## Building Virtual Worlds

#### Moshell - Fall 98

Lecture 18: Human Factors - Vince Chapter 9

These notes consist mostly of a list of concepts which you need to be able to define. I will embed some Queries for you to practice with, as the usual study tools for the Final Exam.

Vision: Terms to Define

Here's how to practice for the final exam: write out your own personal definitions of these terms. It is not acceptable to copy the text's language! If I ask you for a definition on the final exam, and you quote the text to me, I'll give Mr. Vince credit for that problem and you won't get any.

Also, of course, I'm not going to ask you to regurgitate a mere DEFINITION. I'll ask you to do something with your BRAIN. If you redefine these concepts in your own words, you will have a better chance at that mission!

Before we begin with the text, we have to define some basic optical terms:

image - a mapping (R x R --> (r,g,b)) which means "give me two real numbers representing (x,y) in a scene, and I'll give you the (r,g,b) triad of values associated with that location. Thus an image is a 2d array of triples of numbers; or three 2d arrays of numbers.

synthetic camera - a way of specifying an image, by specifying the viewpoint, view direction, viewing cone, near and far clipping planes. The near clipping plane's intersection with the viewing cone defines the image surface. (There are more general ways of doing this, but this way is good enough for our purposes.)

imaging optical system - a device which takes light from some source of an image S, and forms a derived image T that is based on (and resembles) S.

pinhole camera - an imaging optical system which produces an inverted derived image with infinite depth of field (if the pinhole is infinitely small.) The viewing cone is defined by the relationship between the pinhole and the derived image surface.

Depth of field (of an imaging system): the range of distance values in a 3d scene within which, if physical object P is located, the imaging system produces an image of P whose details do not noticeably suffer from defocusing.

Lens: An optical device for increasing the amount of light in an image, compared to a pinhole. A simple convex lens has a focal length, which is the distance at which parallel incoming light forms a point.

Lens equatioin:  1/Obj + 1/Image = 1/f, where obj=distance from lens center to the object; Image=distance from lens center to the image; and f is the lens' focal length. This is an idealized equation, describing the location at which the image is best focused. But except for an infinitely small object (and image), a lens of nonzero size always forms a less than perfect image.

Aperture: the size of the hole through which the image's light must pass. Typically this is located at the face of the lens.

Accomodation: Changing the focal length of the lens to form an image on the retina ("film") of the eye. Since 1/Obj is a real world parameter and 1/image is based on the eye, the only thing one can do to make the image clear is to change 1/f (by changing f.) Us old folks can't accomodate very well.

Pupil: The aperture of the eye. Varies to allow more light in (low light conditions) but this decreases the depth of field of the eye.

Query 18.1. Old man is laying in bed, can't read the book. Hates to use reading glasses. Turns on a bright lamp, text seems to get sharper. Why?

Retina
Fovea - and its relationship to cones
Peripheral vision - and its relationship to rods
Cones and rods: reaction times: rods are 4x faster than cones.
Critical fusion frequency

Query 18.2: Why do movie projectors have a spinning shutter in them that makes each frame (at 24 frames/sec) appear three times (total flicker rate of 72/sec?)

Field of view: Two eyes, with a nose of some height between them. (Asians sometimes have one eyeball that can see the other; us tall-nose Europeans, rarely.) Each eye can cover about 130 degree FOV; their overlap yields perhaps a 60 degree stereo FOV and a 200 degree total FOV.

Acuity: consider it as one arc-minute in a one degree foveal region. Degrades to 10 arc minutes or more at the perimeter. How many pixels would this amount to, if degradation is linear across the FOV? (order of magnitude estimates are OK.)

Sound

Eardrum
Cochlea
Impedence matching
Basilar membrane and sound frequency

Query 18.3: What is a somatotopic mapping? Why does Moshell claim that touch, hearing and vision incorporate somatotopic mappings whereas olfaction uniquely doesn't?

Logarithmic response
Intensity difference for sound localization
phase difference for sound localization

Somatic Senses

I don't like his classification, so let's use mine:

1. Tactile sensations: "touch":
A variety of sensors which we won't try to remember.
Heat, cold
vibration
small differential pressures (ant on the skin)
large broad pressures (sitting on a chair)
2. Postural
Where are my limbs? What is my posture? Stretch sensors in muscles
3. Inertial
Inner ear, and also info from the same sensors as postural.

Haptic Technology

PhanTOM device

Difficulties of haptic force feedback: "get out of the way problem"
Dangers of haptic force feedback: broken fingers or arms, falling down. The skateboard experience.

Equilibrium

The vestibular system: static orientation to gravity via weighted hairs; rotation via fluid in semicircular canals.

When vision, postural and vestibular signals disagree, body decides its being poisoned. Bye bye stomach contents!

VRsick = SimSick, quite similar to space sickness and motion sickness

Query 18.4: Develop a list of five experiences one might have in a virtual environment, sorted from least to most likely to produce VR sickness. Provide a theoretical justification for your answer based on the physiology you learned in this chapter.

=================

Some general queries for this chapter:

Query 18.5: "Veridical" means "indistinguishable from reality." What would the quantitative properties of a veridical visual display for VR be? Gather your notes from the lecture and compute, if you can, the scaled cost in terms of today's graphics devices, for a visually veridical display.

Query 18.6: Holodeck: Assume we have a veridical visual and audio system. List any apparently inherent obstacles to creating a real holodeck, in which one could (virtually)

a) run a marathon
b) kiss one's true love
c) slice off one's own left arm and replace it with a lobster claw
d) swim

Would the fact that the Holodeck was on a starship give you any help with any of these activities? (You may assume that the pleasure or inconvenience of the rest of the crew is irrelevant to this question.)

Back to the course index
Back to the course syllabus
Back to the previous lecture
Onward to the next lecture