Abstract:
A region-based illumination-normalization method and system utilizes a plurality of filters to filter a test image for obtaining a high frequency image and a low pass image. The low pass image is segmented into several regions by an image segmentation unit so that a region-based adjustment unit normalizes the regions based on an illumination reference model, respectively. Finally, an image combination unit is employed to combine the high frequency image and the low pass image that is normalized.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image process and, more particularly, to a region based illumination-normalization method and system.  
         [0003]     2. Description of Corresponding Art  
         [0004]     An image has two primary types of image information, illumination information and object information. During an image analysis or an image processing application (such as a face recognition system or a mechanical vision system), the illumination information in the image is usually removed first to eliminate data deviations caused by different lighting effects on objects, thus bringing lighting effects in the processed image to a minimum.  
         [0005]      FIG. 1  is a schematic drawing of a face image obtained under different lighting conditions. It illustrates how different lighting can cause different image results. Therefore, in order to utilize a plurality of images in the face recognition system, an illumination compensation process must be performed to the plurality of images to bring the lighting effects to a minimum. Currently, there are two types of illumination compensation processes. One is implemented by utilizing a statistical approach, and the other one is implemented by utilizing a space field analysis process or a frequency field analysis process. These two types of illumination compensation processes will be explained hereinafter.  
         [0006]     The statistical approach employs an averaging method to reduce image differences so that the images are presented evenly. The space field analysis process uses a filter to filter image features (F 0 ) and illumination information (I 0 ) from the testing image, and normalizes the illumination information (I 0 ) to serve as illumination information (I a ) so that any effect caused by lighting factors are minimized. The illumination information (I a ) and the image features (F 0 ) are combined to form an illumination normalized image with low light sensitivity. For example, U.S. Pat. No. 6,122,408 discloses such an image processing method by using a formula: 
 
 x ( x,y )=α( x,y )· Y ( x,y )+β( x,y ), 
 
 wherein, x (x,y) is an input test image, Y(x,y) is an output illumination normalized image, α(x,y) is a multiplication factor and β(x,y) is an addition factor. Accordingly, the output illumination normalized image Y(x,y) can be obtained by: 
 
 Y ( x,y )=( x ( x,y )−β( x,y ))/α( x,y ). 
 
 Thus, this patent focuses on how to obtain the multiplication factor and the addition factor. However, obtaining the multiplication factor and the addition factor is not easy and requires a lot of processing time. 
 
