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Chapter 15 Principles for color thematic maps

•CRTs use additive colors (RGB) and printer uses subtractive colors (CMYK)

•Visible Light - is a type of EM (electromagnetic energy), measured in nm (between 380 to 760 nm)

•Shorter wavelength (blue) bent more than longer wavelength (red)

•Rods and cones - specialized nerve cells contain light-sensitive chemicals called visual pigments

•6 million cones - function in bright light and responsible for color vision, 120 million rods function in dim light, not associated with color.

•Most sensitive wavelength - short (blue), medium (green) and long (red). In retina, blue cones cover largest area but is the least sensitive which make it inappropriate for small map features.

Color blindness – 7% male and 1% female

http://www.toledo-bend.com/colorblind/Ishihara.html

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Physical Properties of Colors• Transparent objects• Reflective• Opaque• Desert Island Experiment

– chromatic and achromatic

– Value in achromatic

– Hue

– Saturation (chroma) – less gray is more saturated

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Hue/Value/Chroma(H/S/B in Photoshop)

• Hue – name of the colors. Color wheel (Figure 15.4/Plate 2) can contain millions, but most with 24 colors. 12 being used by artists.

• Value – lightness/darkness, controlled by adding white/black to hues. If white is added, a tint results, a shade is produced if black is added. Against different background will result in different views. (Fig 15.5)

• Chroma(Saturation)– Hue away from Gray. From 0 to 100 % showing the total gray to total hue.

• Achromatic – zero chroma

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Color Interactions• Simultaneous Contrast – eye

spontaneously produces the complementary color (opposite color in color wheel). Induction (lighter in a darker background, darker in a lighter background, fig 15.5) the apparent color of an area will tend to shift toward the opponent color of the surround color. (gray surrounded by green will appear reddish, surrounded by blue will appear yellowish)

• Successive contrast – orange will appear darker and more red on a yellow background and lighter and more yellow on a red background.

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Color Preferences

• Color preference is somewhat developmental

• Warm colors – longer wavelengths (red, orange, yellow)

• Cool colors – shorter wavelength (violet, blue, and green)

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Color Preference• Aged 4-5: prefers warm colors,

Red -> Blue and Green.• School-age children reject fully

saturated colors, and achromatic color.

• Older, prefers shorter wavelength. Preferences of N Am. adults, B-R-G-Violet-O-Y.

• The greenish-yellow hues are the least liked by man and women. Women prefer R over B, and Y over O and men in reversed order.

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Color Combination

• Either light or dark background not intermediate in lightness. Objects stand out from background.

• Pleasant object color – green to blue, little gray. Unpleasant color – Y to YG or considerable gray

• Table 15.2 Connotative Meanings and Color

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Map Use Tasks

• Specific purpose - unordered hues work best.• General purpose -ordered colors (shading) performed

better• Hue-lightness scheme was the best for specific purpose

maps, according to Mersey. • Mixed color come from each of the opposing channels, for

example red (from red-green channel) and yellow (from blue-yellow) merge to form orange.

• Opponent process was successful for selecting colors for unipolar data (such as income for counties in TN)

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Kinds of Data

• Unipolar data - use sequential scheme. (gray scale thematic maps), lightness and lightness+saturation (hue, such as greenish..)

• Bipolar data - use diverging scheme, in which two hues diverge away from a common light hue or a neutral gray., such as a “dark red-> light red-> gray-> light blue -> dark blue” scheme (see demo)

• Balanced data (usually involve tow variables) - use sequential schemes to emphasize the high end of the data, or alternatively a diverging scheme to emphasize the midpoint of the balanced data.

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Types of Maps• In isarithmic map, data are presumed to be part of a

smooth continuous surface where in choropleth maps different data adjacent to each other so that viewers need to consult the legend to understand the value associated with the map elements.

• Spectral scheme (color in em spectrum, or referred as “ROYGBIV”) where colors in red portion of the spectrum should appear near than colors from blue portion - referred as “Color Stereoscpic Effect”. Used only for elevation

• Criticism of spectral scheme - yellow (a light color) is in the middle of the scheme.

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Color Schemes used by Mersey (1990)

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Color Schemes used by Mersey (1990)

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Color Schemes used by Mersey (1990)

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Sequential color schemes

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Sequential color schemes

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HSV (HSB in Photoshop)

• disadvantage: different hues having the same value in HSV will not all have the same perceived value

• mid-value will not represent mid-color between two hues.complementary colors are 180o apart.

link

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HSB in Photoshop

• In HSB color mode, specify saturation and brightness as percentages; specify hue as an angle from 0° to 360° that corresponds to a location on the color wheel. (See About color modes and models (Photoshop) for information on the color wheel.)

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CIE Model(Photoshop : In Lab mode, enter a lightness value (L) from 0 to 100 and a axis (green to magenta) and b axis (blue to yellow) values from -128 to +127)

• color specification in CIE means that anyone in the world should be able to recognize and reproduce a desired color. most colors can be defined by a mixture of three colors (RGB).

• Yxy model - x and y coordinates define a 2-d space where hue and saturation vary.

• To avoid negative values, transform is required to obtain X,Yand Z

• To get coordinates for the Yxy system, the X,Y and Z values were converted to proportional values:

• x = X /(X + Y + Z)

• y = Y /(X + Y + Z)

• z = Z / (X + Y + Z)

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L*a*b model• L*a*b color is designed to be device independent, creating

consistent color regardless of the device (such as a monitor, printer, computer, or scanner) used to create or output the image.

• L*a*b color consists of a luminance or lightness component (L) and two chromatic components: the a component (from green to red) and the b component (from blue to yellow).

• A. Luminance=100 (white) B. Green to red component C. Blue to yellow component D. Luminance=0 (black)

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Practice• Start “Photoshop” and create a new file (go to File > New and

then OK to accept the settings)

• Click the “Set Background Color” in Toolbox to open the Color Picker window

• Adjust color using RGB (R:100, G:100, B:100) and

• Click “Ellipse” tool to draw an Ellipse

• Change color to Blue (R:0,G:0,B:255) and use Paint Bucket to paint over the existing object (if you encounter the problem like this

• Go to Layer> New Layer

• Work on the new layer

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Practice: HSB/L*a*b in Photoshop

• Try to change H from 0 to 359 and see the colors corresponding to the number

• Find the H number for green and blue (slide #8)

• Adjust saturation and brightness

• Do similar adjustment on L*a*b