BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image sensing apparatus, and an optical touch control apparatus and a motion tracking apparatus utilizing the image sensing apparatus, and particularly relates to an image sensing apparatus, and an optical touch control apparatus and a motion tracking apparatus, which can change a size of an image sensing region. 
     2. Description of the Prior Art 
     In the prior art, a sensing region of an image sensor is fixed. For example, a total image sensing region of the image sensor is utilized to sense an image. However, such mechanism consumes large power and wrong image might be detected.  FIG. 1  is a schematic diagram illustrating how a prior art image sensor utilizes a total image sensing region to sense an image. As shown in  FIG. 1 , the image sensor  100  utilizes all sensing region of the image sensor (i.e. a total image sensing region, a matrix with M×N pixels in this example) to sense an image. However, the image sensor may have a disturbance region P caused by dust or dirt, such that the image sensed by the image sensor  100  may include an image caused by the disturbance region P. Therefore, a wrong image is sensed. 
     SUMMARY OF THE INVENTION 
     One objective of the present invention is to provide an image sensing apparatus that can change image sensing regions. 
     Another objective of the present invention is to provide an optical touch control apparatus that can change image sensing regions. 
     Another objective of the present invention is to provide a motion tracking apparatus that can change image sensing regions. 
     One embodiment of the present invention discloses an image sensing apparatus, comprising: a control unit; and an image sensor, wherein the control unit controls the image sensor to utilize a first image sensing region to sense a first image to output a first image signal in a first mode, wherein the control unit controls the image sensor to utilize a second image sensing region to sense a second image to output a second image signal in a second mode. The first image sensing region is smaller than a total image sensing region of the image sensor, and the second image sensing region is smaller than the first image sensing region. 
     Another embodiment of the present invention discloses an optical touch control apparatus, which comprises: a sensing surface; a light source, for emitting light to an object on the sensing surface; a control unit; and an image sensor, wherein the control unit controls the image sensor to utilize a first image sensing region to sense a first image to output a first image signal in a first mode, where the control unit controls the image sensor to utilize a second image sensing region to sense a second image to output a second image signal in a second mode, wherein the control unit computes a touch control operation for the object on the sensing surface according to the first image signal or the second image signal. The first image sensing region is smaller than a total image sensing region of the image sensor, and the second image sensing region is smaller than the first image sensing region. 
     Still another embodiment of the present invention discloses a motion tracking apparatus comprising a display and a remote controller. The display comprises at least one light source provided thereon. The remote controller comprises a control unit and an image sensor, wherein the control unit controls the image sensor to utilize a first image sensing region to sense a first image to output a first image signal in a first mode, where the control unit controls the image sensor to utilize a second image sensing region to sense a second image to output a second image signal in a second mode, wherein the control unit computes a location of the light source according to the first image or the second image and computes relative motion between the remote controller and the light source according to a location of the light source. The first image sensing region is smaller than a total image sensing region of the image sensor, and the second image sensing region is smaller than the first image sensing region. 
     In view of above-mentioned embodiment, proper sensing regions can be utilized corresponding to different modes. By this way, the power consumption can decrease and a correct image can be acquired. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating how a prior art image sensor utilizes a total image sensing region to sense an image. 
         FIG. 2  is a schematic diagram illustrating an optical touch control apparatus according to one embodiment of the present invention. 
         FIG. 3 ,  FIG. 4 ,  FIG. 5  and  FIG. 6  are schematic diagrams illustrating operations of the optical touch control apparatus shown in  FIG. 2 . 
         FIG. 7  is a schematic diagram illustrating a motion tracking apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  is a schematic diagram illustrating an optical touch control apparatus according to one embodiment of the present invention.  FIG. 3 ,  FIG. 4 ,  FIG. 5  and  FIG. 6  are schematic diagrams illustrating operations of the optical touch control apparatus shown in  FIG. 2 . As shown in  FIG. 2 , the optical control apparatus  200  comprises a sensing surface  201 , a light source  203 , a control unit  205  and an image sensor  207 . The light source  203  emits light to an object on the sensing surface  201  (a finger F in this embodiment). The image sensor  207  is arranged to sense an image, thus the light source  203  emits light to the finger F and the light is reflected to the image sensor  207  if the distance between the finger F and image sensor  207  is smaller than a predetermined distance (i.e. a touch control operation occurs). By this way, the image sensor  207  senses an image of the finger F. Also, the control unit  205  computes touch control operation for the finger F on the sensing surface  201  according to an image of the finger F. 
     The image sensor  207  utilizes different image sensing regions to sense images in different modes. Please refer to  FIG. 3 , the image sensor  300  utilizes a first image sensing region W 1  to sense an image to thereby generate a corresponding first image signal in the first mode. The first image sensing region W 1  includes only a part of the total image sensing region of the image sensor  300  and excludes the disturbance region P. By this way, power consumption decreases and a more correct image can be sensed. The size and the location of the first image sensing region W 1  can be determined by various determining mechanisms. For example, it can be determined which part of the image sensor is not suitable for sensing according to the image sensor manufacturing process, such that the first image sensing region W 1  can exclude these parts. Alternatively, the disturbance region P can be determined first and then the first image sensing region W 1  can be designed to exclude the disturbance region P. Many methods can be utilized to determine a location of the disturbance region P. For example, if a fixed image has been found in a plurality of images, the fixed image can be determined to be the disturbance region P. However, disturbance region P is not limited to be determined via this method. Additionally, the first image sensing region W 1  can be other shapes besides a rectangle, and can be non-continuous shapes. Please refer to the embodiment shown in  FIG. 4 , the first image sensing region W 1  can be formed by different bar shape sensing regions. 
