色彩之補色的殘像加諸於後看的色彩，若影像為色彩如中灰時，則無法看到殘像。陽性殘像：注視色塊一段時間，轉移至白背景上，瞬時會看到和原影像色彩相同得影像。陰性殘像：陽性殘像出現後很快會消失，隨即變成一個原狀淡色彩不同之影像。

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Chromatic Adaptation  Have you ever entered a movie theater on a sunny afternoon? The room probably appeared completely dark but as your visual system adjusted to the reduced level of light you were able to see better after a few moments.  This "adaptation mechanism" allows our eyes to recover from an over sensitivity to a particular stimuli. "Chromatic adaptation" occurs when our eyes adjust to certain color stimuli. Follow the instructions below and see how the visual system responds to a color overload. 

Instructions: Fixate upon the black spot in between the uniform cyan and yellow areas for about 30 seconds. Then scroll down and shift your gaze to the black spot in the 2nd image. Note that the image of the seaplane appears approximately uniform after this adaptation.

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Human Retina L:M:Sď &#x20AC;32:16:1

Digital Camera Sensor

A representation of the retinal photoreceptor mosaic artificially colored to represent the relative proportions of L (red), M (green), and S (blue) cones in human retina. Modeled after Williams et al.(1991). ď §

Printer dot

Chromatic Adaptation Large-independent sensitivity regulation of the (three) mechanisms of color vision

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Adaptation  Light Adaptation Decrease in visual sensitivity with increases in luminance. (Automatic exposure control)  Dark Adaptation Increase in visual sensitivity with decreased in luminance. (Automatic exposure control)  Chromatic Adaptation Independent sensitivity regulation of the mechanisms of color vision. (Automatic color balance)

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Color Terminology  Color  Brightness (absolute): how much light.  Lightness (relative): brightness relative to white.  Hue (absolute): red, yellow, green, blue, purple, etc.  Achromatic: without hue.  Colorfulness: (absolute): how much hue. Colorfulness increases with illumination level.  Chroma (relative): Colorfulness relative to the brightness of a white object similarly illuminated. Chroma is not applicable for object surfaces that are not uniformly illuminated. Chroma is approximately constant with illumination level.  Saturation (relative): Colorfulness relative to its own brightness. Saturation is used for isolated color stimuli (unrelated color) and for describing the color of an object surface which is not uniformly illuminated.

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Color Terminology  Helpful ways of thinking about these terms: Brightness Brightness of white Colorfulness Chroma  Brightness of white Colorfulness Saturation  Brightness Lightness 

Chroma Lightness Colorfulness Brightness of white   Brightness of white Brightness Colorfulness  Brightness

Saturation 

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Corresponding colors are two color stimuli that appear the same color under two difference viewing conditions

L1M1S1 First viewing condition

Correspondin g color

X1Y1Z1

3X3 matrix

X2Y2Z2

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Chromatic Adaptation model

LaMaSa

3X3 matrix

L2M2S2 Second viewing condition

Adapted or normalized viewing condition

Chromatic Adaptation model

Von Kries Model, 1902 La  K L L M a  KM M Sa  K L S The effect of chromatic adaptation is to adjust the gain of each photoreceptor system independently and proportionally.

KL 

1 LW

1 KM  MW 1 KL  SW 

Photoreceptor responses to reference white

Von Kries Predictions

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Color Appearance Phenomena  Hunt Effect (1952) Colorfulness increases with luminance 改變照度照射彩色物時，隨著照度的增加會感到彩度增加。

 Stevens Effect (1963) Contrast increases with luminance 改變照度照射多種彩色物時，在高照度下會感到明亮的灰色更白，暗灰色更黑。

 Helson-Judd Effect (1938) Hue of nonselective samples 用彩色光照射灰階色表時，在明亮的灰色處可感受到照明光的色調，在暗的灰色處會感受照明光的補色色調。

 Bartleson-Breneman Equations (1968) Image contrast changes with surround (幻燈片v.s.照片)  Discounting-the-Illuminant (Fairchild, 1992)

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Color Appearance Models ď ś Color appearance models extend basic colorimetry to the prediction of appearance attributes under widely varying viewing conditions.

