# Introduction to Computer Graphics ColorColor. Specifying Color Color perception usually involves three quantities: Hue: Distinguishes between colors like.

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Introduction to Computer Graphics ColorColor

Specifying Color Color perception usually involves three quantities: Hue: Distinguishes between colors like red, green, blue, etcHue: Distinguishes between colors like red, green, blue, etc Saturation: How far the color is from a gray of equal intensitySaturation: How far the color is from a gray of equal intensity Lightness: The perceived intensity of a reflecting objectLightness: 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. Color perception usually involves three quantities: Hue: Distinguishes between colors like red, green, blue, etcHue: Distinguishes between colors like red, green, blue, etc Saturation: How far the color is from a gray of equal intensitySaturation: How far the color is from a gray of equal intensity Lightness: The perceived intensity of a reflecting objectLightness: 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) 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)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)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?)Brightness (Luminance) - How bright is the color (how bright are the lights illuminating the object?) Computer scientists frequently use an intuitive color space that corresponds to tint, shade, and tone: Hue - The color we see (red, green, purple)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)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?)Brightness (Luminance) - How bright is the color (how bright are the lights illuminating the object?)

Intuitive Color Spaces A top-down view of hexcone

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

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

Electromagnetic Spectrum Visible light frequencies range between... Red = 4.3 x 10 14 hertz (700nm)Red = 4.3 x 10 14 hertz (700nm) Violet = 7.5 x 10 14 hertz (400nm)Violet = 7.5 x 10 14 hertz (400nm) Visible light frequencies range between... Red = 4.3 x 10 14 hertz (700nm)Red = 4.3 x 10 14 hertz (700nm) Violet = 7.5 x 10 14 hertz (400nm)Violet = 7.5 x 10 14 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) 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 nmYellow-green at 550 nm What color do we see the worst? Blue at 440 nmBlue at 440 nm Flashback: Colortables (colormaps) for color storage Which RGB value gets the most bits?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 What color do we see the best? Yellow-green at 550 nmYellow-green at 550 nm What color do we see the worst? Blue at 440 nmBlue at 440 nm Flashback: Colortables (colormaps) for color storage Which RGB value gets the most bits?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

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 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

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

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 positive combinations of X, Y, and Z 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 positive combinations of X, Y, and Z x ~ R y ~ G z ~ B

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

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

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

YIQ Color Space YIQ is the color model used for color TV in America. Y is brightness, I (orange-cyan) & Q (green-magenta) are color Note: Y is the same as CIE’s YNote: Y is the same as CIE’s Y Result: Use the Y alone and backwards compatibility with B/W TV!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)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) YIQ is the color model used for color TV in America. Y is brightness, I (orange-cyan) & Q (green-magenta) are color Note: Y is the same as CIE’s YNote: Y is the same as CIE’s Y Result: Use the Y alone and backwards compatibility with B/W TV!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)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)

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 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 uniformCIE, RGB, not perceptually uniform –Example with RGB –LUV was created to be perceptually uniform Color space in which Euclidean distance between two colors in space is proportional to the perceived distance CIE, RGB, not perceptually uniformCIE, 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 whiteWe can use them as filters to subtract from white The space is the same as RGB except the origin is white instead of blackThe 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 reflectedIf you put cyan ink on the page, no red light is reflected Add black as option (CMYK) to match equal parts CMYAdd black as option (CMYK) to match equal parts CMY Cyan, magenta, and yellow are the complements of red, green, and blue We can use them as filters to subtract from whiteWe can use them as filters to subtract from white The space is the same as RGB except the origin is white instead of blackThe 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 reflectedIf you put cyan ink on the page, no red light is reflected Add black as option (CMYK) to match equal parts CMYAdd black as option (CMYK) to match equal parts CMY

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 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 patternPerceived as a moire pattern Instead, randomize halftone patternInstead, randomize halftone pattern Repeated use of same dot pattern for particular shade results in repeated pattern Perceived as a moire patternPerceived as a moire pattern Instead, randomize halftone patternInstead, randomize halftone pattern

Dithering Halftoning for color images

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