Abstract:
A method of waking up a device includes capturing an image by scanning a fingerprint pattern in a sleep mode; analyzing the captured image representing the scanned fingerprint pattern to obtain an amount of pixels in the captured image with respect to each brightness value; transforming the analyzed captured image into brightness distribution; operating the brightness distribution to obtain a feature value; and comparing the feature value with a pre-stored value, the device being woken up when the feature value is matched with the pre-stored value.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to fingerprint authentication, and more particularly to a method of fast activating a mobile device from a sleep mode based on fingerprint authentication. 
     2. Description of Related Art 
     A fingerprint sensor has been commonly used in a mobile device such as a mobile phone to capture a digital image of a fingerprint pattern. The captured digital image is then processed to create a biometric template (also known as a golden template) that is stored and used for matching. Accordingly, individuals operating the mobile device may be identified and their identity or access approval be verified. Compared with conventional user authentication schemes using passwords, the fingerprint authentication provides more convenient and faster means. 
     Due to limited and precious battery power of the mobile devices, sleep mode is adopted to save significantly on power consumption, in stead of leaving the mobile devices fully on all the time. When resumed (i.e., wake up), the operation of the mobile devices continues from the same point it leaves. 
     However, conventional mobile devices equipped with fingerprint authentication suffer slowness and power waste when resuming from a sleep mode, owing to time-consuming matching between a captured fingerprint image and a fingerprint golden template. Therefore, a need has thus arisen to propose a novel method and device of activating a mobile device from a sleep mode in a fast manner. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the embodiment of the present invention to provide a method and device of fast activating a mobile device from a sleep mode based on fingerprint authentication to reduce power consumption. 
     According to one embodiment, an image is captured by scanning a fingerprint pattern in a sleep mode. The captured image representing the scanned fingerprint pattern is analyzed to obtain an amount of pixels in the captured image with respect to each brightness value; and the analyzed captured image is transformed into brightness distribution. The brightness distribution is operated to obtain a feature value, and the feature value is compared with a pre-stored value, the device being woken up when the feature value is matched with the pre-stored value. 
     According to another embodiment, a device includes a host processor, a fingerprint sensor and a local controller. The host processor controls at least one component of the device. The fingerprint sensor is associated with and disposed at a scan area, and the local controller commands the fingerprint sensor. The host processor communicates with the local controller in an operating mode, and is separate from the local controller in a sleep mode. The fingerprint sensor captures an image under control of the local controller by scanning a fingerprint pattern in the sleep mode; the captured image is analyzed to obtain an amount of pixels in the captured image with respect to each brightness value, which is then transformed into brightness distribution by the local controller, from which a feature value is obtained; and the local controller notifies the host processor to wake up when the feature value is matched with a pre-stored value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flow diagram illustrating a method of activating a mobile device from a sleep mode to an operating mode based on fingerprint authentication according to one embodiment of the present invention; 
         FIG. 2A  shows a perspective view exemplifying a mobile device; 
         FIG. 2B  schematically shows a simplified block diagram illustrating the mobile device of  FIG. 2A ; 
         FIG. 3A  shows an exemplary timing diagram demonstrating the full scan performed by the fingerprint sensor of  FIG. 2B ; 
         FIG. 3B  shows an exemplary timing diagram demonstrating the partial scan performed by the fingerprint sensor of  FIG. 2B ; 
         FIG. 4A  schematically shows an exemplary fingerprint sensor that performs a partial scan; 
         FIG. 4B  schematically shows another exemplary fingerprint sensor that performs a partial scan; 
         FIG. 5  shows an exemplary image histogram revealing brightness distribution of a fingerprint pattern; 
         FIG. 6  shows an exemplary image histogram revealing brightness distribution of a null pattern; and 
         FIG. 7  schematically shows an exemplary fingerprint sensor that is divided into four parts. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a flow diagram illustrating a method  100  of activating a mobile device from a sleep mode to an operating mode based on fingerprint authentication according to one embodiment of the present invention. The mobile device of the embodiment may, but not necessarily, be a mobile phone. As exemplified in a perspective view of  FIG. 2A , the mobile device  20  may include at least a touch screen  201  and a scan area  202  associatively equipped with a fingerprint sensor. The technologies adopted to implement the fingerprint sensor of the embodiment may include, but not limited to, capacitive, optical, radio frequency (RF), thermal, resistive, ultrasonic, piezoelectric and micro-electro-mechanical systems (MEMS). In the specification, the term sleep mode may generally refer to a low power mode for the mobile device  20  to save significantly on power consumption, in stead of leaving the mobile device  20  fully on all the time. The term sleep mode may include, for example, stand by, sleep, suspend and hibernation. 
       FIG. 2B  schematically shows a simplified block diagram illustrating the mobile device  20  of  FIG. 2A . Specifically, the mobile device  20  may include a host processor  203  that is configured primarily to operate the components (such as the touch screen  201 ) of the mobile device  20 . The host processor  203  of the embodiment may include, for example, a central processing unit. 
     The mobile device  20  may also include a local controller  204  that is configured to command the fingerprint sensor  205  associated with and disposed at the scan area  202  to authenticate a fingerprint pattern captured by the fingerprint sensor  205 . In the operating mode, the host processor  203  may communicate with the local controller  204 . In the sleep mode, the host processor  203  is separate from the local controller  204 . 
