Colormaps 101 Sources – Russell Taylor Comp 290 course notes Colin Ware – Perception for Design IBM Web Site An Excellent Paper: A Rule-based Tool for Assisting Colormap Selection, L. Bergman, B. Rogowitz and L. Treinish. Proceedings of the IEEE Computer Society Visualization '95 pp , October Rachael Brady Visualization Friday Forum November 22, 2002

Color is useful for classification Color is not needed to understand shape, spatial relationships of objects, movement Laboratory assistant went 21 years without realizing he was color- blind

Color Space: RGB Taken from Russell Taylor II, UNC CS 290 Course Notes

Colorspace: HSV Taken from Russell Taylor II, UNC CS 290 Course Notes

Opponent Process Theory (Hering 1920) The six elementary colors are paired on 3 orthogonal axis: Black-White, Red-Green, Yellow-Blue Modified from Russell Taylor II, UNC CS 290 Course Notes

Color Category Perception Task: Name the colors Regions same > 75% Nonuniform sizes Why “rainbow scale” is so nonuniform The fact that only 8 hues were named out of 210 different colors indicates that there may be only a few colors available for labeling. Taken from Russell Taylor II, UNC CS 290 Course Notes

Other Color Issues Color Blindness –Most red/green color blind (10% of males, 1% females) Field Size –Avoid small spots, especially in yellow/blue –Small areas: strong, highly-saturated colors –Large areas: low saturation with slight differences Conventions –U.S.: Red = danger, Green = life –China: Red = life, Green = death –Some scientific domains have color conventions Taken from Russell Taylor II, UNC CS 290 Course Notes

Designing of a colormap Perceptually Ordered Sequence is required, such as black- white, red-green, blue-yellow, or saturation (dull->vivid) Note: a perceptually ordered sequence will result from a series of colors that monotonically increase or decrease with respect to one or more of the color opponent channels Use a Luminance Ordered Sequence for High Spatial Frequency Data Use a Saturation Ordered Sequence for Low Spatial Frequency Data Use several discrete colors when it is necessary to read back values (to avoid contrast effects)

Example Colormaps Source: Ware, Perception for Design

Not ordered (red at both ends) Taken from Russell Taylor II, UNC CS 290 Course Notes

Luminance (Gray) Scale Taken from Russell Taylor II, UNC CS 290 Course Notes

Saturation Scale Taken from Russell Taylor II, UNC CS 290 Course Notes

Hue Scale Taken from Russell Taylor II, UNC CS 290 Course Notes

Hue+Luminence Taken from Russell Taylor II, UNC CS 290 Course Notes

Hue+Saturation Taken from Russell Taylor II, UNC CS 290 Course Notes

Black Body Radiation Taken from Russell Taylor II, UNC CS 290 Course Notes

Colormap Choice Should be Linked to Task Atmospheric Motion Bergman, Rogowitz, Treinish, IEEE Vis ‘95

Colormap Choice Should be Linked to Task Pollution Levels Bergman, Rogowitz, Treinish, IEEE Vis ‘95

Color Interaction Bergman, Rogowitz, Treinish, IEEE Vis ‘95

Color Interaction Bergman, Rogowitz, Treinish, IEEE Vis ‘95

Trumbo’s Principles Univariate –Order: ordered values should be represented by perceptually-ordered colors –Separation: significantly different levels should be represented by distinguishable colors Modified from Russell Taylor II, UNC CS 290 Course Notes

Ordered (and double-ended) Tufte ‘97, pg. 76. Taken from Russell Taylor II, UNC CS 290 Course Notes

Not ordered Tufte ‘97, pg. 77. Taken from Russell Taylor II, UNC CS 290 Course Notes

Double-ended Scale Two distinct scales joined at neutral middle Characteristics –segments values into two groups –can emphasize both extremes of data range Taken from Russell Taylor II, UNC CS 290 Course Notes

Double-Ended Income Olson ‘97, fig Taken from Russell Taylor II, UNC CS 290 Course Notes

In Summary Interesting values? –Position striking colors at interesting values Zero in range? –Double-ended scale High spatial frequency? –Vary lightness in addition to hue Taken from Russell Taylor II, UNC CS 290 Course Notes

Color Theory in Cartography is well developed. Cynthia Brewer at Penn State has a great color creator for discrete, monotonic color sequences.