CS 544 Human Abilities Color Perception and Guidelines for Design Preattentive Processing Acknowledgement: Some of the material in these lectures is based on material prepared for similar courses by Saul Greenberg (University of Calgary), Ravin Balakrishnan (University of Toronto), James Landay (University of California at Berkeley), monica schraefel (University of Toronto), and Colin Ware (University of New Hampshire). Used with the permission of the respective original authors. 1 UI hall of shame From IBM s RealCD Prompt Button Black on Black? Cool! But you can t see it! click here prompt should not be needed. 2
Why study color? Color can be a powerful tool to improve user interfaces, but its inappropriate use can severely reduce the performance of the systems we build 3 Visible Spectrum 4
Human Visual System Light passes through lens Focused on retina 5 Retina covered with light-sensitive receptors rods sensitive to broad spectrum of light unable to resolve detail overstimulated in all but the dimmest light can t discriminate between colors sense intensity or shades of gray primarily for night vision & perceiving movement 120 million per eye cones less sensitive to light used to sense color 6 million per eye 6
Retina Center of retina (fovea) has most of the cones allows for high acuity of objects focused at center Edge of retina, periphery, is dominated by rods allows detecting motion of threats in periphery 7 Trichromacy theory Cone receptors used to sense color 3 types: blue, green, red (really yellow) each sensitive to different band of spectrum ratio of neural activity of the 3 color other colors are perceived by combining stimulation 8
Color Sensitivity Really yellow 9 Distribution of cones Not distributed evenly mainly reds (64%) & very few blues (4%) insensitivity to short wavelengths cyan to deep-blue Center of retina (high acuity) has no blue cones small blue objects you fixate on disappear 10
Color Sensitivity & Image Detection Most sensitive to the center of the spectrum blues & reds must be brighter than greens & yellows Brightness determined mainly by R+G Shapes detected by finding edges combine brightness (luminance actually) & color differences for sharpness Implications? hard to deal with blue edges & blue shapes 11 Color Sensitivity (cont.) As we age lens yellows & absorbs shorter wavelengths sensitivity to blue is even more reduced fluid between lens and retina absorbs more light perceive a lower level of brightness Implications Blue text on a dark background to be avoided. We have few shortwavelength sensitive cones in the retina and they are not very sensitive. Older users need brighter colors. Blue text on a dark background to be avoided. We have few shortwavelength sensitive cones in the retina and they are not very sensitive. Older users need brighter colors. 12
Focus Different wavelengths of light focused at different distances behind eye s lens need for constant refocusing causes fatigue careful about color combinations Pure (saturated) colors require more focusing then less pure (desaturated) don t use saturated colors in UIs unless you really need something to stand out (stop sign) 13 Peripheral acuity 14 With strict fixation of the center spot, each letter is equally legible because it is about ten times its threshold size. This is true at any viewing distance. Chart shows the increasingly coarse grain of the retinal periphery. Each letter is viewed by an equal area of visual cortex ("cortical magnification factor") (Anstis, S.M., Vision Research 1974) http://www-psy.ucsd.edu/~sanstis/sablur.html
Color Channels Long (red) Med (green) Short (blue) Luminance R-G Y-B 15 Luminance contrast 16 Illustration of simultaneous luminance contrast. The upper row of rectangles are an identical gray. The lower rectangles are a darker gray but also identical.
