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Introduction to Color Appearance Models Outline

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Introduction to Color Appearance Models Outline Introduction to Color Appearance Models Arto Kaarna Lappeenranta University of Technology Department of Information Technology P.O. Box 20 FIN-53851 Lappeenranta, Finland [email protected] Basics of CAM … 1/81 Outline 1. Definitions 3 2. Color Appearance Phenomena 15 3. Chromatic Adaptation 44 4. Color appearance models 59 5. CIECAM02 65 Literature 80 Basics of CAM … 2/81 1 Definitions Needed for uniform and universal description Precise for mathematical manipulation International Lighting Vocabulary (CIE, 1987) Within color science Color Hue Brightness Lightness Colorfulness Chroma Saturation Unrelated and Related Colors Basics of CAM … 3/81 Definitions Color Visual perception of chromatic or achromatic content •Yellow, blue, red, etc •White, gray, black, etc. •Dark, dim, bright, light, etc. NOTE: perceived color depends on the spectral distribution of the stimulus, size, shape, structure, and surround •Also state of adaptation of the observer Both physical, physiological, psyckological and cognitive variables New attempt: visual stimulus without spatial or temporal variations More detailed definition needed using more parameters for numeric expression Basics of CAM … 4/81 2 Definitions Hue Visual sensation similar to one of the perceived colors: red, yellow, green, and blue or their combination Chromatic color: posessing a hue, achromatic no hue Munsell book of color No hue with zero value Achromatic colors Basics of CAM … 5/81 Definitions Brightness A visual sensation according to which an area emits more or less light An absolute level of perception Lightness The brightness of an area judged relative to the brightness of a similarly illuminated area that is white or highly transmitting, thus a relative value For example: A page from a book Office •Brightness: some value •Lightness: high value Sunny day: •Higher brightness value •Roughly the same lightness as in office Basics of CAM … 6/81 3 Definitions Basics of CAM … 7/81 Definitions Colorfulness A visual sensation in which an area appears to be more or less chromatic Defines the intensitivity of the hue of a given stimulus Chroma Colorfulness of an area as a proportion of the brightness of a similarly illuminated area of white or high transmittance The third dimension of 3D color vision (hue+brightness/lightness+colorfulness/chroma) Colorfulness and chroma in similar relationship as brightness and lightness Zero colorfulness or chroma: achromatic colors Basics of CAM … 8/81 4 Definitions Increase in lightness Increase in colorfulness Basics of CAM … 9/81 Definitions Saturation Colorfulness of an area in proportion to its own brightness Relative colorfulness Traffic lights at night Increase in saturation Increase in lightness Increase in colorfulness Basics of CAM … 10/81 5 Definitions Unrelated Colors Seen in isolation of other colors Many visual experiments with unrelated colors Related Colors Seen in relation to other colors Color appearance applications deal with related colors Colors like gray and brown Gray has lower lightness than white Brown is an orange color with low lightness Gray/brown light in dark environment? Basics of CAM … 11/81 Definitions Unrelated Colors Perceptual attributes are hue, brightness, colorfulness, and saturation No attributes related to similarly illuminated white Related colors Perceptual attributes are hue, brightness, lightness, colorfulness, chroma, and saturation Test: White/orange illuminant in dark environment surrounded with an increasing white illuminant? White/orange illuminant in highly illuminated environment? What is the color of the white/orange illuminant? Basics of CAM … 12/81 6 Definitions Finally we get the equations Colorfulness Colorfulness Chroma = Saturation = Brightness(white) Brightness Brightness Lightness = Brightness(white) Chroma Saturation = Lightness Colorfulness Brightness(white) = × Brightness(white) Brightness Five perceptual dimensions for color appearance Brightness, lightness, colorfulness, chroma, hue, saturation is redundant Basics of CAM … 13/81 Definitions Brightness-colorfulness vs. lightness-chroma Example Object in sunny day: hue(red), high brightness, lightness, colorfulness, chroma Object in office: hue, lightness and chroma same, brightness and colorfulness lower Basics of CAM … 14/81 7 Color Appearance Phenomena Relationships in viewing conditions and color appearance, examples Simultaneous Contrast Josef Albers Complex Spatial Structures Hunt Effect Stevens Effect Bartleson-Breneman Surround Effect Basics of CAM … 15/81 Color Appearance Phenomena Equal (XYZ)1 = (XYZ)2 appear similar Similar viewing conditions for both samples: surrounds, backgrouds, size, shape, surface, illumination, etc. Differences