perceiving egospeed in the fog: effects of global optical flow rate and contrast master’s thesis...

Perceiving Egospeed in the Fog: Effects of Global Optical Flow Rate and Contrast

Master’s Thesis Proposal

Andy Schaudt

University of Idaho

April 30th, 2004

Overview

• Introduce egospeed in fog

• Optical factors of influence on egospeed

• Contrast

• Global Optical Flow Rate

• Failure to investigate

• Purpose of proposed research

• Proposed experimental method

Egospeed & Fog

• Fog– 1,387 highway deaths 2001 alone (L.A. Times, 2002) – Many due to misjudgment of speed

Egospeed & Fog

• Egospeed – Perception of the speed of self-motion

• Optical factors that influence perceived egospeed:– Global Optical Flow Rate (GOFR)– Edge Rate (ER)– Motion Parallax– Contrast

Fog• Fog

– Water Vapor– Reflects and scatters light

Fog

• Dispersion of light reduces the transparency of the air reducing contrast and limiting visibility-distance

• Visibility-distance – distance from the eye to the set of visible objects

Contrast

• Gjon (1935) stated fog reduces contrast and reduces a signal’s beam through diffusion

• Brooks (2001) reduction in contrast reduces perceived speed

• Thompson Effect – reduction in contrast also reduces perceived speed

Global Optical Flow Rate

• Dyre, Ballard, and McDevitt (2004) found that GOFR is the primary basis for egospeed estimates

• GOFR – rate of egospeed scaled in eye-heights relative to a planar surface

D

VGOFR p

Example9

000

m

GOFR = 0.028 eye-heights s-1

GOFR = 1.25 eye-heights s-1

• GOFR may be higher while walking and much lower during flight at high speeds and altitudes

• Leads to the misperception of slower travel while flying

Global Optical Flow Rate• A more general distance metric for non-

planar and planar environments, Dyre, Ballard,

and McDevitt (2004)

• GOFR relative to any arbitrary environment can be defined as:

N

iiii

N

iiii ZYX

NV

N

ZYXV

R

VGOFR

1

222

1

222

1

Effect of Fog

• Average visibility distance – average distance from the eye to the set of visible objects

• Contrast gradient – contrast of an object is reduced as a function of depth or distance

• At some distance, contrast is reduced to zero obscuring any objects that lie beyond this distance

Fog’s Effects

• 3 different scenarios of fog

No Contrast Global ContrastLinear Fog

Contrast Gradient

Contrast Gradient - Fog

Contradicting Findings

• Contrast and GOFR influence egospeed

• Fog affects both optical parameters

Contrast Reduction in Egospeed

GOFR Increase in Egospeed

Previous Study

• Snowden, Stimpson, & Ruddle, (1998) – Reduction of global contrast increased perceived speed

• Conclusion – contrast reduction due to fog causes drivers to perceive egospeed as slower and increase speed to compensate

• Failure to investigate GOFR in regards to object distance

Fog’s Effects

• Fog reduces maximum visibility distance in the environment

• Constrains the sampling of optical flow to objects closest to the observer, increasing GOFR

• Currently no research has investigated the effect of fog on perception and control of egospeed that includes this contrast gradient

Purpose of Proposed Research

• Purpose: To examine the effect of GOFR and contrast on the perception of egospeed in a foggy environment

• Contrast will be manipulated to the environment in the following:– Introduce levels of a linear contrast gradient– Introduce levels of global contrast

Hypothesis 1

• Hypothesis 1 – Fog can affect visually perceived speed due to reduced visibility distance, which changes the sampling of optical flow to emphasize nearby objects

– This will create an increase in both GOFR and perceived speed, and a driver may decrease speed to compensate

Hypothesis 2

• Hypothesis 2 – Fog can affect visually perceived speed by reducing overall contrast

– This will create a decrease in perceived speed and a driver may increase speed to compensate

Contradicting Predictions

• These contradicting predictions suggest that the relationship between fog and perceived speed is complex

• Both overall contrast and visibility distance need to be taken into account

Proposed Experimental Methods

• Propose 2 experiments– 1) Two-alternative sequential presentation

method to investigate the effects of a contrast gradient on perceived egospeed while holding overall contrast constant

– 2) Adjustment of speed method to examine whether the effects of overall contrast and visibility distance on perceived speed generalize to control of speed

Both Experiments

• Participants

– Eight undergraduates students for each experiment

– 20/30 or better corrected Snellen acuity

– Naïve to purpose

Experiment 1: Stimuli & Apparatus

• Simulate driving over a flat textured surface

• Presented on one 1.69m diagonal rear projection monitor

• Fog will be presented as levels of a linear contrast gradient (linear contrast)


