1 of 32 Computer Graphics Color

2 of 32 Basics Of Color elements of color:

3 of 32 Basics of Color Physics: –Illumination Electromagnetic spectra –Reflection Material properties Surface geometry and microgeometry Perception –Physiology and neurophysiology –Perceptual psychology

4 of 32 Electromagnetic Spectrum

5 of 32 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: Color tables (color maps) for color storage

6 of 32 Humans and Light when we view a source of light, our eyes respond to –hue: the color we see (red, green, purple) dominant frequency –saturation: how far is color from grey how far is the color from gray (pink is less saturated than red, sky blue is less saturated than royal blue) –brightness: how bright is the color how bright are the lights illuminating the object?

7 of 32 Hue hue (or simply, "color") is dominant wavelength –integration of energy for all visible wavelengths is proportional to intensity of color

8 of 32 Saturation or Purity of Light how washed out or how pure the color of the light appears –contribution of dominant light vs. other frequencies producing white light

9 of 32 Intensity, Brightness intensity : radiant energy emitted per unit of time, per unit solid angle, and per unit projected area of the source (related to the luminance of the source) brightness : perceived intensity of light

10 of 32 Combining Colors Additive (RGB) Shining colored lights on a white ball Subtractive (CMYK) Mixing paint colors and illuminating with white light

11 of 32 Colour Matching Experiment Target colour Mixing of 3 primaries Adjust intensities to match the colour overlap

12 of 32 RGB Color Space (Color Cube) Define colors with (r, g, b) amounts of red, green, and blue

13 of 32 CMY Color Model CMY (short for Cyan, Magenta, Yellow, and key) is a subtractive color model.

14 of 32 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

15 of 32 YIQ Color Space YIQ is the color model used for color TV in America. Y is brightness, I & Q are color –Note: Y is the same as Color space XYZ –Result: Use the Y alone and backwards compatibility with B/W TV! –I and Q are hue and purity –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)

16 of 32 Converting Color Spaces Y = R G B I = R – Y Q = B – Y Converting between color models can also be expressed as such a matrix transform: Note the relative unimportance of blue in computing the Y

17 of 32 Converting Color Spaces

18 of 32 HSV Color Space A more intuitive color space –H = Hue –S = Saturation –V = Value (or brightness) Value Saturation Hue

19 of 32 HSV Color Model Figure 15.16&15.17 from H&B H S V Color Red Green Blue *0.01.0White *0.00.5Gray * *0.0Black ? ? ?

20 of 32 Intuitive Color Spaces

21 of 32 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

22 of 32 Halftoning

23 of 32 Halftoning – dot size

24 of 32 Spatial versus Intensity Resolution Halftone Approximation: Dither –n  n pixels encode n intensity levels The distribution of intensities is randomized: dither noise, to avoid repeating visual artifacts

25 of 32 Dithering Halftoning for color images

26 of 32 No Comment Needed