Abstract:
A method to crop image from an irregularly shaped region. A top-left corner of the best cropping rectangle are determined by sequentially scanning the rows of the irregularly shaped region to find out the largest bounding area. By rolling back the largest bonding area from the x or y direction, the bottom-right corner is determined.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to image cropping and particularly to a method of image cropping for exposure blending, panorama stitching or other multi-shot applications.  
         [0003]     2. Description of the Related Art  
         [0004]     When photographs are taken without due care by a photographer, the camera may not be correctly orientated with respect to the horizontal. This results in differently tilted images in a multi-shot application, such as exposure blending or panorama stitching. Thus, when these images are registered, each of them should be rotated to be aligned with each other. The alignment result must typically be cropped to generate a final image rectangle with a largest possible overlapping area. A problem then arises of how best to select the position of these cropped images.  
       SUMMARY OF THE INVENTION  
       [0005]     The object of the present invention is to provide a method for image cropping which yields a best cropping rectangle.  
         [0006]     The present invention provides a method for image cropping including the steps of A method for image cropping comprising the steps of providing an irregularly shaped region composed of columns and rows of pixels, calculating, within the irregularly shaped region for each of the rows, a first width between a left and right boundary pixel of the row, a height between the left and a boundary pixel of the same column, another height between the left and the other boundary pixel of the same column, and first products of the widths and the heights, and identifying one of the left boundary pixels in the irregularly shaped region as a first corner of the cropping rectangle, wherein the first product calculated for the row of the first corner is the largest, and a semi-finished rectangle region is formed by the first corner, and the length and width which yield the largest first product, calculating, within an overlapping region of the semi-finished rectangle region and the irregularly shaped region, for each of the rows, a width between a right and left boundary pixel of the row, a height between the right and a boundary pixel of the same column, another height between the right and the other boundary pixel of the same column, and second products of the widths and the heights, and identifying one of the right boundary pixels in the overlapping region as a second corner of the cropping rectangle, wherein the second product calculated for the row of the second corner is the largest. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, given by way of illustration only and thus not intended to be limitative of the present invention.  
         [0008]      FIG. 1  shows a flowchart of a method for image cropping according to one embodiment of the invention.  
         [0009]      FIG. 2A ˜ 2 D respectively show four images captured using different exposure times by a camera aimed at a cross.  
         [0010]      FIG. 3  shows the alignment of the four transformed results of the images in FIGS.  2 A˜ 2 D.  
         [0011]      FIG. 4  shows a mask created to determine the cropping rectangle for the registration result of the images shown in FIGS.  2 A˜ 2 D.  
         [0012]      FIG. 5A-5D  show four scanning sequences of the auto-cropping function according to one embodiment of the invention.  
         [0013]      FIG. 6  shows the edge list used in the method of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]      FIG. 1  shows a flowchart of a method for image cropping according to one embodiment of the invention.  
         [0015]     In step  61 , the transformation matrices derived by image registration are applied to the images.  
         [0016]     In step  62 , a mask is created and has columns and rows with the same dimensions as the images. The mask distinguishes an overlapping region of all the transformed images. FIGS.  2 A˜ 2 D respectively show four images  71 ˜ 74  captured by a camera aimed at a cross using different exposure times, wherein the crosses in the images  72 ˜ 74  are results of rotations of the cross in the image  71 .  FIG. 3  shows the alignment of the four transformed results of the images  71 ˜ 74 . It is noted that a region  81  is composed of a conjunction of the pixels of the four transformed images, i.e., the region  81  is an overlapping region of all the transformed images.  FIG. 4  shows the corresponding mask, which has the same dimensions as each image, and the pixel in the mask has a value of zero if it is an element of the pixel conjunction (in the region  81 ); otherwise (outside the region  81 ), the pixel value is not zero.  
         [0017]     In step  63 , an edge list is created as shown in  FIG. 6 . The edge list is a one-dimensioned array with a length the same as the height (number of the rows) of the mask and records the column indices of the left and right boundary pixels of each row of the region  81  in the mask.  
         [0018]     In step  64 , an optimal cropping function is applied to the edge list to derive two corners for determining the cropping rectangle.  
         [0019]     First, as shown in  FIG. 5A , in a sequence from top to bottom, for each row, a width between the left and right boundary pixel of the row and a height between the left boundary pixel and a bottom boundary pixel of the same column are calculated using the edge list. The product of the width and height is also derived. By comparing all the product, the largest is determined.  
         [0020]     Second, as shown in  FIG. 5B ,in a sequence from bottom to top, for each row, a width between the left and right boundary pixel of the row and a height between the left boundary pixel and a top boundary pixel of the same column are calculated using the edge list. The product of the width and height is also derived. By comparing all the products, the largest is determined. Then, the two largest products derived in the two opposite row sequences for the left boundary pixels are compared to identify a corner of the cropping rectangle, wherein the left boundary pixel of the row having the larger one is the corner. If the corner is derived by the sequence from top to bottom, it is a top-left corner of the cropping rectangle; otherwise it is a bottom-left corner. Additionally, a semi-finished rectangle  82  is formed by this corner and the accompanied length and width which yield the largest product, as shown in  FIG. 5C and 5D , respectively when the corner is a bottom-left and top-left one.  
         [0021]     Third, the edge list is updated with a region  81 ′ shown in  FIG. 5C  or  5 D. The region  81 ′ is the overlapping region of the semi-finished rectangle  82  and the region  81 . As shown in  FIG. 5C , in a sequence from top to bottom, for each row, a width between the left and right boundary pixel of the row and a height between the right boundary pixel and a bottom boundary pixel of the same column are calculated using the edge list when the previously determined corner is a bottom-left one. The product of the width and height is also derived. By comparing all the products, the largest is determined. The right boundary pixel of the row having the largest product is the top-right corner of the cropping rectangle. Thus, the cropping rectangle is determined. Alternatively, as shown in  FIG. 5D , in a sequence from bottom to top, for each row, a width between the left and right boundary pixel of the row and a height between the right boundary pixel and a top boundary pixel of the same column are calculated using the edge list when the previously determined corner is a top-left one. The product of the width and height is also derived. By comparing all the products, the largest is determined. The right boundary pixel of the row having the largest product is the bottom-right corner of the cropping rectangle. Thus, the cropping rectangle is determined.  
         [0022]     The foregoing description of the preferred embodiments of this invention has been presented for purposes of illustration and description. Obvious modifications or variations are possible in light of the above teaching. The embodiments were chosen and described to provide the best illustration of the principles of this invention and its practical application to thereby enable those skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.