Abstract:
A process for performing white balancing of an image is performed by subdividing an image into a plurality of subframes, and then analyzing each subframe to determine if that subframe is predominantly monochromatic other than gray. If so, that subframe is excluded from the computation of the gain adjustments in the white balancing operation. As a result, the white balance process is performed using only the multicolored and/or gray subframes, thus allowing the overall white-balance of the image to be shifted only when a change in the color average is due to a change in the spectra of illumination, and not a presence of large monochromatic areas in the image.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a method for adjusting the color gains in an imaging system to compensate for the variations in illumination spectra attributable to different illumination sources. 
   BACKGROUND OF THE INVENTION 
   The human eye and brain are capable of “white balancing.” If a person takes a white card outside, it looks white. If he takes it inside and views it under fluorescent lights, it looks white. When viewed under an incandescent light bulb, the card still looks white. Even when placed under a yellow light bulb, within a few minutes, the card will look white. With each of these light sources, the white card is reflecting a different color spectrum, but the brain is smart enough to make it look white. 
   Obtaining the same result with a camera or other imaging device is harder. When the white card moves from light source to light source, an image sensor “sees” different colors under the different conditions. Consequently, when a digital camera is moved from outdoors (sunlight) to indoor fluorescent or incandescent light conditions, the color in the image shifts. If the white card looks white when indoors, for example, it might look bluish outside. If it looks white under fluorescent light, it might look yellowish under an incandescent lamp. 
   One of the most challenging problems in color image processing is adjusting the color gains of the system to compensate for the variations in the illumination spectra incident on the imaging sensor due to the illumination source, also known as white balance. This problem stems from the fact that spectral emission curves of common sources of illumination are significantly different from each other. For example, in accordance with Plank&#39;s law, the spectral energy curve of the sun is shifted towards the shorter wavelengths relative to the spectral energy curve of an incandescent light source. Therefore, the sun can be considered to be a “blue-rich” illuminator while an incandescent bulb can be considered to be a “red-rich” illuminator. As a result, if the color processing settings are not adjusted, scenes illuminated by the sunlight produce “bluish” imagery, while scenes illuminated by an incandescent source appear “reddish”. 
   In order to compensate for changes in illumination spectra, the gains of the color processing systems and/or imager should be adjusted. This adjustment is usually performed to preserve the overall luminance (brightness) of the image. As a result of proper adjustment, gray/white areas of the image appear gray/white on the image-rendering device (hence the term “white balance”). In the absence of specific knowledge of the spectra of the illumination source, this adjustment can be performed based on inference of the spectra of illumination from an analysis of the image itself. 
   The most commonly used approach to computing the proper adjustment to the color channel gains is based on the premise that in complex images all colors are represented equally. Based on this assumption, the sums of all red, green and blue components in the image should be equal (in other words, the image should average to gray). Following this approach, the overall (average over the entire image) luminance Y, and red (R_avg), green (G_avg) and blue (B_avg) components are evaluated. The color gains (G_red, G_Green, G_blue) are then selected so that
 
 Y=G _red* R _avg =G _green* G _avg =G _blue* B _avg.
 