CIE TC1-34 Definition: To be considered a color appearance model, a model must account for at least chromatic adaptation and have correlated of at least lightness, chroma and hue.

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CIELAB as an Example  CIELAB does:  Model chromatic adaptation  Model response compression  Include correlates for lightness, chroma and hue  Include useful color difference measure  CIELAB doesn’t  Predict luminance dependent effects  Predict background surround effects  Have an accurate adaptation transform

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CIELAB Equations L*  116[ f (Y / Yn )]  16 a *  500[ f ( X / X n )  f (Y / Yn )] b*  200[ f (Y / Yn )  f ( Z / Z n )] 1 3

where f ( X / X n )  ( X / X n ) for X / X n  0.008856 1 3

f (Y / Yn )  (Y / Yn ) for Y / Yn  0.008856 1 3

f ( Z / Z n )  ( Z / Z n ) for Z / Z n  0.008856 and

f ( X / X n )  7.7867( X / X n )  16 / 116 for X / X n  0.008856 f (Y / Yn )  7.7867(Y / Yn )  16 / 116 for Y / Yn  0.008856 f ( Z / Z n )  7.7867( Z / Z n )  16 / 116 for Z / Z n  0.008856 1 * C ab  (a * 2  b* 2 ) 2

hab  arctan( b* / a * ) L* 2 C * 2 H * 2 12 E  [( ) ( ) ( ) ] K L SL K C SC K H SH * 94

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sRGB

sRGB規格(1)  為電腦或網路為中心的多媒體領域所制定的一種假想的RGB色彩空間  1996年，最先是由HP和Microsoft共同提倡, 其後, IEC將其制定成標準化  在標準觀測環境下,定義出標準影像的顯示器特性  色彩變換處理的負擔輕,可以作到彩色影像機器之間的直接連接 

sRGB

sRGB標準觀察環境 sRGB標準觀察環境背景 (background)

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顯示器輝度程度的20% D65 (x=0.3127, y=0.3290), 16 cd/m2

環境 (surround)

周圍輝度的20% D50 (x=0.3457, y=0.3585), 4.1 cd/m2

近側範圍 (proximal field)

顯示器輝度程度的20% D65 (x=0.3127, y=0.3290), 16 cd/m2

周圍照度程度

64 lux

周圍白色點

D50 (x=0.3457,y=0.3585)

炫光 (viewing glare)

0.2 cd/m2

sRGB

觀察環境的設計 Color Element

Proximal Field

Surround

Background

Adapting Field 

sRGB

sRGB規格顯示器特性

sRGB標準影像顯示器特性顯示器輝度程度

80 cd/m2

顯示器白色點

D65 (x=0.3127,y=0.3290)

顯示器RGB色度點

s-=2.2

Radiance

顯示器階調特性

ITU-R BT.709-3

Digital counts

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sRGB sRGB構造

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Structure of the sRGB Profile

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Math for sRGB to PCS Conversion

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Bradford Chromatic Adaptation Math X  Y     Z  dest

 Rdw   0.9870  0.1471 0.1600  Rsw   0.4323 0.5184 0.0493   0   0.0085 0.0400 0.9685   0

Inverse of Lam and Rigg matrix (“Bradford” matrix)

0 Gdw Gsw 0

 0  0.8961 0.2664  0.1614  X    0    0.7502 1.7135 0.0367    Y   Bdw   0.0368  0.0685 1.0296   Z source Bsw  Lam and Rigg

(Bradford University) Cone response matrix

0.2664  0.1614  X   Rdw  0.8961 Gdw   0.7502 1.7135   Y  0 . 0367        Bdw  0.0368  0.0685 1.0296   Z  dest

white

0.2664  0.1614  X   Rsw  0.8961 Gsw   0.7502 1.7135 0.0367    Y      Bsw  0.0368  0.0685 1.0296   Z source

white

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Conversion of XYZD65 to XYZD50 Using the Bradford Transform

