Color Vision: Understanding Colors, Color Spaces, and Color Effects, Slides of Computer Science

Download Color Vision: Understanding Colors, Color Spaces, and Color Effects and more Slides Computer Science in PDF only on Docsity! Color Vision 1 Color Docsity.com Color Vision 2 Review of last week Docsity.com Color Vision 5 Perceptually Uniform Space: MacAdam • In color space CIE-XYZ, the perceived distance between colors is not equal everywhere • In perceptually uniform color space, Euclidean distances reflect perceived differences between colors • MacAdam ellipses (areas of unperceivable differences) become circles 560550540530520 510 490 480 470 460 450 .05 x y .10 .15 .20 .45.50 .55.60 .65 .25 .25 .20 .15 .10 .05 .35 .40 .45 Spectrum locus .50 .55 .60 .65 .70.30 400 nm 700 nm 650 620610 600 590 580570 Purp le lin e Wyszecki, G. and W. S. Stiles. Color science: Concepts and Methods, Quantitative Data and Formulae. Wiley-Interscience, 2nd ed. July 2000. ISBN: 0471399183. Image adapted from: Docsity.com Color Vision 6 Hue Saturation Value • Value: from black to white • Hue: dominant color (red, orange, etc) • Saturation: from gray to vivid color • HSV double cone value saturation saturation hue Docsity.com Color Vision 7 Hue Saturation Value • One interpretation in spectrum space • Not the only one because of metamerism • Dominant wavelength (hue) • Intensity • Purity (saturation) Docsity.com Color Vision 10 Color terms (Fairchild 1998 • Color • Hue • Brightness vs. lightness • Colorfulness and Chroma • Saturation • Unrelated and related colors Docsity.com Color Vision 11 Color – chromatic and achromatic content. This attribute can be described by chromatic color names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic color names such as white, gray, black, etc., and qualified by bright, dim, light, dark, etc., or by combinations of such names. – Note: Perceived color depends on the spectral distribution of the color stimulus, on the size, shape, structure, and surround of the stimulus area, on the state of adaptation of the observer's visual system, and on the observer's experience of the prevailing and similar situations of observations. Docsity.com Color Vision 12 Related and Unrelated Colors • Unrelated Color – Color perceived to belong to an area or object seen in isolation from other colors. • Related Color – Color perceived to belong to an area or object seen in relation to other colors. Docsity.com Color Vision 15 Colorfulness & Chroma • Colorfulness – Attribute of a visual sensation according to which the perceived color of an area appears to be more or less chromatic. • Chroma: – Colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting. Docsity.com Color Vision 16 Saturation – Colorfulness of an area judged in proportion to its brightness. Docsity.com Color Vision 17 Plan • Color Vision • Color spaces • Color effects – Definitions – Spatial sensitivity – Color illusion and color appearance • Producing color Docsity.com Land Retinex and local contrast \ \ \ \ \ 80% 40% B \ 20%, C + \ \ \ \ A / 48/ 94 / / / «32 / / L a 20 s s ~ \ ‘1! / - - SNe Reflectance B Luminance ™32 1 40 60 80 Illumination Level (# photons/unit time) 7o~ 7e\ 48 , 32 _1536_ 4 24° 16 384 Luminance Edge Calculation of A to C (Cereyaatats (on Color Vision 21 Mach Bands • Contrast is enhanced at region boundaries a light Pe rc ei ve d B rig ht ne ss (e .g . s ca le ) A ct ua l I nt en si ty (e .g . l um in a) dark light dark b c d e a b c d e e d c b a Docsity.com Color Vision 22 Hermann Grid - --- - --+ +++ ++++ + + --- - - ----- ----- - - ----- - - ----- --- - - --------- - - - - -- -- --- -- - - ------ --- -- --- - - --- - - Docsity.com Color Vision 25 Brightness vs. lightness • Brightness: subjective amount of light • Lightness: how “white” The white cells in shadow are as dark as the black illuminated cells Docsity.com Color Vision 26 Lightness and transparency (Courtesy of The Department of Brain and Cognitive Sciences. Used with permission.) Docsity.com Color Vision 27 Opponents and image compression • JPG, MPG • Color opponents instead of RGB • Compress color more than luminance Docsity.com Color Vision 30 Contrast Sensitivity Function (CSF) -1 0.01 0.1 1 0 Luminance Red-Green Blue-Yellow Log Spatial Frequency (cpd) C on tra st S en si tiv ity 1 2 10 100 Spatial contrast sensitivity functions for luminance and chromatic contrast. Image adapted from: Mark D. Fairchild. Color Appearance Models. Prentice Hall PTR, November 1997. ASIN: 0201634643 Docsity.com Color Vision 31 JPEG Compression • Perform DCT to work in frequency space – Local DCT, 8x8 blocks • Use CSF for quantization (more bits for sensitivity with more contrast) • Other usual coding tricks Docsity.com Color Vision 32 Plan • Color Vision • Color spaces • Color effects – Definitions – Spatial sensitivity – Color illusion and color appearance • Producing color Docsity.com Color Vision 35 Crispening • Increased sensitivity Docsity.com Color Vision 36 Stevens effect • Stevens effect – Perceived contrast increases with luminance 0.1 1.0 1.0 L (97 dB) L (79 dB)L (63 dB)L (Dark) Image adapted from: Fairchild, Mark D. Color Appearance Models. Prentice Hall PTR. November 1997. ASIN: 0201634643. R el at iv e B rig ht ne ss Relative Luminance Docsity.com Color Vision 37 Hunt and Stevens effect • Stevens effect – Perceived contrast increases with luminance • Bartleson-Breneman effect – Image contrast changes with surround – A dark surround decreases contrast (make the black of the image look less deep) – Important for movies • Hunt effect – Colorfulness increases with luminance • Hence the need for gamma correction (see later) Docsity.com Color Vision 40 Color synthesis Additive Subtractive red, green, blue cyan, magenta, yellow Add light (CRT, video projector) Remove light (e.g. filter) printer, photos Docsity.com Color Vision 41 Color synthesis: a wrong example Additive Subtractive red, green, blue cyan, magenta, yellow RIGHTWRONG Docsity.com Color Vision 42 Device Color Models(Subtractive) Start with white, remove energy (e.g.,w/ ink, filters) Example: CMY color printer Cyan ink absorbs red light Magenta ink absorbs green light Yellow ink absorbs blue light C+M+Y absorbs all light: Black! RGB => CMY conversion Some digital cameras use CMY Docsity.com Color Vision 45 Gamut • Every device with three primaries can produce only  colors inside some (approx.) triangle – Convexity! • This set is called a color gamut – (Why can’t RGB can’t give all visible colors?) • Usually, nonlinearities warp the triangle – Also, gamut varies with luminance 0.1 0.1 0.2 400 480 490 500 510 520 540 o 560 580 600 700 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.7 x x y y 0.7 0.8 R. W. G. Hunt. The Reproduction of Colour. John Wiley & Sons, Incorporated, September 2004. ISBN: 0-470-02425-9. Image adapted from: Docsity.com Color Vision 46 Gamut Mapping • Color gamut of different processes may be different (e.g. CRT display and 4- color printing process) • Need to map one 3D color space into another CIE-Lab Perceptually-uniform Color space Typical CRT gamut 4-color printing gamut Docsity.com Color Vision 47 Gamut Mapping Typical CRT gamut 4-color CMYK printing gamut a* b* Gamut mapping is a morphing of 3D color space according to adopted scheme Docsity.com