Utah School of Computing 8/31/2015 Fall Introduction to Computer Graphics. Goal: have fun and learn graphics!

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1 CS 4600 Introduction to Computer Graphics Prof: Chuck Hansen Goal: have fun and learn graphics! CS4600 Computer Graphics Modified from Rich Riesenfeld Fall 2015 Computer Graphics CS4600 1

2 Two properties of monitors define what colors they produce Gamma maps intensity of light emitted Gamut maps the space of possible colors generated by a display intensity pixels The amount of light emitted from a monitor is nonlinear Computer Graphics CS4600 2

3 Gamma parameter makes light intensity linear Large Gamma Original Small Gamma Computer Graphics CS4600 3

4 Color is complicated! Highly nonlinear No single model to explain all Many simplistic models, explanations Many myths Much new knowledge Wavelength Spectrum infrared light ultraviolet light Wavelength (nm) Seen in physics, physical phenomena (rainbows, prisms, etc) 1 Dimensional color space Computer Graphics CS4600 4

5 Color as a wavelength Light Source Color as a wavelength Light Source Light Computer Graphics CS4600 5

6 Color as a wavelength Light Source Light is composed of photons Color as a wavelength Light Source Photons have specific wavelengths Computer Graphics CS4600 6

7 Wavelength Spectrum Electromagnetic Spectrum: All wavelengths of light Visible Light ( nm): Light we can see (colors) Sun emits entire spectrum Computer Graphics CS4600 7

8 Tone Mapping Take Continuous values (wavelengths) into discrete values Quantization Wavelength (nm) Tone Mapping Computer Graphics CS4600 8

9 Indexed Color (quantizing color) Color Space Navigating, moving around in a color space, is tricky Many color representations (spaces) Can you get to a nearby color? Can you predictably adjust a color? Computer Graphics CS4600 9

10 Color Spaces Device-derived convenient for describing display device levels RGB, CMYk Intuitive (transformations) based in familiar color description terms HSV, HSI Perceptually based device independent, perceptually uniform CIELUV, CIELAB, Munsell R-G-B Color Space Convenient colors (screen phosphors) Decent coverage of the human color Not a particularly good basis for human interaction Non-intuitive Non-orthogonal (perceptually) Computer Graphics CS

11 Color Cube: (r,g,b) is RHS blue (0,0,1 ) cyan (0,1,1) magenta (1,0,1) white (1,1,1) black (0,0,0) gra y green (0,1,0) red (1,0,0) (1,1,0) yellow Computer Graphics CS

12 Color Cube blue (0,0,1 ) magenta (1,0,1) white (1,1,1) cyan (0,1,1) red (1,0,0) yellow (1,1,0) green (0,1,0) Complementary Colors Add to Gray magenta (1,0,1) blue (0,0,1 ) white (1,1,1) cyan (0,1,1) red (1,0,0) yellow (1,1,0) green (0,1,0) Computer Graphics CS

13 Complementary Colors Looking at color cube along major diagonal Additive Primary Colors red (1,0,0) yellow (1,1,0) green (0,1,0) white (1,1,1) magenta (1,0,1) cyan (0,1,1) blue (0,0,1 ) Computer Graphics CS

14 Additive Primary Colors red (1,0,0) green (0,1,0) yellow (1,1,0) Additive Primary Colors red (1,0,0) magenta (1,0,1) blue (0,0,1 ) Computer Graphics CS

15 Additive Primary Colors cyan (0,1,1) green (0,1,0) blue (0,0,1 ) Additive Primary Colors red (1,0,0) yellow (1,1,0) green (0,1,0) white (1,1,1) magenta (1,0,1) cyan (0,1,1) blue (0,0,1 ) Computer Graphics CS

16 Subtractive Primary Colors yellow (1,1,0) black (0,0,0) red (1,0,0) magenta (1,0,1) green (0,1,0) blue (0,0,1 ) cyan (0,1,1) Subtractive Primary Colors yellow (1,1,0) magenta (1,0,1) red (1,0,0) Computer Graphics CS

17 Subtractive Primary Colors yellow (1,1,0) green (0,1,0) cyan (0,1,1) Subtractive Primary Colors magenta (1,0,1) blue (0,0,1 ) cyan (0,1,1) Computer Graphics CS

18 (H, S, V ) Color Space Introduced by Albet Munsell, late 1800s He was an artist and scientist Hue: Color Saturation/Chroma: Strength of a color Neutral gray has 0 saturation Brightness/Value: Intensity of light emanating from image HSV HSL Computer Graphics CS

19 HSV Color Space (Cone) 120 green V yellow cyan 1.0 red 0 blue 240 magenta 0.0 black H S HSV Color Space (Cone) 120 V H S Computer Graphics CS

20 L white 1.0 HLS Color Space (double cone) red black H S 39 HSL/HSV Value/Luminance total amount of energy Saturation degree to which color is one wavelength Hue dominant wavelength Computer Graphics CS

21 Saturation (in RGB) HSV from RGB Max = max(r, G, B) Min = min(r, G, B) S = (Max Min)/Max If R==Max -> h = (G-B)/(Max-Min) If G==Max -> h = 2+(B-R)/(Max-Min) If B==Max -> h = 4 + (R-G)/(Max-Min) If h<0 -> H = h/6 + 1 If h>0 -> H = h/6 Computer Graphics CS