X   Y        Z  D50 

M BFD

 rD 50 1    rD 65   0      0 

0 g D 50 g D 65 0

 0    0      bD 50   bD 65 

M BFD

 X    Y       Z  D65

rD50, gD50, bD50 = cone response of D50 white (MBFD x XYZD50) rD65, gD65, bD65 = cone response of D65 white (MBFD x XYZD65)

 1.0479 0.0229 0.0502  X  X   0.0296 0.9904 0.0171   Y  Y          Z  D50  0.0092 0.0151 0.7519  Z  D65 

Summary of sRGB to PCS Conversion Details

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Fairchild’s Chromatic Adaptation Model 對參考觀看條件：D65，Hunt-Pointer-Estevez轉換為：  L1   0.4002 0.7076  0.0808  X 1   X1  M    0.2263 1.1653 0.0457   Y   M  Y   1   1   1 0.0 0.9182   Z1   S1   0.0  Z1 

下一步驟是應用修正後的von Kries transform：  L1 '  a L 0  M '   0 a M  1  0  S1 '   0 where aL  PL Ln

0   L1  0 M1    aS   S1 

 L1  A M 1     S1 

1  Yn1 / 3  lE PL  1  Yn1 / 3  1 / lE lE 

3( Ln / LE ) , Ln / LE  M n / M E  Sn / S E

and, a M and aS are similarly defined. 

Fairchild’s Chromatic Adaptation Model In applying the model for predicting the corresponding colors the (X1, Y1, Z1) tristimulus values of the first viewing condition are transformed to the (L’, M’, S’) of the reference viewing condition, which is then transformed to the (X2, Y2, Z2) of the second viewing condition by inverse matricies.

X2  X1   L2 '   L1 '   M '  A M  Y  and  M '  A M  Y  1 2  2  1  2  1  Z 2   Z1   S 2 '   S1 '  Therefore, X2  X1   Y   M 1 A 1 A M  Y  2 1  2  1  Z 2   Z1  

Hunt Model, 1994 A comprehensive model capable of handing a wide range of viewing conditions. Color reproduction history. •Input •Absolute XYZ •Surround relative luminance •Discounting •Scotopic luminance •Output •Bright, lightness, colorfulness, chroma, saturation, hue •Advantages •Comprehensive, all appearance attributes •Disadvantages •Complex, not invertible 

  a  f n ( FL F )   D  1 w   a  f n ( FL F )   D  1 w  )  D  1  a  f n ( FL F w

RLAB (Fairchild, 1996) A simple model combining an accurate adaptation transform with CIELAB. Color reproduction applications •Input •Absolute XYZ •Surround relative luminance •Discounting •Scotopic luminance •Output •Lightness, chroma, saturation, hue

La  ( pL  D (1  pL ))(

M a  ( pM  D (1  pM ))(

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M ) Mn

S Sa  ( pS  D (1  pS ))( ) Sn

•Advantages •Simple, Accurate, easy inverted, sufficient for image? •Disadvantages •Only relative appearance attributes

L ) Ln

CIECAM97s CIE color appearance model adopted by TC1-34 in 1997. Best of other models. Response to industry. •Input •Absolute XYZ (stimulus, adapting stimulus) •Surround and background relative luminance •Discounting •Scotopic luminance •Output •Brightness, lightness, colorfulness, chroma, saturation, hue •Advantages •Combination of available knowledge, all appearance attributes, international “standard” •Disadvantages •More complex than necessary, tricky inversion 

Results

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Color Management Systems

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跨媒体色彩處理技術

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跨媒體色彩修正 ‧觀色環境不同光源不同亮度

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工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

跨媒體色彩修正 spectral radiance of sources and illuminants 250

Halogen Xenon

relative radiance

A 200

Xen.-flash ill.D65 ill. A

150 D65

ill. C

100 C 50 0 300

400

500

600 nm

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700

800 工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

跨媒體色彩修正 1 0.8

0.6

0.4 0.2

0.8

0.6

0.4

0.2

0.4

0.6

IT8.7/2 130個色塊在A光源下之x, y分佈

0.4

0.6

0.8

IT8.7/2 130個色塊在D65光源下之x, y分佈

CIE1931色度座標圖

0.2

0.8

0.2

0.4

0.6

0.8

IT8.7/2 130個色塊在D50光源下之x, y分佈

IT8.7/2標準色稿在不同光源下的色度值表現 

工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

跨媒體色彩修正 ‧光源適應性色彩修正光源 A (鹵素燈)

