Visualization Friday Forum Rachael Brady Visualization Friday Forum November 22, 2002 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. 118-125, October 1995.

Color is useful for classification Color is not needed to understand shape, spatial relationships of objects, movement Some objects differ from their surroundings primarily by color Food: Is this fruit ripe? Is this meat putrid? What kind of mushroom is this? Material: What kind of stone is this (for making an axe or knife)? 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 Yet another reason not to use blue to indicate the shapes of objects; it seems to be ignored in the Luminance calculation. 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. 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 Field Size Conventions 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 To enable color-blind people too view these changes as well, avoid maps that differ only in the red/green channel. Large areas, such as map regions, should use low contrast and vary slightly from one another (but have borders between). This enables small, vivid-colored targets to be seen against the background. Conventions: You may not always follow the conventions in a scientific domain, but you want to know when you are not and explicitly state it. 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 XXX This is not the blackbody radiation spectrum 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. 11-8. Taken from Russell Taylor II, UNC CS 290 Course Notes

In Summary Interesting values? Zero in range? High spatial frequency? 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. http://www.personal.psu.edu/faculty/c/a/cab38/ColorBrewerBeta.html