Abstract:
The present invention provides a extracting and correcting method for a wide-angle image which can be a still image or a video stream. The method is able to display the extracted image without distortion. The correction method of the present invention is performed to map sections of the image to a plane with respect to their respective viewing angles. The present invention is adapted to progressively move the view angle and repeat the steps of extracting and correcting, thereby producing the panning effect without distortion.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a technique of image processing. More specifically, the present invention relates to a perspective correction e-panning (electronic panning) method for wide-angle images. 
       DESCRIPTION OF THE PRIOR ARTS 
       [0002]    Wide-angle images have nonstandard resolution (for example, 1800×460), and thus could be difficult to watch on a standard display device (for example, 640×480) without resizing or sacrificing resolution. Conventional e-panning method treats a wide-angel image as regular large resolution image and displays only slices from the image, which is equivalent to zooming at different part of the wide-angle image without any correction process. However, the conventional method produces distorted result because the wide-angle image is actually represented on a cylindrical surface or a spherical surface. 
         [0003]    Panning is a technique often applied to monitor cameras. For example, U.S. Pat. No. 6,977,678 describes a motor panning method for viewing in different directions. Such a method requires physical moving parts (i.e., motors), and is thus susceptible to mechanical degradation and incurs higher power consumption (to drive the motors). In addition, the mechanical components also increase the cost and space requirement for a system. 
       SUMMARY OF THE INVENTION 
       [0004]    In view of the above problems, an object of the present invention is to provide a perspective correction method adapted to display slices of a wide-angle (panoramic) image without distortion. Another object of the present invention is to provide a perspective correction panning method so as to replace a motor. The present invention has achieved the advantageous effects of low cost, small space requirement and low power consumption. 
         [0005]    According to one aspect of the present invention, a perspective correction method for wide-angle images comprises: (a) providing a wide-angle image which is a still image or a video stream; (b) determining a viewing position; (c) extracting a slice of the wide-angle image based on said viewing position; and (d) performing a perspective correction process with respect to the extracted slice of the wide-angle image. 
         [0006]    According to another aspect of the present invention, a perspective correction panning method for wide-angle images comprises: (a) providing a wide-angle image which is a still image or a video stream; (b) determining a starting position of image extracting; (c) extracting a slice of the wide-angle image based on said starting position; (d) performing a perspective correction process with respect to the extracted slice of the wide-angle image; (e) cropping and displaying said slice of the wide-angle image after correction; and (f) creating a new starting position by adding a small displacement increment to the previous starting position and repeating the above steps of (c), (d) and (e). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a flow chart of a perspective correction panning method for wide-angle images according to the present invention. 
           [0008]      FIG. 2  shows image planes respectively obtained from three shooting directions of a camera. 
           [0009]      FIG. 3  is a top view of  FIG. 2  wherein a virtual cylindrical surface is shown. 
           [0010]      FIGS. 4(A) and 4(B)  are illustrative views for the perspective correction according to an embodiment of the present invention. 
           [0011]      FIG. 5  shows sliced images from a wide-angle image to simulate panning effect, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIG. 1  describes an outline of a perspective correction panning method according to the present invention. This method takes a section of data from a wide-angle image and performs a perspective correction process. Finally, the corrected image is processed and displayed. 
         [0013]    As shown in  FIG. 1 , the wide-angle image is firstly input, wherein said input image can be a single image taken from a wide-angle lens camera system or a stitched image composed of a plurality of small images. In addition, said wide-angle image can be a still image or a video stream. Then, a starting position is input as a starting point for a extracted image of said wide-angle image. Based on said starting position, a slice of said wide-angle image is extracted. With respect to said slice of image, a perspective correction process (discussed later) is performed to create a slice of image without distortion. Said slice of image after perspective correction is cropped to remove the blank area in the image, so that an image after processing is displayed on a display device. Next, a small displacement increment is added to the starting position of the extracted image to form a new starting position, and then the subsequent steps of image extracting, perspective correction, cropping, displaying, etc. are repeated. The above step of adding a small displacement increment has achieved the effect equivalent to the progressive rotation of camera in a conventional panning camera system. Thus a camera system with panning function is successfully simulated without a physical moving part (motor). 