         [0009]     Therefore, it is desirable to provide a region based illumination-normalization method and system to mitigate and/or obviate the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0010]     The objective of the present invention is to provide a region based illumination-normalization method and system which can minimize an illumination sensitivity of an image.  
         [0011]     In order achieve the above-mentioned objective, the present invention provides a region based illumination-normalization system which includes a high pass filter, a low pass filter, an image normalization unit, and an combination unit. The high pass filter is used for receiving a test image to obtain a high frequency image. The low pass filter is used for receiving the test image to obtain a low frequency image. The image normalization unit is used for receiving the low frequency image and segmenting the low frequency image into a plurality of regions, and then normalizing each region to obtain a normalized low frequency image. The combination unit is used for combining the normalized low frequency image and the high frequency image. Furthermore, the plurality of regions correspond to an illumination reference model with a plurality of reference regions, so as to normalize the regions according to an illumination statistical distribution.  
         [0012]     The present invention also provides another region based illumination-normalization system which includes a high pass filter, a low pass filter, an illumination reference model, an image normalization unit, and an image normalization unit. The high pass filter is used for receiving a test image to obtain a high frequency image. The low pass filter is used for receiving the test image to obtain a low frequency image. The image segmentation unit is used for receiving the low frequency image and segmenting the low frequency image into a plurality of regions. The illumination reference model is used for dividing the low frequency image into a plurality of reference regions according to the plurality of regions of the image segmentation unit. The image region normalization unit is used for receiving the plurality of regions and normalizing illumination information of the plurality of regions to obtain a normalized low frequency image via the plurality of reference regions of the illumination reference models. The combination unit is used for combining the normalized low frequency image and the high frequency image.  
         [0013]     The present invention also provides a region based illumination-normalization method which includes steps: (A) filtering a test image to obtain a high frequency image and a low frequency image; (B) segmenting the low frequency image to obtain a plurality of regions; (C) normalizing each region to obtain a normalized low frequency image; and (D) combining the normalized low frequency image and the high frequency image. Furthermore, the plurality of regions correspond to an illumination reference model with a plurality of reference regions so as to normalize the regions according to an illumination statistical distribution.  
         [0014]     Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a schematic drawing of a face image obtained under different lighting conditions.  
         [0016]      FIG. 2  is a function block schematic drawing of a preferred embodiment in accordance with the present invention.  
         [0017]      FIG. 3  is a flowchart of the preferred embodiment in accordance with the present invention.  
         [0018]      FIG. 4  is an image processing schematic drawing of the preferred embodiment in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The present invention is a pre-process applicable to many imaging processes, such as face tracking systems, human face recognition system, mechanical vision system, and the like. All these image processing systems need to minimize the effects caused by illumination information, and the present invention can minimize illumination sensitivity for an image. The following embodiment will take a face recognition system for illustrative purposes.  
         [0020]     Please refer to  FIG. 2 .  FIG. 2  is a function block schematic drawing of a preferred embodiment in accordance with the present invention. An illumination-normalization system comprises a high pass filter  21 , a low pass filter  22 , an image normalization unit  23  and an image combination unit  24 . The image normalization unit  23  further has an image segmentation unit  231 , a region normalization unit  232 , an illumination reference model  233 , and a smooth processing unit  234 .  
         [0021]     The high pass filter  21  and the low pass filter  22  are used for filtering an input testing image (such as a face) to obtain a high frequency image and a low frequency image. The high frequency image is a sharpened image of the original image, which primarily includes most of the image features and some illumination information. The low frequency image is a fuzzy image, which primarily includes most of the illumination information and some image features. Therefore, the present invention will focus only on the low frequency image to minimize the illumination sensitivity of an image. The following description will explain how to use the above-mentioned system to perform an illumination normalization procedure.  
         [0022]     Please refer to  FIG. 2 ,  FIG. 3  and  FIG. 4 .  FIG. 3  is a flowchart of the preferred embodiment in accordance with the present invention.  FIG. 4  is an image processing schematic drawing of the preferred embodiment in accordance with the present invention. First, a test image  401  is inputted separately into the high pass filter  21  and the low pass filter  22 , so that the high pass filter  21  outputs a high frequency image  406  with most of the image features, and the low pass filter  22  outputs a low frequency image  402  with most of the illumination information (step  301 ).  
         [0023]     Furthermore, the low frequency image  402  is normalized (step  302 ), which means reducing the illumination sensitivity of the image by adjusting (compensating) illumination information (light information) of the low frequency image  402 . Since the low frequency image  402  has most of the illumination information, change in illumination on the low frequency image  402  is very obvious. The image segmentation unit  231  segments the low frequency image  402  into a plurality of regions  4031 ,  4032 ,  4033 ,  4034  according to an illumination difference to obtain a segmented image  403 . The number of regions is determined by a predetermined parameter of the image segmentation unit  231 .  
         [0024]     Furthermore, the region normalization unit  232  normalizes the illumination information of the plurality of regions  4031 ,  4032 ,  4033 ,  4034  of the segmented image  403 . The region normalization unit  232  compares each region  4031 ,  4032 ,  4033 ,  4034  with a corresponding illumination statistical distribution of a corresponding reference region in the illumination reference models  233 ,  404 . For example, the region  4031 , located at the left side of the image, is compared with an illumination statistical distribution on a corresponding position at the left side of the illumination reference model  404 .  
         [0025]     If the regions are identical with the illumination statistical distribution, the region normalization unit  232  stops normalizing the region  4032  (step  303 ). If the region is different from the illumination statistical distribution, the region normalization unit  232  normalizes the plurality of regions  4031 ,  4032 ,  4033 ,  4034  according to the reference regions (the corresponding position for the plurality of regions  4031 ,  4032 ,  4033 ,  4034 ) on the illumination reference model  404 , so that the plurality of regions  4031 ,  4032 ,  4033 ,  4034  are similar to the illumination reference model  404  and become a normalized low frequency image  405  (step  304 ). The region normalization unit  232  uses the following formula to achieve a region-based illumination-normalization: 
 
 T=Ĝ   −1   °Ĥ 
 
 where Ĝ is a continuous distribution function of the reference region of the illumination reference model  404 , and Ĥ is a continuous distribution function of the plurality of regions  4031 ,  4032 ,  4033 ,  4034 . Therefore, a normalized region can be expressed as: 
 
 R   a ″( x,y )= T ( R   a ( x, y ))= Ĝ   −1   °Ĥ ( R   a ( x,y )), 
 
 where R a ″ is the normalized region, R a  is a test region, and T is a transfer formula. 
 
         [0028]     In this embodiment, the illumination reference model  404  is an object image (such as a face), which combines light information from a plurality of the same type of object images with front face lighting by averaging or weighted averaging of the root-mean square of the light information, in order to divide image features as far as possible and enhance the illumination information. Therefore, a greater number of object images is preferred.  
         [0029]     In the aforementioned steps, the low frequency image  402  is segmented, and the plurality of regions  4031 ,  4032 ,  4033 ,  4034  are normalized. However, discontinuities occur at the boundaries among the plurality of regions  4031 ,  4032 ,  4033 ,  4034 . Therefore, in step  305 , the smoothing unit  234  performs a smoothing process to the normalized low frequency image  405  to eliminate the discontinuities. In addition, step  305  can be preformed after step  303 , if the regions are identical with the illumination statistical distribution.  
         [0030]     Finally, the image combination unit  24  combines the smoothed low frequency image  402  and the high frequency image  406  to generate a normalized image  407  (step  306 ).  
         [0031]     The normalized image can then be inputted into the face recognition system for a recognition process. According to experimental results, the normalized image provided by the present invention has a high recognition rate of 82.73% in the face recognition system, whereas a non-normalized image has a relatively low recognition rate of 44.93% in the face recognition system. Moreover, an image provided by the prior art light normalization system using the statistical method has a recognition rate of 73.2% in the face recognition system, and an image provided by the technique provided in U.S. Pat. No. 6,122,408 has a recognition rate of 76.6% in the face recognition system. Therefore, the region based illumination-normalization system and method of the present invention can minimize the illumination sensitivity for images, and increase the recognition rate of the face recognition system.  
         [0032]     Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.