   This conventional approach produces reasonable color rendition for images containing a large number of objects of different colors or large gray areas, such as that shown in region “A” indicated in  FIG. 1 . However, if the image contains any large monochrome regions, such as that shown in region “B” indicated in  FIG. 1 , the conventional approach fails. This is the case in many practical situations. Typical examples of such images with a large area having only one color include landscapes in which a significant portion of the image is occupied by either blue sky or green vegetation. Other examples include close-up images of people, wherein flesh tones occupy a significant portion of the image. Yet another example is a non-gray wall serving as a background of the image. 
   In all of the above scenarios, the averages of the color components of the image would not be equal. An adjustment of the gains based on such proportions would not produce a properly white-balanced image. In other words, the conventional approach to white balancing an image does not distinguish between shifts in the spectra of illumination or the presence of large monochromatic regions in the image. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention addresses the problems with the prior art approach by performing a statistical analysis of the image that distinguishes between effects on the color averages due to a change in the spectra of illumination and those due to the presence of large monochromatic regions in the image. 
   Specifically, the method of the present invention subdivides an image frame into a plurality of subframes, and each subframe is analyzed to determine if that subframe is predominantly monochromatic other than gray or white. If so, that subframe is excluded from the computation of the gain adjustments. As a result, the white balance process is performed using only the multicolored and/or gray subframes, thus allowing the overall white-balance of the image to be shifted only when a change in the color average is due to a change in the spectra of illumination, and not a presence of large monochromatic areas in the image. 
   Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  demonstrates an image having large monochromatic areas and to which the method of the present invention is applicable for a white balancing operation; 
       FIG. 2  illustrates the method of the present invention; 
       FIG. 3  is a processing flowchart for illustrating method of the present invention; 
       FIG. 4  is an illustration of an imaging apparatus incorporating a circuit for performing automatic white balance in accordance with the method of the present invention; and 
       FIG. 5  is an illustration of a processing system communicating with an imaging apparatus of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  illustrate a processing method which can be carried out by a microprocessor or a microcontroller of an image sensor. In the method according to the present invention, an image obtained from an imaging device such as an image sensor at processing segment  102  is subdivided into a plurality of subframes as shown in  FIG. 2  and indicated as processing segment  104  in  FIG. 3 . In the exemplary processing method illustrated in  FIG. 3 , the image frame is divided into K subframes, wherein K is the number of subframes. 
   Each subframe is then individually analyzed to determine whether the given subframe includes multicolored or gray areas at processing segments  108  and  110 . At processing segment  106 , before any subframes are analyzed, a counter variable C is set to 0 for keeping track of the number of subframes in the image frame that have been analyzed. 
   Each subframe is analyzed at processing segment  108  by first obtaining the value of hue for each pixel in the subframe using Eqs. 1–10 as follows:
 
 M =max( R,G,B )  (Eq. 1) 1 
 
 m= min( R,G,B )  (Eq. 2) 1 
 
 r= ( M−G )/( M−m )  (Eq. 3) 1 
 
 g= ( M−G )/( M−m )  (Eq. 4) 1 
 
 b= ( M−B )/( M−m )  (Eq. 5) 1 
 
if  R=M  then Hue=60( b−g )  (Eq. 6) 1 
 
if  G=M  then Hue=60(2+ r−b )  (Eq. 7) 1 
 
if  B=M  then Hue=60(4 +g−r )  (Eq. 8) 1 
 
if Hue&gt;360 then Hue=Hue−360  (Eq. 9) 1 
 
if Hue&lt;0 then Hue=Hue+360  (Eq. 10) 1 
 
 1  Reference: Keith Jack, “Chapter  3 , Color Spaces,”  Video Demystified , pp. 50–51 (HighText 2d ed. 1996).
 
   In the equations above, R, G and B are the signal levels for R, G and B, respectively, for each pixel, and M and m determine the highest and lowest values, respectively, of R, G and B. Thus, if R has the highest value, then the hue value of the pixel is calculated using Eq. 6. If G has the highest value, then the hue value of the pixel is calculated using Eq. 7. Similarly, if B has the highest value, then the hue value of the pixel is calculated using Eq. 8. 
   Based on the hue values for each of the pixels, a mean hue is calculated, and then the standard deviation of hue across each subframe is calculated using the difference between each hue value and the mean value as set forth in Eq. 11 below.
 
