Lift Up Arm Phone Rotate Fine-tuning and Shooting Lay Down Arm 00 Time (0.1s) Fig.1.The Process of Photographing activities.By maintaining an activity state machine,we can classify the user's activities in a very accurate approach.Third, we have implemented a system prototype in android-powered smart camera phones,the experiment results shows that our solution is able to perceive the user's activities with an average accuracy of 95.5%and reduce the overall energy consumption by 46.5%for smart camera phones compared to that without ■■ ra ovro m noo energy-saving scheme. (a)Energy Consumption Compo-(b)Energy Consumption of Built-in nents Sensors II.SYSTEM OVERVIEW In order to adaptively reduce the power consumption based Fig.2.Energy Consumption.(a)pl-Samsung GT-19250.p2-Huawei H30- T10.p3-Samsung GT-19023,p4-Huawei G520-T10:(b)acc-accelerometer,la- on activity sensing,we first use the built-in sensors of the linear accelerometer,gra-gravity sensor,gyro-gyroscope.mf-magnetic field, phone to observe the human activities and discuss the energy pro-proxity,cam-camera. consumption during the process of photographing.Then,we introduce the system architecture of our proposed energy- saving scheme. preview mode.For the four phones,we can find that keeping screen on increases the power consumption by 20%,18%,27% and 26%,respectively.While using camera increases power A.Human activities related to photographing consumption by 63%,70%,53%and 57%,respectively.There- Usually,the users tend to have similar activities during pho- fore,in preparation for photographing,energy consumption is tographing.As shown in Fig.1,we use the linear accelerometer wasted on camera and screen. to detect the activities.Before or after the user takes photos, 2)Energy consumption of turning on/off the camera: he/she may stay motionless,walking or jogging.While taking Frequently turning on/off the phone is annoying and energy photos,the user usually lifts up the arm,rotates the phone, wasting.When we need to take photos frequently for a period makes fine-tuning,shoots a picture,then lays down the arm. of time,the camera may switch between on/off frequently and We categorize all the eight actions into the following three so is to screen.And user needs to press button and unlock parts: the screen with scrolling which results in large energy waste Body movement:Motionless,walking and jogging [3].Based on Table I and Eq.(1),we can find that energy TABLE I. ENERGY DATA OF TURNING ON/OFF PHONE(SAMSUNG Arm movement:Lifting up the arm and laying down GT-I9250) the arm Subject Wrist movement:Rotating the phone,making fine Tumn On (Total) Turn Off (Total) Preview(1s) tuning and shooting a picture Energy (uAh) 769.71 400.71 160.22 B.Energy consumption related to photographing consumption of turning on/off the phone one time can keep camera working in previewing mode for seven seconds,which Before we propose the energy-saving scheme to reduce the means one picture can be shot power consumption based on the user's activities,we observe the energy consumption of the phone by using Monsoon power (769.71uAh+400.71uAh)/160.22uAh=7.305s(1) monitor [2]firstly It indicates that frequently turning on/off the phone manually 1)Energy consumption in preparation for photographing: is indeed inconvenient and rather energy-consuming. Power consumption for shooting a picture is large.We observe 3)Energy consumption of the sensors:Fig.2(b)shows the the power consumption in the following four android-based power consumption of a phone when a sensor is turned on. phones,i.e.,Samsung GT-19250,Huawei H30-T10,Samsung All these sensors work in their maximum sampling frequency, GT-19230 and Huawei G520-T10.In Fig.2(a),"Base"repre- i.e.,100Hz.When we turn on the camera and phone will be sents the phone's basic power when the phone is off."Display" in preview state,its increase of power is much larger than that represents the power when phone is idle and only keeps screen of other sensors.Therefore,low power-consuming sensors can on."Camera"represents the power when camera works in be used to reduce the energy consumption of photographing. 