Introduction to Computer Graphics

1. Introduction to Computer Graphics Color

2. Specifying Color • Color perception usually involves three quantities: • Hue: Distinguishes between colors like red, green, blue, etc • Saturation: How far the color is from a gray of equal intensity • Lightness: The perceived intensity of a reflecting object • Sometimes lightness is called brightness if the object is emitting light instead of reflecting it. • In order to use color precisely in computer graphics, we need to be able to specify and measure colors.

3. Combining Colors Additive (RGB) Shining colored lightson a white ball Subtractive (CMYK) Mixing paint colors andilluminating with white light

4. How Do Artists Do It? • Artists often specify color as tints, shades, and tones of saturated (pure) pigments • Tint: Gotten by adding white to a pure pigment, decreasing saturation • Shade: Gotten byadding black to a pure pigment, decreasing lightness • Tone: Gotten by adding white and black to a pure pigment White Tints Pure Color Tones Grays Shades Black

5. HSV Color Space • Computer scientists frequently use an intuitive color space that corresponds to tint, shade, and tone: • Hue - The color we see (red, green, purple) • Saturation - How far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue) • Brightness (Luminance) - How bright is the color (how bright are the lights illuminating the object?)

6. Intuitive Color Spaces A top-down view of hexcone

7. HSV Color Space • A more intuitive color space • H = Hue • S = Saturation • V = Value (or brightness) Saturation Value Hue

8. Precise Color Specifications • Pigment-mixing is subjective --- depends on human observer, surrounding colors, lighting of the environment, etc • We need an objective color specification • Light is electromagnetic energy in the 400 to 700 nm wavelength range • Dominant wavelength is the wavelength of the color we “see” • Excitation purity is the proportion of pure colored light to white light • Luminanceis the amount (or intensity) of the light

9. Electromagnetic Spectrum • Visible light frequencies range between ... • Red = 4.3 x 1014 hertz (700nm) • Violet = 7.5 x 1014 hertz (400nm) Figures 15.1 from H&B

10. Visible Light • Hue = dominant frequency (highest peak) • Saturation = excitation purity (ratio of highest to rest) • Lightness = luminance (area under curve) White Light Orange Light Figures 15.3-4 from H&B

11. How well do we see color? • What color do we see the best? • Yellow-green at 550 nm • What color do we see the worst? • Blue at 440 nm • Flashback: Colortables (colormaps) for color storage • Which RGB value gets the most bits? • Can perceive color differences of 10 nm at extremes (violet and red) and 2 nm between blue and yellow • Metamers – different energy radiations look like the same color • Color perception also affected by surrounding light and adaptation

12. Just noticeable difference (JND) • 128 fully saturated hues can be distinguished • Cannot perceive hue differences with less saturated light. • Sensitivity to changes in saturation for a fixed hue and brightness ranges from 16 to 23 depending on hue. • Talked about representing intensities last lecture

13. Color Spaces • Three types of cones suggests color is a 3D quantity. How to define 3D color space? • Idea: • Shine given wavelength () on a screen • User must control three lasers producing three wavelengths (say R=700nm, G=546nm, and B=436nm) • Adjust intensity of RGB until colors are identical • Note phosphors of TV are not perfect RGBemitters as the results to right demonstrate

14. CIE Color Space • No standard set of three wavelengths can be combined to generate all other wavelengths. • The CIE(Commission Internationale d’Eclairage) defined three hypothetical lights X, Y, and Z with these spectra: • Idea: any wavelength  can be matched perceptually by positivecombinations of X, Y, and Z x ~ R y ~ G z ~ B

15. CIE Color Space • The gamut of all colors perceivable is thus a three-dimensional shape in X, Y, Z • Color = xX + yY + zZ

16. CIE Chromaticity Diagram (1931) • For simplicity, we often project to the 2D plane x + y + z = 1 • x = x / (x+y+z) • y = y / (x+y+z) • z = 1 – x - y

17. RGB Color Space (Color Cube) • Define colors with (r, g, b) amounts of red, green, and blue

18. YIQ Color Space • YIQis the color model used for color TV in America. Y is brightness,I (orange-cyan) & Q (green-magenta) are color • Note: Yis the same as CIE’s Y • Result: Use the Y alone and backwards compatibility with B/W TV! • These days when you convert RGB image to B/W image, the green and blue components are thrown away and red is used to control shades of grey (usually)

19. Converting Color Spaces • Converting between color models can also be expressed as such a matrix transform: • Note the relative unimportance of blue in computing the Y

20. Perceptually Uniform Color Space • Color space in which Euclidean distance between two colors in space is proportional to the perceived distance • CIE, RGB, not perceptually uniform • Example with RGB • LUV was created to be perceptually uniform

21. The CMY Color Model • Cyan, magenta, and yellow are the complements of red, green, and blue • We can use them as filters to subtract from white • The space is the same as RGB except the origin is white instead of black • This is useful for hardcopy devices like laser printers • If you put cyan ink on the page, no red light is reflected • Add black as option (CMYK) to match equal parts CMY

22. Halftoning • A technique used in newspaper printing • Only two intensities are possible, blob of ink and no blob of ink • But, the size of the blob can be varied • Also, the dither patterns of small dots can be used

23. Halftoning

24. Halftoning – dot size

25. Halftoning – Moire Patterns • Repeated use of same dot pattern for particular shade results in repeated pattern • Perceived as a moire pattern • Instead, randomize halftone pattern

26. Dithering • Halftoning for color images