Color in Image and Video

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Color in Image and Video

ByRazia Nisar Noorani

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ContentsContents

Physical and perceptual aspects of colorPhysical and perceptual aspects of color

Several color modelsSeveral color models

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The Physics of Color

• The electromagnetic spectrum

• The spectrum of visible light

• Human response to color

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The electromagnetic spectrum, of which visible light is a very thin band

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The spectrum of visible light

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The spectrum as seen in nature

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Human response to color

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Color ModelsColor Models

A A color modelcolor model is an abstract is an abstract mathematical model describing the way mathematical model describing the way colors can be represented as can be represented as tuples of of numbers, typically as three or four values numbers, typically as three or four values or color components. or color components.

When this model is associated with a When this model is associated with a precise description of how the precise description of how the components are to be interpreted components are to be interpreted (viewing conditions, etc.), the resulting (viewing conditions, etc.), the resulting set of colors is called set of colors is called color space. .

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Color ModelsColor Models

An artist’s color wheel: red, yellow, and An artist’s color wheel: red, yellow, and blue (RYB)blue (RYB)

Additive color: red, green, blue (RGB)Additive color: red, green, blue (RGB)

Subtractive color: cyan, magenta, yellow, Subtractive color: cyan, magenta, yellow, and black (CMYK)and black (CMYK)

Hue, saturation, and brightness (HSB)Hue, saturation, and brightness (HSB)

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The artist’s model: red, yellow, and The artist’s model: red, yellow, and blueblue

Primary colors are sets of colors that can be combined to make a useful range of colors.

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The secondary colorsThe secondary colors

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The tertiary colorsThe tertiary colors

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Additive color: things that emit light, Additive color: things that emit light, especially monitors (RGB)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.

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In additive color . . .In additive color . . .

Red + Green = YellowRed + Green = Yellow Red + Blue = MagentaRed + Blue = Magenta Green + Blue = CyanGreen + Blue = Cyan

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Subtractive color: things that reflect Subtractive color: things that reflect (and selectively absorb) light (and selectively absorb) light

(CMYK)(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

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In subtractive color . . .In subtractive color . . .

Cyan subtracts redCyan subtracts red Magenta subtracts greenMagenta subtracts green Yellow subtracts blueYellow subtracts blue In photography, that’s it: all three together In photography, that’s it: all three together

subtract all light, giving blacksubtract all light, giving black In print, the dyesIn print, the dyes/inks/inks aren’t that good, and we aren’t that good, and we

need black ink tooneed black ink too Hence, four-color printing: CMYKHence, four-color printing: CMYK K from blacK; B already means BlueK from blacK; B already means Blue

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HSB: Hue, Saturation, and HSB: Hue, Saturation, and BrightnessBrightness

Hue: where a color lies around a color Hue: where a color lies around a color wheel: red, green, yellow, blue-green, etc.wheel: red, green, yellow, blue-green, etc.

Saturation: the “purity” of a color; a fully-Saturation: the “purity” of a color; a fully-saturated color has no white mixed with it, saturated color has no white mixed with it, in paint termsin paint terms

Brightness: light, dark, or in between?Brightness: light, dark, or in between?

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The color cone: hue, saturation, The color cone: hue, saturation, and brightness in relation to each and brightness in relation to each

otherother

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HSB: Hue, Saturation, and HSB: Hue, Saturation, and BrightnessBrightness

White, black, and all grays are zero percent White, black, and all grays are zero percent saturatedsaturated

A color becomes more saturated as it moves A color becomes more saturated as it moves away from gray to a pure coloraway from gray to a pure color

A pure (fully-saturated) color, in RGB terms, in A pure (fully-saturated) color, in RGB terms, in one that contains:one that contains: Only red, green, or blue, orOnly red, green, or blue, or Only yellow (= red + green), orOnly yellow (= red + green), or Only magenta (= red + blue)Only magenta (= red + blue) Only cyan (= blue + green)Only cyan (= blue + green)

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More on saturation, continuedMore on saturation, continued