22 HSV User Interaction HSL from RGB S = sqrt( ((R-G) 2 + (R-B) 2 + (G-B) 2 )/2 ) I = (R + G + B)/3 H = (a arctan((r I)b/(G-B)))/(2 ) --- angle a = /2 if G>B 3* /2 if G<B H = 1 if G=B a=sqrt(3) Computer Graphics CS

23 (Hue, Saturation, Value/Intensity) (H, S, V ) Color Space The hue of an object may be blue, but the terms light and dark distinguish the brightness of one object from another. Using Color JS: Color is Explicit (ctx is canvas context): ctx.fillstyle = "rgb(200,0,0)"; ctx.fillrect (10, 10, 55, 50); ctx.fillstyle = "rgba(0, 0, 200, 0.5)"; ctx.fillrect (30, 30, 55, 50); WebGL (OpenGL): Color is stored as a vertex attribute (we ll get to this later) Computer Graphics CS

24 Tristimulus Color Theory Any color can be matched by a mixture of three fixed base colors (primaries) Eye has three kinds of color receptors called cones Eye also has rods (low light receptors) Computer Graphics CS

25 Computer Graphics CS

26 Retina is a bunch of sensors Cones Rods Rods: Shades of grey Sensitive to low-light Densest in the periphery Cones: Responsible for color vision Long (L), Medium (M), Short (S) Densest in the fovea (center) Computer Graphics CS

27 Color Receptors in Eye Fraction of light absorbed by each type of cone Wavelength λ (nm) Red, Green, Blue Long, Medium, Short Computer Graphics CS

28 Color Receptors in Eye 1.0 Relative sensitivity Wavelength λ (nm) CIE* Color Space ( X, Y, Z ) represents an imaginary basis that does not correspond to what we see Define the normalized coordinates: x = X / ( X + Y + Z ) y = Y / ( X + Y + Z ) z = Z / ( X + Y + Z ) * Commission Internationale de l'êclairage Computer Graphics CS

29 CIE Color Space of Visible Colors x = X / ( X + Y + Z ) y = Y / ( X + Y + Z ) z = Z / ( X + Y + Z ) y x + y + z = 1 z The projection of the plane of the triangle onto the (X,Y) plane forms the chromaticity diagram that follows. x Color Gamuts: CIE Color Chart Ideal green 520 nm 540 nm 510 nm 560 nm 500 nm 490 nm green cyan blue yellow white 580 nm red 600 nm 700 nm ideal blue 400 nm ideal red Computer Graphics CS

30 Color Gamuts: CIE Color Chart Color Gamuts: CIE Color Chart 510 nm 520 nm 540 nm 560 nm 500 nm 580 nm 600 nm 490 nm 700 nm 400 nm Computer Graphics CS

31 Color Gamuts: CIE Color Chart C 2 C 3 C The additive colors C 1 and C 2 combine to form C 3 on the line connecting C 1 and C Color Gamuts: CIE Color Chart y cyan green 490 blue yellow white magenta 580 red x Computer Graphics CS

32 Color Gamuts: CIE Color Chart G R B The Color Gamuts of different displays and printers are not likely to match. Printers usually have smaller gamuts. Gamuts Computer Graphics CS

33 CIE L*a*b* Color Space white L*=1 lightness Equal distances represent approximately equal color difference. black L*=0 Opponent Color Theory Humans encode colors by differences E.g R-G, and B-Y Differences Color blindness Long (R) - R - G Achromatic Medium (G) Short (B) + Yellow - Y-B Computer Graphics CS

34 Trichromatic Theory Shortcomings Color blindness R-G, B-Y, All Yellow seems primary Color constancy Color Blindness Normal Protan (L-cone) Deutan (M-cone) Tritan (S-cone) Computer Graphics CS

35 Mondrian Color Patches Colors look different depending on their neighbors Adjacency/black lines Color edges are critical to color perception Can determine color in non-white lighting conditions Computer Graphics CS

36 Perceptual Distortions Color-deficiency Interactions between color components brightness - hue (Bezold-Brucke Phenomenon) saturation - brightness (Helmholtz-Kohlraush effect) Simultaneous contrast brightness hue Small field achrominance Effects of color on perceived size Computer Graphics CS

37 Bezold-Brucke Phenomenon Hurvich 81, pg. 73. Bezold-Brucke Phenomenon Hurvich 81, pg. 73. Computer Graphics CS

38 Helmholtz-Kohlrausch effect Helmholtz-Kohlrausch effect Computer Graphics CS

39 Simultaneous Contrast Simultaneous Contrast Computer Graphics CS

40 Simultaneous Contrast Mach Banding (lateral inhibition) Computer Graphics CS

41 Mach Banding Mach Banding Computer Graphics CS

42 Chromatic Adaptation Computer Graphics CS

43 Computer Graphics CS

44 Color Applets edu/brown/cs/exploratories/applets/combinedcolormixing /combined_color_mixing_java_browser.html Nice overview Computer Graphics CS