• Linear fog parameter equation used by OpenGL:

startend

zendendzf

,

Parameter Descriptionz Depth

end Defines the fog

start Always equal to zero

f 1 – fog-density


• f = 1 - fog-density

• It is always at its maximum value when: z = start (a value of 1)

• It decreases linearly to completely opaque (no contrast) when:

z = end (a value of 0)


• The manipulation of linear fog will also affect global contrast

• Necessary to maintain a constant global contrast across contrast gradients

• How?• Compensating for the covariance between

the contrast gradient and overall contrast with the use of a translucent object similar to a lens from a pair of sunglasses

Translucent Object


• Contains 2 important parameters

21

22211 11,,

end

endendend

Parameter DescriptionX2 Standard term

X1 Comparison term

Determines translucency


2 end2 end1 1 .85 125 25 .25 .85 125 50 .625 .85 125 75 .75 .85 125 100 .813

• For example, if we enter the following values for these parameters:2 = .85

end2 = 125

• we can enter in values for end1 and find the overall contrast level necessary

Experiment 1: Design (Two-Alternative)

• A 2 x 5 x 5 within-subjects factorial design will be used

• Independent Variables:– Temporal Position of the standard stimulus

(first or second)– Linear Contrast– Observer Velocity

• Dependent Variable:– Proportion of faster judgments for the

comparison stimulus

Experiment 1: Procedure• Experiment will span 2 one-hour and

fifteen minute sessions– Each session contains 500 trials– 50 unique displays- 20 blocks– Each trial will last approximately 7 seconds– Each trial will present two displays, standard

and comparison

• Observers indicate– Which display in each set resulted in a faster

perception of egospeed


• Apparatus and general format of the displays will be identical to Experiment 1

Experiment 2: Design (Adjustment)

• A 3 x 3 x 3 within-subjects factorial design will be used

• Independent Variables:– Linear Contrast– Overall Contrast– Observer Velocity

• Dependent Variable:– Adjusted speeds

Experiment 2: Procedure• Experiment will span 2 one-hour and

fifteen minute sessions– Each session contains 500 trials– 50 unique displays- 20 blocks– Each trial will last approximately 7 seconds– Each trial will present two displays, standard

and comparison

• Observers indicate– Which display in each set resulted in a faster

perception of egospeed

ReferencesBlakemore, M.R., & Snowden, R.J., (1999). The effect of contrast upon perceived speed:

a general phenomenon? Perception, 28, 33-48.

Blakemore, M.R., & Snowden, R.J., (2000). Textured backgrounds alter perceived speed. Vision Research, 40, 629-638

Brooks, K. (2001). Stereomotion speed perception is contrast dependent. Perception, 30, 725-731.

Dyre, B.P, Ballard, T.G., & McDevitt, J.R., (2004). Perception of egospeed: Effect of flow rate on element density. Manuscript in preparation, University of Idaho.

Dyre, B.P., Kludt, K., & Schaudt, W.A., (2004). Onset of vection in simulated environments. Unpublished raw data, University of Idaho..

Gibson, J.J. (1950). The perception of the visual world, Boston: Houghton Mifflin.

Gjon, M. (1935). Visibility of signals through fog. Journal of the Optical Society of America, 25, 237-240.

McDevitt, J.R., (2001). Quantifying the biasing effect of motion parallax on speed perception (Masters Thesis, University of Idaho).

References Continued..Owen, D.H., Wolpert, L., & Warren, R. (1984). Effects of optical flow acceleration, edge

acceleration, and viewing time on the perception of egospeed acceleration. In d.H. owen (Ed.) Optical flow and texture variables useful in detecting decelerating and accelerating self-motion. (Interim technical report for Contract No. F33615-83-K-0038, Task 2313-T3, pp. 79-133). Columbus: Ohio State University, Department of Psychology, Aviation Psychology Laboratory.

Snowden, R.J., Stimpson, N., & Ruddle, R.A. (1998). Speed perception fogs up as visibility drops. Nature, 392(6675), 450.

Thompson, P. (1982). Perceived rate of movement depends on contrast. Vision Research, 22, 377-380.

Vartabedian, R. (2002). Clear need for response to fatal fog: Sensor systems could ease the toll of horrific crashes amid road-obscuring weather conditions on highways in California and elsewhere. L.A. Times, October 23 2002.

Warren, R. (1982). Optical transformation during movements: review of the optical concomitants of egomotion. (AFOSR Grant Proposal number 81-0108). Columbus: Ohio State University, Department of Psychology, Aviation psychology Laboratory.

perceiving egospeed in the fog: effects of global optical flow rate and contrast master’s thesis...

Documents

egospeed slide

egospeed fog fog

contrast gradient fog

gofr influence egospeed

contrast gjon

reduction of global

object distance slide

light slide