Note: the previous slide said nothing about Note: the previous slide said nothing about the brightness of those pure colorsthe brightness of those pure colors

A saturated color can be a brilliant yellow, A saturated color can be a brilliant yellow, butbut

It can be a deep crimson, or midnight blueIt can be a deep crimson, or midnight blue

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Varying saturation, with brightness Varying saturation, with brightness held constantheld constant

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Varying brightness, with saturation Varying brightness, with saturation held constantheld constant

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Color models in computerColor models in computer

RGBRGB HSLHSL

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Color models in computerColor models in computer

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Color models in computerColor models in computer

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Color models in computerColor models in computer

For MonitorsFor Monitors RGBRGB HSLHSL

For PrintingFor Printing CMYKCMYK

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HP 364 Photo Black Ink Cartridge HP 364 Cyan Ink Cartridge

HP 364 Magenta Ink Cartridge HP 364 Yellow Ink Cartridge

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The HP 28 tri-color inkjet cartridge provides brilliant The HP 28 tri-color inkjet cartridge provides brilliant .... ....

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ContrastContrast

ContrastContrast is the difference in visual is the difference in visual properties that makes an object (or its properties that makes an object (or its representation in an image) representation in an image) distinguishable from other objects and the distinguishable from other objects and the background. background.

ContrastContrast is determined by the difference in is determined by the difference in the color and brightness of the object and the color and brightness of the object and other objects within the same field of view. other objects within the same field of view.

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ContrastContrast

Contrasting colors in terms of a computer's Contrasting colors in terms of a computer's representation of an image, means the "primary representation of an image, means the "primary colors" or the colors with color components of 0 colors" or the colors with color components of 0 or 255 (Min and Max).  or 255 (Min and Max). 

Black, White, Red, Green, Blue, Cyan, Magenta, Black, White, Red, Green, Blue, Cyan, Magenta, and Yellow are the high contrast colors.  and Yellow are the high contrast colors. 

When all the colors in an image are around one When all the colors in an image are around one single color, that image has low contrast.  single color, that image has low contrast. 

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ContrastContrast

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Color Models in Video

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Color Models in Video

Largely derive from older analog methods of coding color for TV. Luminance is separated from color information.

For example, a matrix transform method called YIQ is used to transmit TV signals in North America and Japan.

This coding also makes its way into VHS video tape coding in these countries since video tape technologies also use YIQ.

In Europe, video tape uses the PAL or SECAM codings, which are based on TV that uses a matrix transform called YUV.

Finally, digital video mostly uses a matrix transform called YCbCr that is closely related to YUV

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Sample Input

Gamma Corrected Input

Monitor Output

Graph of Correction L' = L ^ (1/2.5)

Gamma Correction

• Gamma correction provides displaying an image accurately on a computer screen. • Images which are not properly corrected can look either bleached out, or too dark.• Trying to reproduce colors accurately also requires some knowledge of gamma. • Varying the amount of gamma correction changes not only the brightness, but also

the ratios of red to green to blue.

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YUV Color Model

YUV codes a luminance signal (for gamma-corrected signals) equal to Y’.

Chrominance refers to the difference between a color and a reference white at the same luminance.

U = B’ − Y’; V=R’−Y’

R' , G' , B' : gamma correction applied

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YIQ Color Model

YIQ is used in NTSC color TV broadcasting.

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Y component

I component

Q component

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From RGB to YIQ:

Y = 0.299 R + 0.587 G + 0.114 B

I = 0.5957161349127745 R − 0.2744528378392564 G − 0.3212632970735180 B

Q = 0.211456402120117 R − 0.5225910452916111 G + 0.3111346431714933 B

From YIQ to RGB:

R = Y + 0.9562948323208939905 I + 0.6210251254447287141 Q

G = Y − 0.2721214740839773195 I − 0.6473809535176157223 Q

B = Y − 1.106989908567128216 I + 1.704614975498829329 Q

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YCbCr Color Model

The Rec. 601 standard for digital video uses another color space, YCbCr.