Abstract:
A method of performing non-linear transformation of a digital image for contrast modification. The original video input is compared to a linear transformation with gain and offset; and, the differential Δ multiplied by a factor m obtained from a look-up table and the product mΔ added to the original input to obtain the output video.

Description:
BACKGROUND 
       [0001]    The present disclosure relates to digital imaging and particularly in devices employed for scanning, copying and printing in enterprise or office environments where the user can input a document either as a printed sheet or an electronically created document such as from a desktop computer into a multi-function device for effecting scanning, copying and printing as the case may be. Heretofore, the technique of dynamic range adjustment of the image has been performed for image compression and the purpose of background suppression; however, this can result in changing of the colors of the image in the case of a colored document thereby producing undesired shifts in the appearance of the image. The linear transformation employed for image compression and background suppression has utilized a gain change and an offset based upon a mapping of the pixel content distribution in the image. This technique has provided image compression and background suppression but as mentioned before, has not been altogether satisfactory when applied to color images. 
         [0002]    Thus, it has been desired to provide a way or means of image compression and background suppression of digital imaging in any manner which can provide desired contrast changes and background suppression without distorting the colors or overall appearance of the image. 
       BRIEF DESCRIPTION 
       [0003]    The present disclosure describes and illustrates a technique of dynamic range adjustment in digital imaging which allows adjustment of the contrast in portions of the image without distortion of the color content of the image. 
         [0004]    The method of the present disclosure maps the pixel count of the image against the luminance on a scale of 0 to 255 and performs a transform which includes a non-linear relationship between the input and the output for the region of pixels for which it is desired to alter the contrast or provide background suppression. If desired, a portion of the transform or pixels having a luminance within a selected band may be subjected to a linear transform with the remaining pixel content utilizing a transform which employs a correction factor obtained from a look-up table of the curve values and an adjustment factor computed with respect to the difference between the original input and the linear transform using the correction factor obtained from the look-up table. 
         [0005]    In the present practice, the look-up table of the values of the graph utilizes a non-linear curve of a selected shape in the regions to be modified; although, a correction factor may be applied from the look-up table for the entire range of pixel luminance if it is desired to completely alter the appearance of the image. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an exemplary histogram of the distribution of pixel luminance for a digital image to be modified; 
           [0007]      FIG. 2  is a graphical representation of a linear transform with no correction, with a gain and offset and with the transformation of the present disclosure; 
           [0008]      FIG. 3  is a graphical representation of the values to be employed for the correction factor; 
           [0009]      FIG. 4  is a block flow diagram of the operation of the system for performing the image modification in accordance with the present disclosure; 
           [0010]      FIG. 5  is a diagram of the procedure for determining the adjustment factor; 
           [0011]      FIG. 6  is a print of an exemplary image in its original form; and 
           [0012]      FIG. 7  is a print of the image of  FIG. 6  after transformation in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , the pixel count of an exemplary image is plotted in a histogram of the luminance of the pixels on a scale of 0 to 255. The region of the black point is chosen as pixels having a luminance of less than 25; and, the white point is chosen as pixels having a luminance of less than 210. 
         [0014]    Referring to  FIG. 2 , the direct linear transformation is shown graphically as a plot having a slope of 45 degrees or output equal to the input is shown in dashed line; and, a linear transformation in the region of pixel luminance of 25-210 is shown in solid line indicating the offset and the change in gain or slope and is identified as V g/o . An exemplary transformation in accordance with the present disclosure is indicated in dashed and dotted line which is curvilinear outside of a linear region denoted by the vertical lines in  FIG. 2  and identified by the m factor legend and will hereinafter be described in greater detail. 
         [0015]    Referring to  FIG. 5 , the procedure for computing the adjustment factor is indicated in block diagram generally at  10  wherein the input video V INPUT  is along line  12  and is inputted to block  14  along line  13  for the computation of the V g/o  as follows. 
         [0000]        V   g/o =(V INPUT −black point)*255(white point−black point) 
         [0016]    V g/o  is inputted along line  16  along with the input along line  18  to the block  20  where the differential Δ is computed as follows. 
         [0000]      Δ= V   g/o   −V   INPUT    
         [0017]    The differential Δ is then inputted along line  22  to block  24  where the system determines the m factor from a look-up table of the selected type of transformation values and Δ is then multiplied by the factor m. An exemplary graphical representation of a plot of a look-up table of values is shown in  FIG. 3  where the m factor is employed for pixel values less than 65 and greater than 187. 
         [0018]    An adjustment factor determined by the operation of block  24  comprising m×Δ is then added to the value of V INPUT  at block  26  to provide the output V OUT . Thus, the transformation in the region outside of the linear portion is performed in accordance with the procedure described with respect to  FIGS. 3 and 5  and produces the transformation shown in dashed and dotted line in  FIG. 2 . 
         [0019]    Referring to  FIG. 4 , a block flow diagram of the operation of the system is indicated generally at  50  where the user inputs the document at step  52  and the system proceeds to step  54  and scans the document to generate a digital image at step  54 . The digital image is then rasterized at step  56  and the pixel count is mapped against pixel value in the range between the image black point and the image white point at step  58 . The system then proceeds to step  60  and a desired transformation is implemented for the pixel values over the mapped range. 
         [0020]    An exemplary transformation is illustrated in  FIG. 3 , where values of the m factor are plotted as a function of V INPUT  corresponding to pixel luminance values. 
         [0021]    The system then proceeds to step  62  to make a determination as to whether the selected transformation is linear; and, if the determination is affirmative, the system proceeds to step  64  and selects a portion of the transformed range to be maintained linear. However, if the determination at step  62  is negative, the system proceeds to step  66  to select a non-linear transformation from a look-up table and the attendant m factor is determined as described hereinabove with respect to  FIGS. 3 and 5 . The system then proceeds at step  68  to modify the image contrast over the mapped range based on the selected non-linear transformation. 
         [0022]    Where the transformation is to include a linear portion, the system proceeds from step  64  to step  70  to select a non-linear transformation from a look-up table for values outside of the linear range. This is the determination of the m factor as exemplified in  FIG. 3 . The system then proceeds to step  72  and performs a selected non-linear transformation for the values outside the selected linear portion and at step  74  the image contrast is modified based upon the non-linear transformation of step  72 . 
         [0023]    An example of an image which has not been modified is shown in  FIG. 6 ; and, an image which has been modified in accordance step  74  based upon the transformation of  FIG. 3 , is illustrated in  FIG. 7 . 
         [0024]    The present disclosure thus describes and illustrates modification of a digital image for image compression and contrast adjustment without distorting the color of the image. 
         [0025]    It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.