Luminance channel Visual system extracts surface information Discounts illumination level Discounts color of illumination 17 Luminance is not Brightness Luminance refers to the measured amount of light coming from some region of space Receptors bleach and become less sensitive with more light Takes up to half an hour to recover sensitivity We are not light meters Brightness refers to perception of amount of light coming from a source Brightness non linear 18
Color blindness Trouble discriminating colors around 9% of population (8% males, 1% females) Different photopigment response reduces capability to discern small color diffs particularly those of low luminance Red-green deficiency is best known lack of either green or red photopigment can t discriminate colors dependent on R & G Color blind acceptable palette? Yellow-blue, and grey variation ok 19 Color components Hue property of the wavelengths of light (i.e., color ) Lightness (or value) how much light appears to be reflected from a surface some hues are inherently lighter or darker Saturation purity of the hue e.g., red is more saturated than pink color is mixture of pure hue & achromatic color portion of pure hue is the degree of saturation 20
Color components (cont.) Hue, Saturation, Value (HSV) model from http://www2.ncsu.edu/scivis/lessons/colormodels/color_models2.html#saturation 21 Color components (cont.) Lightness (Value) Saturation from http://www2.ncsu.edu/scivis/lessons/colormodels/color_models2.html#saturation. 22
Color great for classification Rapid visual segmentation Helps determine type Only about six categories white black red green yellow yellow green blue brown pink purple orange grey 23 Color great for classification (cont.) Scatterplot example 24
Color coding/labeling Large areas: low saturation Small areas: high saturation Recommended colors for coding: Widely agreed upon names First 4 + black & white are unique and mark ends of opponent color axes Entire set correspond to most common color names found across cultures Choose from set of first six, then from second set of six 25 Color coding/labeling (cont.) The same rules apply to color coding text and other similar information. Small areas should have high saturation colors. Large areas should be coded with low saturation colors Avoid high saturation colors for large areas Maintain luminance contrast. 26
Color guidelines Avoid simultaneous display of highly saturated, spectrally extreme colors e.g., no cyans/blues at the same time as reds, why? refocusing! desaturated combinations are better pastels Opponent colors go well together (red & green) or (yellow & blue) 27 Color guidelines (cont.) Pick non-adjacent colors on hue circle 28
Color guidelines (cont.) Size of detectable changes in color varies hard to detect changes in reds, purples, & greens easier to detect changes in yellows & blue-greens Older users need higher luminance levels to distinguish colors Hard to focus on edges created by color alone use both luminance & color differences 29 Color guidelines (cont.) Avoid red & green in the periphery - why? lack of RG cones there yellows & blues work in periphery Avoid pure blue for text, lines, & small shapes blue makes a fine background color avoid adjacent colors that differ only in blue Avoid single-color distinctions mixtures of colors should differ in 2 or 3 colors e.g., 2 colors shouldn t differ only by amount of red helps color-deficient observers 30
Perception primitives Whole visual field processed in parallel Can tell us what kinds of information is easily distinguished Popout effects (attention) Segmentation effects (division of the visual field) 31 Machinery of perception A B Parallel processing D of orientation, texture, color and motion features Detection of 2D patterns, contours and regions C Object Identification, Working Memory 32
Preattentive processing 10msec or better 897390570927940579629765098294 9812412412349asdasd1234918241231241249182313 asd1fa98 08028085080830802809850-13195sd0934-gj2-09-0999653681ASgg878188425158237ASDFG 802808 414251509sdfkjw9725792857osg72588419990123520597205920 567847298872t y4582020947577200 57u0sfg98760dSDF215723208SG2826029582019dfsg79827-0555 21789843890r 455790456099272188 897594797902855892594573979209 33 Preattentive processing 10msec or better 897390570927940579629765098294 9812412412349asdasd1234918241231241249182313 asd1fa98 080280850808 13195sd0934-gj2-09-0999653681ASgg878188425158237ASDFG 3 0802809850-802808 414251509sdfkjw9725792857osg72588419990123520597205920 567847298872t y4582020947577200 57u0sfg98760dSDF215723208SG2826029582019dfsg79827-0555 21789843890r 455790456099272188 897594797902855892594573 979209 34
Color 35 Orientation 36
Motion 37 Size 38
Simple shading 39 Conjunction (does not pop out) 40
Compound features (do not pop out) 41 Surrounded colors do not pop out 42
More Preattentive channels Shape Length Width Parallelism Enclosure Curvature Spatial grouping Added marks Number 43 Laws of preattentive display Must stand out on some simple dimension color, simple shape = orientation, size motion, depth Lessons for highlighting one of each 44
Highlighting Texture Using color Using underlining A flying box leads attention Blinking momentarily attracts attention Motion elicits an orienting response 45 Preattentive conjunctions Stereo and color Color and motion Color and position Shape and position In general: spatial location and some aspect of form (color or shape) 46
Preattentive lessons Rapid visual search (10 msec/item) Easy to attend to Makes symbols distinct Based on simple visual attributes Faces, etc are not pre-attentive Rules for making things distinct can be used for individual symbols or areas Do not use large areas of strong color Orthogonality - use a different channel for a different type of information 47 Scale matters Parafovea 48
UI hall of shame What is the empty button above MC for? Can t resize Blue for numbers! goes against all we know hard to focus on combined with red eye strain Grey on grey! Difficult for some users Contrast changes after user clicks the buttons! 49