Half Toning Dithering RGB CMYK Models Colors in Graphics Half Toning Dithering RGB CMYK Models

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

Halftoning

Halftoning – dot size

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

Dithering Halftoning for color images

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.

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

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?)

HSV Color Model Hue (H) is the angle around the vertical axis Saturation (S) is a value from 0 to 1 indicating how far from the vertical axis the color lies Value (V) is the height of the hexcone”

HSV Color Model H S V Color 0 1.0 1.0 Red 120 1.0 1.0 Green 240 1.0 1.0 Blue * 0.0 1.0 White * 0.0 0.5 Gray * * 0.0 Black 60 1.0 1.0 ? 270 0.5 1.0 ? 270 0.0 0.7 ? Figure 15.16&15.17 from H&B

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

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

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

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

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

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

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

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