What is color? Color vocabulary Chapter 9: Color Color mixtures Intensity-distribution curves Specifying colors Hue, saturation and brightness Color trees RGB color specification Chromaticity What is Color? Wavelength is a property of an electromagnetic wave in the frequency range we call light Coloris a psychological phenomenon that occurs when light waves of different wavelengths interact with the human visual system Light wave with wavelength 650 nm Human eye Color Human visual system and brain Color Mixtures A spectral color is a single line on this graph: Color Mixtures Almost everything we see is not a spectral color but a mixture of many spectral colors. Green laser pointer 532 nm 1
Intensity-Distribution Curves Additive Color Mixing How can we describe the sensation of a mixture of lights? Both of these lights would appear white, so we can see there may be several intensity distributions that generate the same color Additive Mixing: Yellow Additive Mixing: Cyan 2
Additive Mixing: Magenta Additive Primary Colors Red, green and blue are called the additive primaries We want to select primary colors that allow us to create the largest possible number of other colors using just those primary colors 530-nm green yellow cyan 650-nm red magenta 460-nm blue Most colors can be described in terms of mixtures of red, green and blue RGB Color Specification RGB Colors: Example RGB (red green blue) is another way of specifying colors. White is made by maximum intensity of all three colors Instead of hue, saturation and brightness, you can specify the amount of each color 530-nm green yellow cyan 650-nm red magenta 460-nm blue Hue: 35 Saturation: 100% Brightness: 100% 3
RGB Colors: Example RGB Colors: Example Hue: 60 Saturation: 0% Brightness: 40% Hue: 60 Saturation: 100% Brightness: 35% Complementary Colors We ve seen that red light plus green light plus blue light equals white light: R + G + B = White But we also know that red light plus green light equals yellow light: R + G = Y If we do some dodgy color math, we get that yellow light plus blue light equals white light: Y + B = White Complementary Colors We then define blue and yellow as complementary colors: When additivelymixed, complementary colors produce white. Additive mixing NOT additive mixing 4
Chromaticity Chromaticity This is a chromaticity diagram (see color plate 9.2 in the book) We are not going to discuss where this comes from, please read that section of the book A chromaticity diagram has a fixed brightness or lightness for all colors The most saturated colors (of different hues) are around the rim (perimeter) Insideare the less saturated colors, including white at the interior Saturated colors Unsaturated colors Chromaticity hue Chromaticity: Lightness The colors on the curved part of the edge are wavelength colors Wavelength colors hue The colors on the straight part are non-wavelengthcolors, but are still 100% saturated. saturation lightness saturation lightness Non-wavelength colors 5
Uses of Chromaticity Diagrams To identify colors with three numbers To predict the results of additive mixing To understand complementary colors To find the dominant hue of a color To understand color gamuts and the significance of RGB mixing Chromaticity: Identifying Colors The numbers that we use to identify a color are its x-value and y-value inside the diagram and a z-value to indicate its brightness or lightness x and y specify the chromaticity of a color Example: Apple pickers are told around the country that certain apples are best picked when they are a certain red (see black dot) x = 0.57 y = 0.28 Chromaticity: Identifying Colors The "purest" white is at x = 0.33 y = 0.33 Chromaticity: Color Mixtures An additive mixtureof two wavelength colors lies along the line joining them Example: The colors seen by mixing 700 nm red and 500 nm green lie along the line shown Wherealong the line is the color of the mixture? 6
Chromaticity: Color Mixtures This depends on the relative intensities of the 700 nm red and the 500 nm green. Much more green than red gives a green Much more red than green gives an orange-red Slightly more red than green gives a yellow This also works for mixing colors not at the edges Chromaticity: Complementary Colors Recall that a color combined with its complement produces white If mixtures lie on the line between the two colors, then we can find the complement by drawing a line through white to the other side Chromaticity: Complementary Colors Using this diagram, we can see that the complement of 700 nm red is 490 nm cyan And the complement of 520 nm green is magenta (a non-wavelength color) Chromaticity: Dominant Hues Unsaturated colors are related to their saturated counterparts by the additive mixture with white Ex: Pink (unsaturated) = red (saturated) + white 7
Chromaticity: Dominant Hues To find the dominant hue of the color indicated by the black dot Draw straight line from whitethrough the point to get dominant wavelength, and hence, hue (547 nm green) Chromaticity: Dominant Hues If hue is in the nonwavelength purples, find the complementary dominant wavelength by extending backwards through white (get 495 nm cyan) There is no dominant wavelength color for magentas and purples Chromaticity: Color Gamuts The gamut of colors which can be reproduced by an additive mixture of red, green and blue light is inside the triangle we now draw at right. 530-nm green 460-nm blue 650-nm red Chromaticity: Color Gamuts Other gamuts are generally even smaller Say we picked red blue and yellow, the artists primaries: We would not be able to make cyans or greens 8
Color Gamut of Flat Panel Displays Display Pixels: Partitive Mixing In additive mixing, we assume that the different wavelength colors reaching your eye are all coming from the same place: The intensity distribution of the primary colors used in an LED backlit display The color gamutsof LED vs. CCFL backlit displays Display Pixels: Partitive Mixing Partitive Mixing: Pointillism What it instead, we make the different colors coming from separate, very small, very closely spaced points You eye cannot see them as separate sources, so the colors mix and you see the same color. This is called partitive mixing 9
Subtractive Mixing In additive mixing, we added the wavelengths that were hitting the eye: say red light and green light Subtractive Mixing Suppose we have a range of wavelengths hitting some object: What about things like filters and dye pigments? These mix by subtractive mixing Three things can happen to each wavelength of light: Reflection: that particular wavelength bounces off the object Transmission: that wavelength passes through the object Absorption: that wavelength is soaked up by the object In general different things happen at different wavelengths Colored Filters Recall that magenta is the additive mixture of blue and red If a light looks magenta, it means that red and blue light is reaching our eyes, thus a magenta filter must transmitblue and red light, and subtract, by reflection or absorption, green light. Colored Filters What happens if we layer colored filters? Filters subtract light by absorption or reflection = = Incident white light Magenta filter subtracts green Cyan filter subtracts red Only blue gets through 10
Another example: = Colored Filters Subtractive Mixing: Primaries The subtractive primaries are Cyan Magenta Yellow Incident white light Magenta filter subtracts green Yellow filter subtracts blue Only red gets through In subtractive mixing, combining complementary colors produces black 11
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