Intro to Human Visual System and Displays

1. Intro to Human Visual System and Displays • Fundamental Optics • Fovea • Perception These slides were developed by Colin Ware, Univ. of New Hampshire

2. Why Should We Be Interested In Visualization • Hi bandwidth to the brain (70% of all receptors ,40+% of cortex, 4 billion neurons) • Can see much more than we can mentally image • Can perceive patterns (what dimensionality?)

3. Perceptual versus Cultural

4. Basic Pathways

5. The machinery

6. Human Visual Field

7. Visual Angle

8. Acuities Vernier super acuity (10 sec) Grating acuity Two Point acuity (0.5 min)

9. Human Spatial Acuity

10. Cutoff at 50 cycles/deg. • Receptors: 20 sec of arc • Pooled over larger and larger areas • 100 million receptors • 1 million fibers to brain • A screen may have 30 pixels/cm – need about 4 times as much. • VR displays have 5 pixels/cm

11. Acuity Distribution

12. Brain Pixels

13. Brain pixels=retinal ganglion cell receptive fields Field size = 0.006(e+1.0) - Anderson Characters = 0.046e - Anstis Ganglion cells Tartufieri

14. 0.8 BP Pixels and Brain Pixels 1 bp 0.2 BP Small Screen Big Screen

15. Perception • Many, many ways to trick the vision system.

22. Intro to Color for Information Display • Color Theory • Color Geometries • Color applications • Labeling • Pseudo-color sequences

23. Trichromacy Three cones types in retina

24. Cone sensitivity functions

25. Color measurement • Based on the “standard observer” • CIE tristimulus values XYX • Y is luminance. • Assumes all humans are the same

26. Short wavelength sensitive cones Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive. Chromatic aberration in the eye is also a problem Blue text on dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive Blue text on a dark background is to be avoided. We have very few short-wavelength sensitive cones in the retina and they are not very sensitive

27. Color Channels

28. Luminance “channel” • Visual system extracts surface information • Discounts illumination level • Discounts color of illumination • Mechanisms • 1) Adaptation • 2) Simultaneous contrast

29. Luminance is not Brightness • Eye sensitive over 9 orders or magnitude • 5 orders of magnitude (room – sunlight) • Receptors bleach and become less sensitive with more light • Takes up to half an hour to recover sensitivity • We are not light meters

30. Luminance contrast

31. Contrast for constancy

32. Contrast for constancy

33. Brightness refers to perception of lights Brightness non linear Monitor Gamma Lightness refers to perception of surfaces Perceived lightness depends on a reference white Brightness Lightness and Luminance

34. Luminance for Shape-from-shading

35. Luminance Channel Detail Form Shading Motion Stereo Chromatic Channels Surfaces of things Labels Berlin and Kay Categories (about 6-10) Red, green, yellow and blue are special (unique hues) Channel Properties

36. Chromatic Channels have Low Spatial Resolution • Luminance contrast needed to see detail 3:1 recommended 10:1 idea for small text

37. Color phenomena Chromatic contrast Small field tritanopia

38. Color “blindness” • A 3D to a 2D space • 8 % of males • R-G color blindness • Can generate color blind acceptable palette • Yellow blue variation OK

39. Implications • Color perception is relative • We are sensitive to small differences- hence need sixteen million colors • Not sensitive to absolute values- hence we can only use < 10 colors for coding

40. Color great for classification • Rapid visual segmentation • Color helps us determine type • Only about six categories

41. Applications • Color interfaces • Color coding • Color sequences • Color for multi-dimensional discrete data

42. Color Coding Large areas: low saturation Small areas high saturation Break isoluminance with borders

43. Color Coding 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 Luminance contrast should be maintained

44. Visual Principles • Sensory vs. Arbitrary Symbols • Pre-attentive Properties • Gestalt Properties • Relative Expressiveness of Visual Cues

45. Sensory vs. Arbitrary Symbols • Sensory: • Understanding without training • Resistance to instructional bias • Sensory immediacy • Hard-wired and fast • Cross-cultural Validity • Arbitrary • Hard to learn • Easy to forget • Embedded in culture and applications

46. American Sign Language • Primarily arbitrary, but partly representational • Signs sometimes based partly on similarity • But you couldn’t guess most of them • They differ radically across languages • Sublanguages in ASL are more representative • Diectic terms • Describing the layout of a room, there is a way to indicate by pointing on a plane where different items sit.

47. Pre-attentive Processing • A limited set of visual properties are processed pre-attentively • (without need for focusing attention). • This is important for design of visualizations • What can be perceived immediately? • What properties are good discriminators? • What can mislead viewers? All Preattentive Processing figures from Healey 97http://www.csc.ncsu.edu/faculty/healey/PP/PP.html

48. Example: Color Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in color.

49. Example: Shape Selection Viewer can rapidly and accurately determine whether the target (red circle) is present or absent. Difference detected in form (curvature)

50. Pre-attentive Processing • < 200 - 250ms qualifies as pre-attentive • eye movements take at least 200ms • yet certain processing can be done very quickly, implying low-level processing in parallel • If a decision takes a fixed amount of time regardless of the number of distracters, it is considered to be pre-attentive.