can be computed from (XYZ) pairs Basics of CAM … 16/81 8 Color Appearance Phenomena If two samples with (XYZ)1 = (XYZ)2 look different then there is a change in viewing conditions Basics of CAM … 17/81 Color Appearance Phenomena Basics of CAM … 18/81 9 Color Appearance Phenomena Cognition through vision Basics of CAM … 19/81 Hermann Grid Basics of CAM … 20/81 10 Scintillation Effect Basics of CAM … 21/81 Basics of CAM … 22/81 11 Basics of CAM … 23/81 Simultaneous Contrast The background influences the apparent color of the sample Bg change Sample change Darker Lighter Lighter Darker Red Green Green Red Yellow Blue Blue Yellow Basics of CAM … 24/81 12 Simultaneous Contrast Constant gray-level of a sample Basics of CAM … 25/81 Simultaneous Contrast Constant color of a sample Robertson (1996): yellow stripe ĺ square gets bluer Basics of CAM … 26/81 13 Josef Albers Studies on simulteneous contrast Josef Albers: Interaction of Color, 1963 Basics of CAM … 27/81 Crispening Similar gray-level at different backgrounds Differences between squares change with different backgrounds Basics of CAM … 28/81 14 Spreading High-frequency sample close to background Left: more greenish, right: more reddish Basics of CAM … 29/81 Bezold-Brucke Hue cannot be determined directly from wavelength Hue shift: hue changes with luminance Largest change in red-yellow area 650nm ĺ 620 nm when luminance is 1/10th of the original Basics of CAM … 30/81 15 Abney effect Mixing of monocromatic light with white light does not preserve constant hue Y 0.8 0.6 Contours at constant hue Constant hue produced by various wavelengths 0.4 0.2 0.2 0.4 0.6 0.8 x Basics of CAM … 31/81 Helmholtz-Kohlrausch Brightness depends on luminance and chromaticity Correction factor: F = 0.256 - 0.184 y - 2.527 xy + 4.656 x 3 y + 4.657 xy 4 Equal brightness: log( L1 ) + F1 = log( L 2 ) + F2 Y 0.8 0.6 1.2 Contours at constant brightness- 1.3 1.1 to-luminance ratio 1.4 Constant luminance ĺ perceived 0.4 1.0 brightness increases when 1.5 stimulus becomes more chromatic 0.2 0.2 0.4 0.6 0.8 x Basics of CAM … 32/81 16 Hunt Effect When luminance level changes the color appearance of objects changes Objects vivid and contrasty in summer afternoon Subdued in dusk Y 10000 0.8 1000 For a constant chromaticity, perceived colorfulness increases 100 0.6 with luminance 10 With luminance increase, a lower colorimetric purity is needed to 0.4 1 match with the reference stimulus 10000 1000 100 10 1 0.2 0.2 0.4 0.6 0.8 x Basics of CAM … 33/81 Hunt Effect At low level of illumination, the colorfulness is low In brighter viewing environment, the elements will be more colorful Absolute luminance level is needed in an appearance model Basics of CAM … 34/81 17 Stevens Effect Contrast increases with luminance: perceived contrast increases in with increasing luminance Lower luminance Luminance level increases: light colors become lighter, dark colors become darker Higher luminance 0.1 Relative0.1 Brightness 1.0 0.1 Relative luminance 1.0 Basics of CAM … 35/81 Stevens Effect Black-and-white image: white is more white and black is more black in higher level luminance conditions Basics of CAM … 36/81 18 Bartleson-Breneman Complex stimuli while varying luminance level and surround Percieved contrast of images increased when the surround was changed from dark to dim to light 1.0 Dark Dark background makes dark colors darker without affecting to light colors Dark background lowers and light background adds contrast Average 0.0 Lightness 0.0 0.0 Relative luminance 1.0 Basics of CAM … 37/81 Bartleson-Breneman Basics of CAM … 38/81 19 Spatial Interactions Spatial variables influence appearance Surround effects Basics of CAM … 39/81 Spatial Interactions Basics of CAM … 40/81 20 Dale Purves, Brown Brown patch under changing illumination See demonstration: http://www.purveslab.net/main/ Basics of CAM … 41/81 Viewing Conditions Color appearance depends on the simulus itself and other stimuli nearby Spatial Temporal Laboratory vs. practical applications and measurements Stimulus Proximal field Background Surround Basics of CAM … 42/81 21 Viewing Conditions Stimulus The area for which the color appearance is desired 2 degree angle (CIE 1931) Color in the angle vs. color of the object Proximal Field Immediate field around stimuli in all directions Local contrast effects: crispening, spreading Nearest pixels in digital images In many cases same as background Background About 10 degree field For simultaneous contrast definition Surround Area outside background Basics of CAM … 43/81 Chromatic Adaptation Light adaptation Decrease in visual sensitivity upon increase in overall illumination level Switch the lights on when you wake up Dark adaptation Increase in visual sensitivity upon decrease in illumination level From sunlight to dark room Chromatic adaptation Adaptation to changes in chromatic
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