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Color Models for Images. By Razia Nisar Noorani. Contents. Physical and perceptual aspects of color Several color models. The Physics of Color The electromagnetic spectrum The spectrum of visible light Human response to color.
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Color Models for Images By Razia Nisar Noorani
Contents • Physical and perceptual aspects of color • Several color models
The Physics of Color • The electromagnetic spectrum • The spectrum of visible light • Human response to color
The electromagnetic spectrum, of which visible light is a very thin band
Color Models A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components. When this model is associated with a precise description of how the components are to be interpreted (viewing conditions, etc.), the resulting set of colors is called color space.
Color Models • An artist’s color wheel: red, yellow, and blue (RYB) • Additive color: red, green, blue (RGB) • Subtractive color: cyan, magenta, yellow, and black (CMYK) • Hue, saturation, and brightness (HSB)
The artist’s model: red, yellow, and blue Primary colors are sets of colors that can be combined to make a useful range of colors.
Additive color: things that emit light, especially monitors (RGB) For additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green, and blue.
In additive color . . . • Red + Green = Yellow • Red + Blue = Magenta • Green + Blue = Cyan
Subtractive color: things that reflect (and selectively absorb) light (CMYK) For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are cyan, magenta, and yellow
In subtractive color . . . • Cyan subtracts red • Magenta subtracts green • Yellow subtracts blue • In photography, that’s it: all three together subtract all light, giving black • In print, the dyes/inks aren’t that good, and we need black ink too • Hence, four-color printing: CMYK • K from blacK; B already means Blue
HSB: Hue, Saturation, and Brightness • Hue: where a color lies around a color wheel: red, green, yellow, blue-green, etc. • Saturation: the “purity” of a color; a fully-saturated color has no white mixed with it, in paint terms • Brightness: light, dark, or in between?
The color cone: hue, saturation, and brightness in relation to each other
HSB: Hue, Saturation, and Brightness • White, black, and all grays are zero percent saturated • A color becomes more saturated as it moves away from gray to a pure color • A pure (fully-saturated) color, in RGB terms, in one that contains: • Only red, green, or blue, or • Only yellow (= red + green), or • Only magenta (= red + blue) • Only cyan (= blue + green)
More on saturation, continued • Note: the previous slide said nothing about the brightness of those pure colors • A saturated color can be a brilliant yellow, but • It can be a deep crimson, or midnight blue
Color models in computer • RGB • HSL
Color models in computer • For Monitors • RGB • HSL • For Printing • CMYK
HP 364 Yellow Ink Cartridge HP 364 Magenta Ink Cartridge HP 364 Cyan Ink Cartridge HP 364 Photo Black Ink Cartridge
The HP 28 tri-color inkjet cartridge provides brilliant ....
Contrast • Contrast is the difference in visual properties that makes an object (or its representation in an image) distinguishable from other objects and the background. • Contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view.
Contrast • Contrasting colors in terms of a computer's representation of an image, means the "primary colors" or the colors with color components of 0 or 255 (Min and Max). • Black, White, Red, Green, Blue, Cyan, Magenta, and Yellow are the high contrast colors. • When all the colors in an image are around one single color, that image has low contrast.