         [0014]    Since a wide-angle image is represented on a cylindrical surface or a spherical surface, a slice of image directly extracted from the wide-angle image is distorted or deformed, especially on both sides of the slice of image. In order to solve the problem of distortion, the inventor has provided a resolution called “perspective correction”. A perspective correction method according to an embodiment of the present invention is described with reference to  FIGS. 2 to 5  as follows. For easy understanding of the inventive content, the following description is provided by taking the cylindrical surface as an example. 
         [0015]    Firstly, a method of forming a wide-angle image is described with reference to  FIG. 2  and  FIG. 3 .  FIG. 2  illustrates three image planes (P 1 , P 2  and P 3 ) taken from three different directions (D 1 , D 2  and D 3 ) respectively by a camera.  FIG. 3  is a top view of  FIG. 2 , wherein the dotted line illustrates a cylindrical surface C. These three image planes (P 1 , P 2  and P 3 ) are mapped onto the cylindrical surface C as a wide-angle image after coordinate transformation, stitching and other processing. 
         [0016]    Next, slices of image are extracted from specified positions of the wide-angle image on the cylindrical surface C. The specified positions are directly related to the starting points of image extracting and thus will affect the view angle of the resulting image. 
         [0017]    Conventional e-panning methods simply display the above-mentioned slices of image directly on a display device without further processing, which is equivalent to displaying sections of the cylindrical surface C on a planar display device. The objects in the resulting image are inevitably distorted and unable to present normal viewing perspective. 
         [0018]      FIGS. 4(A) and 4(B)  are planar and perspective views, respectively, for illustrating the perspective correction method according to the present invention. In  FIGS. 4(A) and 4(B) , C′ denotes a cylindrical section extracted from the cylindrical surface C; T denotes a tangent plane; f denotes the focal distance; (x, y) denotes the coordinate of pixels on the tangent plane T; and (x c ,y c ) denotes the corresponding points on the cylindrical surface. According to an embodiment of the present invention, a reverse transformation is performed to map the image from cylindrical surface C′ to the tangent plane T, as shown by the arrows in  FIGS. 4(A) and 4(B) , by the following formula: 
         [0000]    
       
         
           
             
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         [0019]    For each integer pixel (x,y), the corresponding (x c ,y c ) will be a fraction (i.e., non-integer). Thus a Bi-linear Interpolation is applied to calculate the image data on (x c ,y c ). In the present embodiment, said image data is the color information of the pixel. With the same formula, it is then possible to map each sections of the image from the cylindrical plane to the tangent plane with respect to their respective viewing angle, which is referred to as “perspective correction”. 
         [0020]      FIG. 5  shows four slices of image (V 1 , V 2 , V 3  and V 4 ) extracted from different angles of a wide-angle image, wherein V 1  and V 4  are similar to P 1  and P 2  of  FIG. 2 , respectively. After the perspective correction, an image cropping process is performed to remove the blank area in the transformed image; a zooming process is performed to obtain a suitable size; and then the processed image can be displayed on a display device. 
         [0021]    The above-mentioned steps can be repeated with a small displacement increment added to the starting position of the other extracted image, as the order of V 1 →V 2 →V 3 →V 4  shown in  FIG. 5 . Accordingly, the image obtained by the perspective correction method of the present invention closely resembles the image obtained by a camera system with a panning motor. 
       Effects of the Invention 
       [0022]    Wide-angle images have unavoidable fish-eye like distortion in nature. Besides, wide-angle images have large resolution and do not fit on the normal display device. For the purpose of viewing, the wide-angle images are typically reduced in resolution or partially displayed. The present invention discloses a method to allow display of slices of images from the wide-angle images in the most natural perspective without distortion. The present invention also allows panning through a wide area without involving a motor in a camera system, and thus results in a more cost-effective and power-saving solution.