Abstract:
An apparatus for and a method of removing a moire pattern of a digital imaging device are provided. It is determined whether or not a moire pattern is created based on an image frequency of a preview image and a spatial frequency of an image sensor, and when it is determined that a moire pattern is generated, an optical path of light incident to an image sensor is slightly changed to remove the moire pattern. Accordingly, image quality deterioration can be improved, and a problem of unnecessary filtering that causes deterioration of resolution even when there is no moire pattern can be enhanced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Korean Patent Application No. 10-2008-0006291, filed on Jan. 21, 2008, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for removing a moire pattern, and more particularly, to an apparatus and method for removing a moire pattern from a digital image of a digital imaging device by means of low-pass filtering. 
     2. Description of the Related Art 
     A digital imaging device generally photographs an object using an image pick-up device which has a grid structure. A charge-coupled device (CCD) is a typical image pick-up device used as a digital imaging device. A CCD is an image sensor that projects an image of an object onto a particular solid-state device by use of an optical system and electrically scans the projected image to convert light signals into electric signals. 
     Since a digital imaging method using an image pick-up device enables an image to be obtained as soon as an object is photographed and the photographed image is easy to store, it is widely used in personal cameras, cameras mounted in mobile phones, and the like. 
     However, unlike an analog imaging device such as a film camera, a digital imaging device can have a moire pattern due to the grid structure of an image sensor. A moire pattern is an undesirable irregular image, which is caused by interference between high frequency components of a repeated pattern of an object to be photographed in a region around a spatial frequency of an image sensor. 
     Conventionally, a method using an optical low-pass filter (OLPF), that is, a birefringent device, is used to remove a moire pattern. In this method, incident light is divided into more than two light beams with different refraction angles by using the OLPF and then the light beams are focused on an image sensor. 
     However, since an additional birefringent device is required, the size of an optical system increases. In particular, filtering is always performed even when there is no moire pattern generated so that the sharpness of an image can deteriorate. 
     Recently, although a method of processing image data in a digitized manner in an image processing chip has been suggested in order to remove a moire pattern, this method can reduce the overall performance of the digital imaging device due to the additional image processing procedure. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention provides an apparatus and method for removing a moire pattern only when necessary, without deteriorating resolution, for a digital imaging device. 
     According to an aspect of the present invention, when it is determined that a moire pattern is generated, the moire pattern can be removed by slightly changing an optical path of light incident to an image sensor to perform low-pass filtering. 
     Additional aspects of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     An aspect of the present invention discloses an apparatus for removing a moire pattern of a digital imaging device, the apparatus comprising: a moire pattern detecting unit which determines whether or not a moire pattern is generated, using an image frequency of a preview image and a spatial frequency of an image sensor; and an optical path adjusting unit which changes an optical path of light incident to the image sensor to perform low-pass filtering when it is determined by the moire detecting unit that a moire pattern is generated. 
     The optical path adjusting unit may include an optical filter or total reflection prism and the optical path may be changed by the rotation of the optical filter or the total reflection prism. The optical path adjusting unit may move a position of the image sensor or a lens unit that transfers light to the image sensor to change the optical path. 
     The optical path adjusting unit may move the optical path by a distance of at least one pixel while the image sensor is exposed to light under the control of the moire pattern detecting unit. 
     The moire pattern detecting unit may determine that a moire pattern is created when an image frequency of the preview image is in a particular region around a spatial frequency of the image sensor. The moire pattern detecting unit may determine that a moire pattern is created if the image frequency of the preview image is present in a region ranging between ±25 percent of a spatial frequency of the image sensor. 
     An aspect of the present invention also discloses a method of removing a moire pattern of a digital imaging device, the method comprising: obtaining an image frequency of a preview image; determining whether or not a moire pattern is created by using the obtained image frequency of the preview image and a spatial frequency of the image sensor; and removing a moire pattern by changing an optical path of light incident to the image sensor when it is determined that a moire pattern is present. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the invention, and together with the description serve to explain the aspects of the invention. 
         FIG. 1  is a block diagram showing a structure of an apparatus for removing a moire pattern of a digital imaging device according to an embodiment of the present invention; 
         FIGS. 2A and 2B  show a structure of the optical path adjusting unit; 
         FIG. 3  shows an image sensor in order to explain a principle of how a moire pattern is removed when an optical path of light is changed according to an embodiment of the present invention. 
         FIG. 4  is a block diagram of a moire pattern detecting unit in  FIG. 1 . 
