CS 450: Color presents about Color Receptors; The Luminous-Efficiency Function; The RGB Color Model; Luminance and Chromaticity; Luminance; Luminance; The CIE Chromaticity Diagram; Color Gamuts; The RGB Model; The CMYK Model; Tints and Shades.
CS 450 Color Color Receptors Visible light is in the range 400 nm (blue) to 700 nm (red) Cones have three different kinds of color-sensitive pigments, each responding to a different range of wavelengths These are roughly “red”, “green”, and “blue” in their peak response but each responds to a wide range of wavelengths The combination of the responses of these different receptors gives us our color perception This is called the tristimulus model of color vision CS 450 Color The Luminous-Efficiency Function Because the three receptors respond with different amplitudes and different peak frequencies, the apparent brightness of a color depends not only on the luminance but on the wavelength Plotting perceived luminance as a function of wavelength gives the luminous-efficiency function of the eye CS 450 Color The RGB Color Model The simplest color model is to attempt to model these three stimulus values: red, green, and blue 24-bit color: • one byte for red • one byte for green • one byte for blue Each is called a channel Problem: Machine word sizes are usually 32 bit, not 24 bit Solution: Use 32 bits per pixel: 24 color (3 channels), “alpha” channel CS 450 Color Luminance and Chromaticity RGB isn’t the most intuitive model of color Artists usually think of dark/light and color as two different things: • Luminance • Chromaticity CS 450 Color Luminance • • • • • • Luminance Intensity Brightness Lightness Luma Value all refer to the light/dark properties of the stimulus Some models use linear, non-linear, or perceptually linear encoding CS 450 Color Chromaticity Requires two parameters Usually: Hue The dominant wavelength Saturation How pure that color is Some models use different chromaticity parameters Examples: Red vs Blue? Red vs Pink? Dark red vs Bright red? CS 450 Color The CIE Chromaticity Diagram CIE uses three primaries X, Y , and Z to replace red, green, and blue The Y component was chosen to correspond to luminous efficiency, so Y = luminance X, Z = chromaticity And normalized x = X X +Y +Z y = Y X +Y +Z z = Z X +Y +Z CS 450 Color The CIE Chromaticity Diagram Consider the plane x+y+z =1 or equivalently X +Y +Z =1 Every point on this plane can be characterized by unique x and y (z = − x − y) Plotting on this plane the color of each wavelength gives the CIE chromaticity diagram CS 450 Color Color Gamuts The color space spanned by a set of primary (base) colors is called a color gamut Example: the space of all colors that can be displayed by a device with three color phosphors is the gamut of that device No three primary colors (with positive weights) spans the full space of perceivable colors! CS 450 Color The RGB Model When light is mixed, wavelengths combine (add) The RGB model is used most often for additive models Adding a color and its complement create white Primary Complement Red Cyan Green Magenta Blue Yellow 10 CS 450 Color 11 The CMYK Model Paint subtracts color from white light and reflects the rest Mixing paint pigments subtracts multiple colors The most common subtractive model is the Cyan-Magenta-Yellow model C R M = − G Y B Adding a subtractive color and its complement create black Problem: paints/inks/dyes are imperfect, so it’s hard to make pure black Solution: add black (K) as a fourth primary CS 450 Color Tints and Shades Tint: adding more white to a color Shade: adding more black to a color Tints and shades are not inverses! 12 CS 450 Color 13 The NTSC YIQ Model The NTSC standard uses a color model called YIQ: Y 0.299 0.587 0.114 R I = 0.596 −0.275 −0.321 G Q 0.212 −0.532 0.311 B Y luminance, I and Q chromaticity Color TV broadcasts are YIQ; black-and-white TV uses Y only CS 450 Color The HSV Model Another color model is the Hue-Saturation-Value model More intuitive to understand and manipulate Nonlinear computation to convert to RGB Singularities at pure black and pure white 14 CS 450 Color Other Color Models CIE LUV Perceptually linear version of CIE XYZ HLS Hue-Lightness-Saturation HVC Hue-Value-Chroma and many others 15 CS 450 Color Pseudocolor Many color-encoding schemes use some finite number of colors from a larger palette Example: 8-bit color uses a 256-element look-up table to map to 24-bit values Problem: how to convert 24-bit image to 8-bit pseudocolor? (Color quantization) 16 CS 450 Color Color and Visualization Color can make information visualizations more interesting, but be careful using color to convey information Specific color meanings: useful, but keep them limited Color scales are harder to interpret than intensity scales (Heat scale is one of the few that partially works.) 17 CS 450 Color Color Image Processing Pick a color space that makes sense for your application If you want to manipulate intensity without changing chromaticity, use an intensity-chromaticity based model If you want perceptual linearity (interpolation, etc.) use CIE LUV or similar If you want user interaction, try HSV or something similar Don’t just think that RGB is the only way to manipulate color 18 ... on this plane the color of each wavelength gives the CIE chromaticity diagram CS 450 Color Color Gamuts The color space spanned by a set of primary (base) colors is called a color gamut Example:... white 14 CS 450 Color Other Color Models CIE LUV Perceptually linear version of CIE XYZ HLS Hue-Lightness-Saturation HVC Hue-Value-Chroma and many others 15 CS 450 Color Pseudocolor Many color- encoding... luminous-efficiency function of the eye CS 450 Color The RGB Color Model The simplest color model is to attempt to model these three stimulus values: red, green, and blue 24-bit color: • one byte for red •