Transcript ClearType: A culprit or savior for chromatic aberration?

Effects of Transverse Chromatic
Aberration (TCA) on Peripheral
Reading
Shun-nan Yang, Ph.D
Collaborators
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•
•
•
James Sheedy (OD, PhD)
Yu-chi Tai (PhD)
Hannu Laukkenan (OD)
David Glabe, Wendy Zhi, and Jennifer
Ziegler (Research Assistants)
Outline
• Limits on peripheral reading and likely TCA
effect
• Experimental design
• Effect of TCA on letter recognition
• Effect of TCA on word identification
• Implications on facilitating reading with
ClearType (CT) rendering
Effective Visual Span in Reading
xxxxxx xxxxxxx xxxx Briony Lodge was in x xxxxxxxx xxxxxxxx
effective visual span
increasing TCA
increasing TCA
McConkie & Rayner (1975)
Limiting Factors on Peripheral Processing
• Eccentricity (Low, 1970; Berkeley et al.,
1975):
– Retinal neuronal density (Croner & Kaplan,
1995)
– Cortical magnification (Cowey & Rolls, 1970)
– Visual attention allocation (Bahcall & Kowler,
1999)
– Optical aberration (Marcos et al., 1999).
Limiting Factors on Text Processing
• Lateral interference:
– Spacing (Dakin and Hess, 1997)
– Neighbor interference (Bouma, 1973)
• Contextual facilitation
– Top-down predictability (McClelland & Johnson, 1979)
– Orthographic facilitation (Jonathan et al., 1992)
TCA Effect in Periphery
• Refraction of different-wavelength Light induces
natural TCA (Thibos et al., 1990).
• TCA in periphery can be theoretically computed,
but it is not clear whether TCA in the near
periphery surpasses the spatial resolution and
actually impedes letter recognition (Thibos, 1987).
A
chie
f
B
ray
f ray
chie
red
blue
right periphery
blue
red
left periphery
– It generates vertical color
bands in red, green and blue
(RGB) or reverse order (BGR).
– RGB-rendered image should
increase TCA when it is to the
left of the fovea, and reduce
it when it is to the right.
– BGR-rendered image should
have the opposite effect.
RGB
BGR
Magnified image
• TCA effect can be indirectly
assessed using ClearType
text:
Perceived image
Effect of Color Fringe on TCA
expected
left periphery image
actual RGB-CT stimulus
expected
right periphery image
Magnified image
Perceived image
RGB
BGR
– It generates vertical color
bands in red, green and blue
(RGB) or reverse order (BGR).
– RGB-rendered image should
increase TCA when it is to the
left of the fovea, and reduce
it when it is to the right.
– BGR-rendered image should
have the opposite effect.
RGB
BGR
Magnified image
• TCA effect can be indirectly
assessed using ClearType
text:
Perceived image
Effect of Color Fringe on TCA
expected
left periphery image
actual RGB-CT stimulus
expected
right periphery image
Expected Change in Visual Span
xxxxxx xxxxxxx xxxx Briony Lodge was in x xxxxxxxx xxxxxxxx
effective visual span
significant TCA
significant TCA
Hypotheses
• RGB- and BGR-rendered letters should
reduce and exacerbate TCA effect in the
periphery, and affect the accuracy of letter
recognition there.
• TCA on letter recognition might be affected
by the amount of lateral interference
(spacing and neighbor similarity).
• TCA effect on word identification should be
affected by word familiarity.
Experiment I: Letter Recognition
• Subjects
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–
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31 college students
Native English speaker
20/20 or better visual acuity
Normal or contact lenses correction
• Task
– Recognizing the middle of five horizontally placed
letters
– One-hour testing
• Equipment: LCD monitor with RGB subpixels
Text Stimulus
• Font (Consolas, 11 point, equal spacing)
• Font type (b&w, GS, RGB-CT, BGR-CT)
• Types of neighboring distractor (group A [aceos], B [bdhk], G
[gpqy], F [fijlt], M [mnur], and W [wvxz])
• Spacing (normal [2 pixels, 5.4 min of arc] vs. wide [4 pixels or
10.8 min of arc]).
target
distracting
neighbor
Trial Sequence
Percentages of Correct Responses
84% interval
Choice Probability (narrow spacing)
Effect of Neighboring Distractors
TCA effect on word identification
• Word identification relies on both bottomup feature analysis and top-down
contextual facilitation.
• TCA effect on word identification might be
reduced with greater word familiarity.
Experiment II: Word Identification
• Subjects:
– 30 subjects required, 21 included in the data.
– Same recruitment criteria as in exp. 1.
• Task:
– 480 high- and low-frequency words (Frances &
Kucera, 1982)
– 6- to 7-letter length
– 6, 4, 2, 1 degrees to the left or right of fixation
– 2 degree fixation zone to prevent foveation (eyetracking enforced)
• Measure: oral naming latency and accuracy.
Examples
Trial sequence
Response Accuracy
Correct Percentage (%)
High-frequency Words
120
100% CT
110
50% CT
100
B&W
90
80
70
60
50
40
30
20
10
0
-6
-4
-2
-1
1
2
4
Eccentricity (degree of visual angle)
6
Response Accuracy
Correct Percentage (%)
Low-frequency Words
120
110
100
90
80
70
60
50
40
30
20
10
0
100% CT
50% CT
B&W
-6
-4
-2
-1
1
2
4
Eccentricity (degree of visual angle)
6
Latency for Accurate Response
High-frequency Words
100% CT
1560
50% CT
Response time (ms)
1550
B&W
1540
1530
1520
1510
1500
1490
1480
-6
-4
-2
-1
1
2
4
Eccentricity (degree of visual angle)
6
Latency for Accurate Response
Low-frequency Words
100% CT
1560
50% CT
B&W
Response time (ms)
1550
1540
1530
1520
1510
1500
1490
1480
-6
-4
-2
-1
1
2
4
Eccentricity (degree of visual angle)
6
Frequency Effect on RT
Normalized Frequency Effect
B&W
10
50% CT
Percentage, [RTl - RTh]/RTl
9
100% CT
8
7
6
5
4
3
2
1
0
-6
-4
-2
-1
1
2
4
Target Position (degree of visual angle)
6
Subjective Preference
Subjective Preference
Summary
• As expected, RGB-CT facilitated letter
recognition in the right and impeded it in
the left; opposite effects were found for
BGR-CT.
• Spacing but not neighbor similarity
exacerbated TCA effect.
• RGB-CT had an effect on response latency
but not accuracy, with low-frequency word
identification facilitated in the right.
• Reduced TCA helped attenuate frequency
effect, and increased TCA enhanced it.
Implications
• With the typical RGB LCD display, RGB-CT text
appeared to minimize TCA and extend visual
span in the right periphery, where text
previewing in English reading takes place.
• Insignificant impedance was found in the left
periphery with RGB-CT text.
• RGB-CT rendering was especially useful for
reading narrowly spaced and unfamiliar text.
• RGB-CT text was overall rated as easier to
read, consistent with its effect on peripheral
reading.