R,G,B

色差≧20 影像

未校正影像色彩偏差大

Visual Match 數位相機經光源適應性色校後色彩偏差小

光源特性轉換模式

光源 B (螢光燈)

色差≦8.0 光源適應性 R,G,B 色校程式

X,Y,Z

色度空間轉換

CRT or Printer R',G',B'

Color Difference

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工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

跨媒體色彩修正 ‧跨媒體對色

彩色輸入設備

Scanner

彩色輸出設備

Laser Printer

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Video camera

DSC

CRT

彩色輸出設備

Injet Printer

PTV

工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

跨媒體色彩修正彩色複製方式 • 頻譜複製 (Spectral Colour Reproduction) – 反射率相同 ‧ 優點 : 在不同光源下仍可對色 ‧ 缺點 : 成本及困難度高

• 色度值複製 (Colorimetric Colour Reproduciton) – 色度值相同 ‧限制 :彩色輸出設備之色域限制 ‧缺點 :在某一光源下可對色,其它則否

•

色知覺複製 (Appearance Colour Reproduciton) – 色知覺模型的準確度

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色度空間色差公式： E*ab (L*)2 (a*)2 (b*)2

× Observer 

均勻色度空間 CIELAB

CIExyY

sRGB

Surface

Source

Color Reproduction

LCH=L’C’H’

XYZ

Target

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X’Y’Z’

Reproduction

Color Reproduction

• Colorimetric Color Reproduction – Equality of tristimulus values (X, Y, Z)

E()

R()

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E()R()  X  K  E (  ) R (  ) x (  ) d Y  K  E (  ) R (  ) y (  ) d   120 

 50

Z  K  E (  ) R (  ) z (  ) d  15

Color Reproduction

造成色彩不一致的原因  彩色輸出入設備的色彩元素之定義及響應不同  彩色輸出入設備的色調及色域不同  色彩產生方式不同  加成性色彩(Additive Color)重現  減法性色彩(Subtractive Color)重現

 不同媒體的對色  紙張 (亮面,物面,...,底色)  反射,穿透稿,發光物...

 彩色輸出入設備的穩定性  人類色視覺作用  同色異譜現像 (metamerism) 

Color Reproduction Colour constancy: reminder

The spectra reaching the eye have different SPDs but the perceived surface colour is the same

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工業技術研究院光電工業研究所 Industrial Technology Research Institute Opto-Electronics & Systems Laboratories

Color Reproduction

•Metamerism • We define a pair of objects having different spectral reflectance curves but the same color coordinates for one set of conditions as metameric objects, or a metameric pair. 

Color Reproduction

‧同色異譜（Metamerism）

Reflectances of four objects that look alike

Objects with these spectral reflectance curves would match in color for the CIE 1931 standard observer when viewed under illuminant C. 

Color Reproduction

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Color Reproduction

Color Reproduction in Each Medium CRT(RGB)

Photography (CMY)

Printing (CMYK)

Scanner(RGB)

New Medium(?)

Digital Camera(RGB) ď §

Color Reproduction

Color Reproduction in Cross Medium CRT(RGB)

Printing (CMYK)

Photography (CMY)

Scanner(RGB)

New Medium(?) Digital Camera(RGB)

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Color Reproduction

Device Independent Color CRT(RGB)

Printing (CMYK)

Photography (CMY) XYZ Scanner(RGB)

New Medium(?) Digital Camera(RGB)

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Color Reproduction

Problems in Color Reproduction • Device Calibration – Adjust a device to the standard condition

• Colorimetric Characterization – Characterize color response

• Gamut Mapping – Compensate a different between target & destination

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Color Reproduction

Problems in Color Reproduction • Chromatic Adaptation – Reference White, Environmental condition

• Color Transformation – Calculate color signal from one to the other

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Color Reproduction

Colorimetric Color Reproduction • Colorimetric Color Reproduction = Matching of colorimetric values(XYZ) E*ab