     The image sensor  207  utilizes a second image sensing region W 2  smaller than the first image sensing region W 1  to sense an image to thereby generate a corresponding second image signal in the second mode, as shown in  FIG. 5 . The locations of the first image sensing region W 1  and the second image sensing region W 2  can be that the first image sensing region W 1  includes the second image sensing region W 2 , as shown in  FIG. 3  and  FIG. 5 . Additionally, the locations of the first image sensing region W 1  and the second image sensing region W 2  can be that the first image sensing region W 1  and the second image sensing region W 2  do not overlap to each other, as shown in  FIG. 6 . In one embodiment, the control unit  205  determines if a fixed image exists in a sensing region of the image sensor  207  according to the first image signal or the second image signal, and compensates the fixed image while utilizing the first image sensing region W 1  or the second image sensing region W 2  to sense an image. Such compensation step can be regarded as a step for decreasing an image noise, which can help the image sensor sense a more correct image. How to determine a fixed image and compensate it is well known by persons skilled in the art, thus it is omitted for brevity here. 
     In one embodiment, the first mode is a normal mode and the second mode is a sleeping mode. That is, the image sensor  207  utilizes the first image sensing region W 1  to sense the image while in the normal mode and utilizes the second image sensing region W 2  smaller than the first image sensing region W 1  to sense the image while in the sleeping mode. Furthermore, the control unit  205  controls the image sensor  207  to utilize the second image sensing region W 2  to sense if a distance between the finger F and the sensing surface  201  is smaller than a threshold value (i.e. sense if a touch control operation occurs). If yes, the control unit  205  switches the optical touch control apparatus  200  back to the first mode. If not, the control unit  205  keeps the optical touch control apparatus  200  in the second mode. For more detail, if the distance between the finger F and the sensing surface  201  is larger than the threshold value (i.e. no touch control operation occurs), the image sensor  207  does not receive light reflected from the finger F, such that the brightness is low. On the contrary, if the distance between the finger F and the sensing surface  201  is smaller than the threshold value (i.e. a touch control operation occurs), the image sensor  207  receives light reflected from the finger F, such that the brightness is high. Accordingly, the control unit  205  controls the image sensor  207  to utilize the second image sensing region W 2  to sense brightness of the image and switches the image sensing apparatus  200  back to the first mode if variation of the brightness is larger than a threshold value, since it means the finger F or other objects are close to the sensing surface if the brightness variation is large. The smaller second image sensing region W 2  can be utilized in the second mode, since the step of sensing brightness variation for the whole image does not need a large sensing region. 
     The image sensing mechanisms shown in  FIG. 3  to  FIG. 6  can be applied to other electronic apparatuses. That is, the control unit  205  and the image sensor  207  shown in  FIG. 2  can be regarded as an image sensing apparatus, which can be applied to other electronic apparatuses.  FIG. 7  is a schematic diagram illustrating a motion tracking apparatus according to an embodiment of the present invention. As shown in  FIG. 7 , the motion tracking apparatus  700  comprises a display  701  and a remote controller  703 . The display  701  comprises at least one light source  705  provided thereon (in this example, only one light source is illustrated), and the remote controller  707  comprises a control unit  707  and an image sensor  709 . The image sensor  709  can utilize the image sensing mechanisms shown in  FIG. 3  to  FIG. 6  to sense the image. In one embodiment, the first mode is a normal mode and the second mode is a sleeping mode. In the first mode, the control unit  707  computes a location of the light source  705  according to the sensed image and computes relative motion between the remote controller  703  and the light source  705  according to a location of the light source  705 . In one embodiment, the relative motion between the remote controller  703  and the light source  705  is utilized to control a cursor Cr, but it does not mean to limit the present invention. The control unit  707  determines if the sensed image comprises alight source image generated by the light source  705  in the second mode. If yes, the control unit  707  switches the motion tracking apparatus  700  back to the first mode. If not, the control unit  707  controls the motion tracking apparatus to keep in the second mode. 
     In view of above-mentioned embodiment, proper sensing regions can be utilized corresponding to different modes. By this way, the power consumption can decrease and a correct image can be acquired. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.
An image sensing apparatus, comprising: a control unit; and an image sensor, wherein the control unit controls the image sensor to utilize a first image sensing region to sense a first image to output a first image signal in a first mode, wherein the control unit controls the image sensor to utilize a second image sensing region to sense a second image to output a second image signal in a second mode. The first image sensing region is smaller than a total image sensing region of the image sensor, and the second image sensing region is smaller than the first image sensing region.