Principles of Color. Visible Light OVERVIEW GEOGRAPHY 462: DIGITAL CARTOGRAPHY. Fall Dr. Gregory Elmes 1

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1 Principles of Color How Color is Used in Cartography. GEOG 462 Slides in part derived from Slocum text Thanks to Aileen Buckley for use of her slides OVERVIEW Visible Light Human Color Perception Hardware Fundamentals Color Models Disseminating Maps Visible Light Electromagnetic Energy Wave and particle properties Wave more useful in color theory Wavelength (illustrations) 10-3 nm to nm nanometer (10-9 m) Visible nm solar maximum violet blues 400nm reds 700nm Dr. Gregory Elmes 1

2 Color Sensation and Perception Human Color Vision» Trichromatic Theory Physiological structures of eye, rod cells and cone cells, Fovea α, β, γ cones peak sensitivity at 440nm, 530 nm and 570nm (blue, green, red] Opponent-Process Theory Signals from cones cells transmitted to brain indirectly» Opponent-Process Theory Interaction of signals from αβγ cones and rods Excitation and inhibition of signals sent by ganglion (nerve) cells BY, GR, WBK GREEN See Slocum, Fig 5.6, p. 87 Simple color circle Cyan BLUE Yellowgreen Magenta YELLOW Orange RED Color Models RGB Additive CMYK Subtractive Blue (0, 0, 255) 0, 255, 255??? Green (0,255,0) Red (255, 0, 0) (255,255,0) Disadvantages of RGB (and CMYK) Hue, Saturation and Lightness Equal steps in color model do not correspond to equal visual steps. Eye discerns changes in higher values much more readily than low ranges. Color Models Scientific description of color Precise replication HSV Hue, saturation and value (lightness) Munsell Commission International de l Eclairage (CIE) Rigorous but complex Dr. Gregory Elmes 2

3 Color Models Munsell Color Model User-orientated HUE VALUE» Perceived lightness CHROMA (Similar to Saturation)» Based on relationship to gray tone of same value Munsell Color Solid» Perceptual relationship between three dimensions Applications Color Choices Perceptual effects Preferred hues Variation, classification and color symbolization Graduated color series Univariate - monochromatic Bichromatic Color Use Color is used on maps to create aesthetically pleasing design distinguish classes of features increase number of features that can be mapped create visual hierarchy distinguish amount in classes promote figure-ground relationships promote affective design Symbolizing Qualitative Features Color variations in hue, not value or chroma, do not convey magnitude message Use conventions when possible blue for water features green for vegetated areas brown for land surface symbols (e.g. contour lines) yellow or tan for dry, sparsely vegetated regions Symbolizing Qualitative Features Color standards Pattern conventions and standards geologic structures, climatic regimes, administrative jurisdictions generally used along with hue, without systematic variation in texture types of patterns include Coarse line patterns Dot patterns (stippling) Pictographic patterns Reverse patterns Dr. Gregory Elmes 3

4 Symbolizing Qualitative Features Unique and mixture hues unique -- R,G,B,Y mixture hues -- combinations (except BY and RG) Hue-value combinations Four-Color Theorem Symbolic connotations of hue Physiological limitations hue sensitivity (8-15 hues is the limit, depending on size and complexity of areas) simultaneous contrast -- don t use similar hues or use patterns or category codes Symbolizing Quantitative Features Color variations in value or chroma, not hue, convey magnitude Color progression should allow map user to match color on map to color in legend convey correct impression of changes in magnitude across the map area allow for the comparison of quantitative data for features on two or more maps of the same area Approaches to color selection Symbolizing Quantitative Features Single-hue progressions -- one hue that varies only value and/or chroma Bi-polar progressions -- sequence of color grading between two endpoint hues Complementary hue progressions -- sequence of color grading between two complementary endpoint hues Partial spectral hue progressions -- sequence of colors that matches part of the solar spectrum Blended hue progressions -- sequence of colors grading between several hues, at least one of which is not in the spectrum Value progressions -- grading between white and black Full-spectrum progressions -- sequence of colors of the solar spectrum (doesn t inherently carry a magnitude message) Symbolizing Quantitative Features Color Brewer Cindy Penn State Two-variable (bi-variate) progressions Three-variable (tri-variate) progressions These require prominent and clear legend small black and white versions of the two themes that were composited notes on color scheme and attention to types of information that can be obtained from the map Dr. Gregory Elmes 4

5 Hue Guidelines for Quantitative Progressions Darker = more For bi- and tri-variate maps, the extremes = more You can use more color increments for a layertinted than you can for a choropleth map The number of distinguishable tints varies by hue Continuous tone mapping is sometimes effective for continuous distributions Remember simultaneous contrast! Value Guidelines for Quantitative Progressions The greater the value contrast between symbols, the greater the clarity and legibility Try to limit number of equal contrast value steps to 4-5, plus solid black and white Pattern-Value Guidelines for Quantitative Progressions Pattern and value are related -- differences are related to size and texture of marks (dots and lines) value only = > 75 lines/in value & pattern = lines/in pattern only = < 40 lines/in Pattern-texture progressions can be used to symbolize quantitative data Dr. Gregory Elmes 5

6 Pattern-Value Guidelines for Quantitative Progressions Line patterns and parallel lines at the same orientation (especially vertical) can be irritating to the eye If using line patterns, orient them at 45 Use lines separated by spacing that s greater than the line thickness Dr. Gregory Elmes 6