65Time (0.1s) 0 20 40 60 80 100 120 140 150 Linear Accelerometer -6 -4 -2 0 2 4 6 8 x-axis Walk Lift Up Arm Phone Rotate Fine-tuning and Shooting Lay Down Arm Walk A B C D Fig. 1. The Process of Photographing activities. By maintaining an activity state machine, we can classify the user’s activities in a very accurate approach. Third, we have implemented a system prototype in android-powered smart camera phones, the experiment results shows that our solution is able to perceive the user’s activities with an average accuracy of 95.5% and reduce the overall energy consumption by 46.5% for smart camera phones compared to that without energy-saving scheme. II. SYSTEM OVERVIEW In order to adaptively reduce the power consumption based on activity sensing, we first use the built-in sensors of the phone to observe the human activities and discuss the energy consumption during the process of photographing. Then, we introduce the system architecture of our proposed energy￾saving scheme. A. Human activities related to photographing Usually, the users tend to have similar activities during pho￾tographing. As shown in Fig. 1, we use the linear accelerometer to detect the activities. Before or after the user takes photos, he/she may stay motionless, walking or jogging. While taking photos, the user usually lifts up the arm, rotates the phone, makes fine-tuning, shoots a picture, then lays down the arm. We categorize all the eight actions into the following three parts: • Body movement: Motionless, walking and jogging • Arm movement: Lifting up the arm and laying down the arm • Wrist movement: Rotating the phone, making fine￾tuning and shooting a picture B. Energy consumption related to photographing Before we propose the energy-saving scheme to reduce the power consumption based on the user’s activities, we observe the energy consumption of the phone by using Monsoon power monitor [2] firstly. 1) Energy consumption in preparation for photographing: Power consumption for shooting a picture is large. We observe the power consumption in the following four android-based phones, i.e., Samsung GT-I9250, Huawei H30-T10, Samsung GT-I9230 and Huawei G520-T10. In Fig. 2(a), “Base” repre￾sents the phone’s basic power when the phone is off. “Display” represents the power when phone is idle and only keeps screen on. “Camera” represents the power when camera works in p1 p2 p3 p4 Power (mW) 0 500 1000 1500 2000 2500 Base Display Camera (a) Energy Consumption Compo￾nents acc la gra gyro mf pro cam Power (mW) 0 500 1000 1500 (b) Energy Consumption of Built-in Sensors Fig. 2. Energy Consumption. (a) p1-Samsung GT-I9250, p2-Huawei H30- T10, p3-Samsung GT-I9023, p4-Huawei G520-T10; (b) acc-accelerometer, la￾linear accelerometer, gra-gravity sensor, gyro-gyroscope, mf-magnetic field, pro-proxity, cam-camera. preview mode. For the four phones, we can find that keeping screen on increases the power consumption by 20%, 18%, 27% and 26%, respectively. While using camera increases power consumption by 63%, 70%, 53% and 57%, respectively. There￾fore, in preparation for photographing, energy consumption is wasted on camera and screen. 2) Energy consumption of turning on/off the camera: Frequently turning on/off the phone is annoying and energy wasting. When we need to take photos frequently for a period of time, the camera may switch between on/off frequently and so is to screen. And user needs to press button and unlock the screen with scrolling which results in large energy waste [3]. Based on Table I and Eq. (1), we can find that energy TABLE I. ENERGY DATA OF TURNING ON/OFF PHONE (SAMSUNG GT-I9250) Energy Subject Turn On (Total) Turn Off (Total) Preview (1s) Energy (uAh) 769.71 400.71 160.22 consumption of turning on/off the phone one time can keep camera working in previewing mode for seven seconds, which means one picture can be shot. (769.71uAh + 400.71uAh)/160.22uAh = 7.305s (1) It indicates that frequently turning on/off the phone manually is indeed inconvenient and rather energy-consuming. 3) Energy consumption of the sensors: Fig. 2(b) shows the power consumption of a phone when a sensor is turned on. All these sensors work in their maximum sampling frequency, i.e., 100Hz. When we turn on the camera and phone will be in preview state, its increase of power is much larger than that of other sensors. Therefore, low power-consuming sensors can be used to reduce the energy consumption of photographing. 65