         FIG. 5  is a graph showing a result of analyzing a frequency of a preview image and a spatial frequency of an image sensor. 
         FIG. 6  is a flowchart of a method of removing a moire pattern according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. Hereinafter, in describing the present invention, detailed descriptions of relevant functions or structures well-known to those skilled in the art will be omitted when it is considered that the descriptions obscure the point of the present invention. The terms used herein are defined in consideration of the functions of elements in the present invention, and may be varied according to the intentions or the customs of a user and an operator. 
       FIG. 1  is a block diagram showing a structure of an apparatus  100  for removing a moire pattern of a digital imaging device according to an embodiment of the present invention. Referring to  FIG. 1 , the digital imaging device includes a lens unit  103 , an image sensor  104 , an image processing unit  105 , an image display unit  106 , and a storage unit  107 . The digital imaging device may be a personal digital camera, a built-in camera in a mobile phone, and a reverse guide camera mounted in the rear of a vehicle. 
     When the digital imaging device photographs a particular object, light from the outside passes through the lens unit  103  and is transferred to the image sensor  104 , and the image sensor  104  converts the transferred light signal into an electric image signal. The converted image signal is processed by the image processing unit  105  and then displayed on the image display unit  106  or stored in the storage unit  107 . 
     In this case, when an object to be photographed has repetitive patterns, a moire pattern can be created due to interference between high-frequency components in the course of digitizing an image by the image sensor  104 , since the image sensor  104  contains a plurality of image pick-up devices (for example, CCD sensors or complementary metal oxide semiconductor (CMOS) sensors) which are arranged in a grid pattern. 
     The apparatus  100  for removing a moire pattern includes an optical path adjusting unit  101  and a moire pattern detecting unit  102  as illustrated in  FIG. 1 . 
     The optical path adjusting unit  101  performs low-pass filtering on light incident to the image sensor  104  and slightly changes the optical path of the light incident to the image sensor  104  under the control of the moire pattern detecting unit  102 . 
       FIGS. 2A and 2B  show a structure of the optical path adjusting unit  101 . Referring to  FIGS. 2A and 2B , the optical path adjusting unit  101  includes an optical filter  201  or total reflection prism  202  which is interposed between the lens unit  103  and the image sensor  104  to transfer light to the image sensor  104 . 
     In this case, the optical filter  201  or the total reflection prism  202  is capable of changing an optical path of light incident to the image sensor  104  by rotating within a predetermined angle, as shown in  FIGS. 2A and 2B . The optical path adjusting unit  101  may further include an additional driving device (not shown) to rotate the optical filter  201  or the total reflection prism  202 . 
     Although not illustrated in drawings, the optical path adjusting unit  101  may move the lens unit  103  or the image sensor  104  to change the optical path. For example, the optical path can be changed by slightly moving the image sensor  104  upwards or downwards perpendicularly to an optical axis. 
     The optical path adjusting unit  101  moves an optical path by a distance of at least one pixel, while the image sensor  104  is exposed to light, in order to perform low-pass filtering under the control of the moire pattern detecting unit  102 . The pixel is referred to a unit image pick-up element of the image sensor  104 . 
     With reference to  FIG. 3 , a description will be given of a principle of how a moire pattern is removed by means of low-pass filtering when an optical path of light incident to the image sensor  104  is changed. 
     Referring to  FIG. 3 , light is originally made incident to a pixel A  301  at a time t 1  and then the light is made incident to an adjacent pixel B at a time t 2  by the optical path adjusting unit  101  so that the optical path is changed. Here, the time between t 1  and t 2  indicates a time period when the image sensor  104  is exposed to light to photograph an object, and the pixel A  301  and the pixel B  302  indicate unit image pick-up elements of the image sensor  104 . 
     That is, since the optical path of the light incident to the image sensor  104  is changed from one pixel (for example, the pixel A  301 ) to an adjacent pixel (for example, the pixel B  302 ) for a particular period of time to photograph an object, it is possible that the incident light is temporally divided and focused on the image sensor  104 . As the result, the light is blurred and the high-frequency components are cut off, and hence the moire pattern can be removed. 
     Referring to  FIG. 1  again, the moire pattern detecting unit  102  determines if a moire pattern occurs, and controls the optical path adjusting unit  101  to change the optical path when it is determined that there is a moire pattern, as described above. That is, the operation of the optical path adjusting unit  101  depends on the determination result of the moire pattern detecting unit  102 . 
