1 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.

2 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.

3 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

4 Visible Spectrum

5 Human Visual System Light passes through lens Focused on retina

6 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

7 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

8 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

9 Color Sensitivity Really yellow

10 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

11 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

12 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 short- wavelength 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 short- wavelength sensitive cones in the retina and they are not very sensitive. Older users need brighter colors.

13 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)

14 Peripheral acuity 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)

15 Color Channels

16 Luminance contrast Illustration of simultaneous luminance contrast. The upper row of rectangles are an identical gray. The lower rectangles are a darker gray but also identical.

17 Luminance “channel” Visual system extracts surface information Discounts illumination level Discounts color of illumination

18 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

19 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

20 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

21 Color components (cont.) Hue, Saturation, Value (HSV) model from

22 Color components (cont.) Lightness (Value) Saturation from

23 Color great for classification Rapid visual segmentation Helps determine type Only about six categories white black red green yellow green bluebrown pink purple orange grey

24 Color great for classification (cont.) Scatterplot example

25 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

26 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.

27 Color guidelines Avoid simultaneous display of highly saturated – 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)

28 Color guidelines (cont.) Pick non-adjacent colors on hue circle

29 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

30 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

31 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)

32 Machinery of perception

33 Preattentive processing 10msec or better asdasd –asd1fa sd0934-gj ASgg ASDFG sdfkjw osg u0sfg98760dSDF SG dfsg

34 Preattentive processing 10msec or better asdasd –asd1fa sd0934-gj ASgg ASDFG sdfkjw osg u0sfg98760dSDF SG dfsg

35 Color

36 Orientation

37 Motion

38 Size

39 Simple shading

40 Conjunction (does not pop out)

41 Compound features (do not pop out)

42 Surrounded colors do not pop out

43 More Preattentive channels Length Width Parallelism Curvature Number Added marksSpatial grouping Shape Enclosure

44 Laws of preattentive display Must stand out on some simple dimension – color, – simple shape = orientation, size – motion, – depth Lessons for highlighting – one of each

45 Highlighting Blinking momentarily attracts attention A flying box leads attention Blinking momentarily attracts attention Motion elicits an orienting response

46 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)

47 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

48 Scale matters Parafovea

49 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!