     In step  11  ( FIG. 1 ), while in the sleep mode, the fingerprint sensor  205  captures an image by scanning a fingerprint pattern. In one embodiment, the fingerprint sensor  205  performs full scan under control of the local controller  204  in a manner that substantial entirety of the fingerprint sensor  205  performs the scan.  FIG. 3A  shows an exemplary timing diagram demonstrating the full scan performed by the fingerprint sensor  205  of  FIG. 2B . Specifically speaking, the fingerprint sensor  205  performs periodic full scans under control of the local controller  204  during, for example, period t1-t2 and period t3-t4. A finger touches the fingerprint sensor  205  at a time between t2 and t3. 
     In another embodiment, the fingerprint sensor  205  performs partial scan under control of the local controller  204  in a manner that part of the fingerprint sensor  205  performs the scan.  FIG. 3B  shows an exemplary timing diagram demonstrating the partial scan performed by the fingerprint sensor  205  of  FIG. 2B . 
     Specifically speaking, the fingerprint sensor  205  performs periodic partial scans under control of the local controller  204  during, for example, period t1-t2 and period t3-t4. Compared with the embodiment performing full scan ( FIG. 3A ), the embodiment performing partial scan ( FIG. 3B ) may execute with higher speed and lower power consumption. 
       FIG. 4A  schematically shows an exemplary fingerprint sensor  205  that performs a partial scan. In the exemplary embodiment, one line  2051  out of some successive lines (say 10 lines) is scanned.  FIG. 4B  schematically shows another exemplary fingerprint sensor  205  that performs a partial scan. In the exemplary embodiment, one block of lines  2052  is scanned, while other blocks of lines  2053  are not scanned. 
     In step  12 , the local controller  204  analyzes the captured image representing the scanned fingerprint pattern. In the embodiment, the captured image is analyzed by a statistical method. Specifically, an amount of pixels in the captured image with respect to each brightness value is obtained. 
     In step  13 , analysis results collected from step  12  are then transformed into brightness distribution by the local controller  204 . According to one aspect of the embodiment, the analysis results are transformed into an image histogram. As the amount of pixels is represented in vertical axis with respect to each brightness value represented in horizontal axis, an image histogram may then be plotted in a conceptual manner. In an exemplary embodiment, larger brightness value represents brighter pixel.  FIG. 5  shows an exemplary image histogram revealing brightness distribution of a fingerprint pattern. 
     A high (or first) thresholding value for the image histogram of the fingerprint pattern need be determined beforehand as exemplified in  FIG. 5 . In determining the high thresholding value, a background histogram revealing brightness distribution of a null (i.e., without fingerprint) pattern, as exemplified in  FIG. 6 , need be obtained in advance. As no fingerprint is present, pixels of a captured image mainly reside in a narrow background range of high brightness values. In the embodiment, the left (or lower) boundary value of the background range ( FIG. 6 ) is set as the high thresholding value for the image histogram of the fingerprint pattern ( FIG. 5 ). 
     Referring to  FIG. 5 , a low (or second) thresholding value, being less than the high thresholding value, may be further determined for the image histogram of the fingerprint pattern. As the brightness distribution near the left (or lower) portion of the image histogram rises monotonically, the low thresholding value may be set at a brightness value with a correspondingly significant amount (e.g., 250) of pixels. The pixels of the image histogram with brightness values less than the low thresholding value are mainly noise, and should be discarded. 
     It is observed that various parts of the fingerprint sensor  205  have different response due to dissimilar signal attenuation.  FIG. 7  schematically shows an exemplary fingerprint sensor  205  that is divided into four parts, e.g., part A, part B, part C and part D. Signals collected from part A are amplified by an amplifier  206  via wires  207 A. Similarly, signals collected from part B, part C and part D are amplified by the amplifier  206  via wires  207 B,  207 C and  207 D, respectively. As signals in the (longer) wires  207 B and  207 C suffer greater attenuation than the signals in the (shorter) wires  207 A and  207 D, the high thresholding value and the low thresholding value are set different for the part A, part B, part C and part D. For example, the high/low thresholding value of part B/C is set smaller than the high/low thresholding value of part A/D. 
     Subsequently, in step  14 , the amounts of pixels for the brightness values (i.e., histogram data) between the low thresholding value and the high thresholding value are operated (or calculated) by the local controller  204  to result in a feature value. For example, histogram data between the low thresholding value and the high thresholding value are summed up to result in a feature sum. Alternatively, standard deviation or maximum/minimum value may be obtained instead as the feature value. 
     In step  15 , the feature value resulted from step  14  is compared, by the local controller  204 , with a previously stored feature value derived according to a fingerprint pattern of a user of the mobile device  20 . If the two feature values match, the local controller  204  notifies the host processor  203 , which then wakes up the mobile device  20  from the sleep mode to the operating mode (at time t4,  FIG. 3A / 3 B), or otherwise the flow goes back to step  11 . According to the embodiment, there is no need of keying password or pushing a button in order to wake up as for the conventional mobile devices. 
     After entering the operating mode, the fingerprint sensor  205  may perform full scan, for example, in the period t5-t6 as exemplified in  FIG. 3A / 3 B, to capture an image by scanning the fingerprint pattern. The captured image may then be authenticated by the host processor  203  using a conventional technique (e.g., by comparing with a pre-stored fingerprint golden template) that is distinct from the histogram technique discussed above, in order to make sure that the individual operating the mobile device  20  is an admitted user of the mobile device  20 . 
     Although specific embodiments have been illustrated and described, it will be appreciated by those skilled in the art that various modifications may be made without departing from the scope of the present invention, which is intended to be limited solely by the appended claims.