Adaptive Display Power Management for Mobile
GamesBhojan Anand‡, Karthik Thirugnanam†, Jeena Sebastian‡,
Pravein G. Kannan‡, Akhihebbal L. Ananda‡, Mun Choon Chan‡ and Rajesh Krishna Balan†
‡ National University of Singapore (NUS) and † Singapore Management University (SMU)
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Problem: Display draws significant phone power
Key Challenge: No loss in end user experience
Key problem and Solution
Display45-50%
Network35-40%
CPU4-
15%Measured on HTC Magic while streaming a Youtube Video
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Dynamically adjust image brightness and LCD display backlight levels
50-70% display power savings with no significant user experience impact
Key problem and Solution
Key problem and Solution
Background
System Design
Evaluation
Discussion/Future Work
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Talk Outline
LCD displays have two components:
Power consumed mostly by the Backlight Thus brightening the image, and darkening
the backlight saves power.
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Background - LCD Displays
Backlight - Provides light, and consumes power
LCD Panel - Filters light based on image to be displayed
Method 1: Naively dim the display◦ Creates visible artifacts (flicker, brightness loss,
etc)◦ Especially noticeable in high frame rate
applications
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Background: Saving Display Power
Method 2: Compensate with increased brightness● Linearly apply same transform to entire image
Leads to saturated images
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Background: Saving Display Power
Method 2: Compensate with increased brightness● Non linear approaches prevent saturation but cause
contrast loss
Our solution uses this approach intelligently
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Background: Saving Display Power
Gamma Correction, or gamma, is a tone mapping function used to brighten scenes● Very Low Saturation relative to linear● Low computational overhead
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Non Linear Gamma Correction
Before After: Gamma 2
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Effect of Gamma on Image Quality
Original Image after Gamma Increase
(gamma=2)
Image after Gamma Increase andbacklight reduction
Games are popular and resource intensive● Extremely high frame rates● Flicker and brightness changes very noticeable to users
We use two representative games● Quake III – Commercial First Person Shooting (FPS) game ● Planeshift – Massively Multiplayer Role Playing Game
(MMORPG)
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Test Applications
Key problem and Solution
Background
System Design
Evaluation
Discussion/Future Work
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Talk Outline
Ultimate goal: Save significant power with no loss in end user experience
Challenge 1: Understanding the relationship between the backlight intensity, gamma, image brightness, and the power consumed
Challenge 2: Identifying human thresholds for brightness compensation
Challenge 3: Dynamically applying the solution
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System Design: Key Challenges
Relationship found to be linear. No other major contributing factors
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Challenge 1: Backlight vs Power
0 50 100 150 200 2500
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60
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Backlight Value
% D
isp
lay P
ow
er
Co
nsu
me
d
HTC Hero
Laptop (W500)
HTC Magic
Perceived brightness kept constant Non linear compensation necessary Useful gamma range is 1 to 4
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Challenge 1: Backlight vs Gamma
0 1 2 3 4 5 6 7 8 9 100
50
100
150
200
250
Gamma Value
Ba
cklig
ht
Va
lue
Obtained via small user study●5 postgraduate students
Each user shown a range of images●Covered a full range of brightness
For each image, users had to boost gamma to obtain two quality thresholds ●Described in next slide●Tool provided boosted gamma at .1 intervals
with automatic backlight compensation
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Challenge 2: Human Thresholds
Conservative: Image quality comparable to original
Aggressive: Image quality is affected but acceptable
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Challenge 2: Two Thresholds
0 2 4 6 8 10 12 141
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Image Brightness Levels (Darkest to Brightest)
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Challenge 3: Runtime Algorithm
Start
Calculate Average Brightness of last X Samples. Is there a change?
Mode + Brightness -> Gamma & Backlight Leave Settings as it is
Sleep Thread for Y ms
Yes No
Test platform● Most of the evaluation on Laptop● Prototype Mobile implementation available
(demo)
Objective Analytical Experiments ● Power measurements ● Measured power saved in different modes
Perceived User Impact● Large scale user study (60 users) with Quake III● Measured perceived quality loss in different
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Evaluation Methodology
0 2 4 6 8 10 120
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2
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Image Brightness Levels (Darkest to Brightest)
Ga
mm
a V
alu
e
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Evaluation: Three Test Modes
Unmodified
Conservative
Aggressive
Two bounding modes tested but omitted for simplicity
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A recorded trace used to measure power
Evaluation: Power Savings
Default
Conservative
Aggressive
0 10 20 30 40 50 60 70 80
0
49
68
% Display Power Saved
Large Scale User study● 60 Singapore Management University
undergrads.● 34 Male and 26 Female students with differing
background and game experiences
Participants trained on an unmodified version of the game● They then played the 3 different versions of the
game● Play order randomized with recalibration at every
step
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User Study Methodology
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Users rated each version by 6 criteria● Covered different quality dimensions
Evaluation: User Study Results
Aggressive Conservative Default1
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4
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Ave
rag
e A
cce
pta
bilit
y
Sco
re
Bad
Good
Strongly Agree =
Strongly Disagree =
Neutral =
Aggressive Conservative Default1
2
3
4
5
Avg
Acc
ep
tab
ilit
y S
core
Default
Conservative
Aggressive
0 10 20 30 40 50 60 70 80
% Display Power Saved
Difference between Aggressive and Conservative significant
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Evaluation: Aggressive vs Conservative
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Aggressive Conservative Default1
2
3
4
5
Avg
Acc
ep
tab
ilit
y S
core
Default
Conservative
Aggressive
0 10 20 30 40 50 60 70 80
% Display Power Saved
Difference betw. Conservative and Default
Evaluation: Power vs Perception
Conservative (Dynamic Conservative)● High Quality
Perceived quality comparable to default.● Significant Savings – 49%
Aggressive (Dynamic Aggressive)
● High Power Saving – 68%● Acceptable Quality
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Evaluation: Conclusions
OLED Displays● Do not use Backlight● Power consumption depends on displayed
content● Algorithm needs to be rethought
Other Limitations & Future Work● Initial User studies were small scale, and in a
controlled environment● Power Measurements could be more accurate,
esp for mobile phone● Evaluation of mobile implementation
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Discussion
General systematic approach to save power on LCD screens
Identified key parameters determining the quality vs power savings tradeoff
Implemented and tested with 60 end users
System achieves significant power savings with minimal overhead and quality loss
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Contributions
Thank youAny Questions
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