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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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

1

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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

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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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3

4

5

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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