     To this end, the moire pattern detecting unit  102  may include a frequency analyzing unit  110  and a frequency comparing unit  120  (referring to  FIG. 4 ). 
     The frequency analyzing unit  110  analyzes a preview image based on a frequency domain to obtain an image frequency of the preview image. The preview image may be an image displayed in the image display unit  106  before the object is actually photographed, and the image frequency of the preview image can be obtained by analyzing an image signal of the preview image based on a frequency domain. For instance, the frequency analyzing unit  110  can obtain an image frequency of the preview image in a spectrum form by performing a fast-Fourier transform on the image signal obtained by the image sensor  104 . 
     The frequency comparing unit  120  compares the image frequency of the preview image obtained by the frequency analyzing unit  110  and a spatial frequency of the image sensor  104 . The spatial frequency of the image sensor  104 , which is a unique property of the image sensor  104 , can be obtained by taking the inverse of a distance between unit image pick-up devices which comprise the image sensor  104 . 
     Furthermore, the frequency comparing unit  120  compares the image frequency of the preview image and the spatial frequency of the image sensor  104  in order to determine whether the image frequency of the preview image is present in a particular region around the spatial frequency of the image sensor  104 . The particular region may range between about ±25 percent of the spatial frequency of the image sensor  104 . 
     If the image frequency of the preview image is present in the particular region around the spatial frequency of the image sensor  104 , there is a high possibility of the occurrence of a moire pattern due to frequency interference between the image and the sensor. Therefore, it can be determined that the moire pattern is created when the image frequency is present in the particular region around the spatial frequency of the image sensor  104 . 
       FIG. 5  is a graph showing a spectrum obtained by analyzing the preview image based on a frequency domain. Referring to  FIG. 5 , a moire pattern generating region  401  is set to range between ±25 percent of a spatial frequency of the image sensor. 
     The moire pattern detecting unit  102  analyzes the preview image based on a frequency domain to obtain the image frequency of the preview image, as shown in  FIG. 5 , and determine if the obtained image frequency is present in the moire pattern generating region  401 . When the image frequency of the preview image is present in the moire pattern generating region  401 , it is determined that the moire pattern is generated. 
     A method of removing a moire pattern of a digital imaging device according to an embodiment of the present invention will now be described with reference to  FIG. 6  in conjunction with  FIG. 1 . 
     When a user operates a digital imaging device to photograph an object, light from outside is focused on the image sensor  104  so that a preview image can be obtained. 
     The moire pattern detecting unit  102  analyzes the preview image based on a frequency domain in order to obtain an image frequency of the preview image (operation S 401 ). Image information of the preview image can be input from the image sensor  104 , and an image frequency in a spectrum form, as shown in  FIG. 5 , can be obtained by fast-Fourier transforming the input image information. 
     Then, the moire pattern detecting unit  102  determines whether the moire pattern is created based on the obtained image frequency of the preview image and the spatial frequency of the image sensor  104  (operation S 402 ). The spatial frequency of the image sensor  104  may be an inverse value of a distance between unit image pick-up devices that comprise the image sensor  104 . In this case, the moire pattern detecting unit  102  may determine whether the moire pattern is generated depending on if a particular frequency of the preview image that has been obtained in operation S 401  is detected around the spatial frequency of the image sensor  104 . 
     For example, if the image frequency of the preview image is present in a particular region around the spatial frequency of the image sensor  104 , it can be determined that the moire pattern is generated, and the particular region may range between ±25 percent of the spatial frequency of the image sensor  104 . 
     When it is determined that there is the moire pattern, the moire pattern detecting unit  102  issues a predetermined control signal to the light optical path adjusting unit  101 , and the optical path adjusting unit  101  changes an optical path of light incident to the image sensor  104  to remove the moire pattern (operation S 403 ). In this case, the optical path may be moved by a distance of at least one pixel while the image sensor is exposed to light in order to photograph an object, and thus low-pass filtering is performed. 
     Thus, according to the structure and method described above, the moire pattern detecting unit  102  determines whether a moire pattern is generated and then controls the optical path adjusting unit  101  only when it is determined that a moire pattern is created or when a moire pattern is highly likely to be generated, and hence clearness of an image is secured and the moire pattern can be removed under an optimum environment. 
     In addition, since the optical path is changed while the image sensor  104  is exposed to light, low-pass filtering is automatically performed simultaneously upon finishing exposure, and hence there is no need for additional image correction. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.