σ 2   hue =1/ N·Σ (ΔHue i ) 2 ,  (Eq. 11)
 
wherein N is the number of pixels in the subframe, and ΔHue i =Hue i ;−  Hue ; if ΔHue i ;&gt;180 then ΔHue i =360−ΔHue i 
 
   For this calculation, any pixels at the saturation level for any or all of the color components in the image are not considered since they do not convey useful color information for white balance purposes. For example, a white pixel which results from all of the color components being at a saturated level are not considered in the deviance calculation. However, white-appearing pixels which are simply a bright shade of grey are considered in the calculation. 
   Also, it is noted that gray colors are typically grayscale tones composed of very low levels of one or more of the color components. At a pixel level, such gray areas in the image appear as multi-colored areas due to spatial noise, which therefore creates very large deviations in hue values from pixel to pixel. This is very beneficial for the stability of the white balance algorithm, as color gain computation requires the adjustment of gains so that the image averages to gray. 
   The standard deviation obtained in Eq. 11 is then compared against a predetermined threshold H th  at processing segment  110 . Subframes standard deviations of hue less than H th  are considered to be monochromatic and are excluded from the statistics gathering process used to calculate the white balance of the image in processing segment  112 . For Eq. 11, the threshold of comparison H th  to determine if a subframe is monochromatic is approximately in the range of 10 to 40 for 8-bit depths of pixel color, although this number may vary according to the system parameters in the implementation of the invention. 
   Once a subframe has been analyzed at segments  108  and  110 , and a subframe is excluded (segment  112 ) or not from use in a white balance connection, the counter variable is incremented by one at processing segment  114  and then it is determined whether or not the number of subframes which have been processed is equal to K, the total number of subframes, at processing segment  116 . If not, the process returns to segment  108  to analyze the next subframe. 
   After the monochromatic subframes have been identified and excluded from the white balancing process in segment  112 , the white balancing process may be performed in processing segment  118  using the non-excluded subframes in any known method or algorithm that effectuates the adjustment of the balance between the color components in the image in processing segment  118 . For example, color balancing is typically performed by summing each of the values for red, green and blue, respectively, and weighting the sums so that the three components are equal, whereupon each of the individual pixel datum is adjusted by the weighted value for the corresponding color component. 
   Alternative embodiments of the method of the present invention may be used in which the calculation of hue and its standard deviation for each subframe may be approximated with simpler schemes, to simplify hardware implementation. 
   In the embodiment discussed above, Eqs. 6–10 for calculating the hue value of each pixel are based on the commonly used color wheel representation of hue in the visible spectrum, wherein each color, or hue, is expressed as a value within the range of 0° to 360°. In one alternative embodiment of this invention, the range of hue is redefined to have values along a scale of 0 to 96. The hue value determinations of this embodiment are executed at processing segment  108  using the modified Eqs. 12–16 below.
 
if  R=M  then Hue=16( b−g )  (Eq. 12)
 
if  G=M  then Hue=16(2+ r−b )  (Eq. 13)
 
if  B=M  then Hue=16(4+ g−r )  (Eq. 14)
 
if Hue&gt;96 then Hue=Hue−96  (Eq. 15)
 
if Hue&lt;0 then Hue=Hue+96  (Eq. 16)
 
   Although redefining the hue range in this manner results in a coarser evaluation of the hue value for each pixel, it does not modify the general concept behind the present invention, but allows the analysis of processing segment  108  to be performed with 6-bit wide hues and requires only 4 bits for the multiplication by 16 in Eqs. 12–14, instead of the 6 bits needed for multiplication by 60 in Eqs. 6–8. Such modification therefore makes the computations more efficient. 
   In another embodiment of the present invention, the calculation of hue variances in the analysis of the subframes at processing segment  108  uses sums of the absolute value of the differences between the current hue and the mean hue, as set forth in Eq. 17, instead of the squares of the same as employed in Eq. 11.
 
σ 2   hue =1 /N·Σ| ΔHue i |,  (Eq.17)
 
wherein N is the number of pixels in the subframe, and
 
ΔHue i =Hue i ;−  Hue i   ; if ΔHue i ;&gt;48 then ΔHue i =96−ΔHue i 
 
   If σ 2   hue &gt;H th , then all the pixels in the subframe are determined to be monochromatic at processing segment  110 , and hence are excluded from the computation of the average color values of the image. In this embodiment, preferable values for H th  are in the range from 3 to 10 for 8-bit color pixel depth. More preferably, the value of the threshold for determining whether or not the subframes are monochromatic is approximately 5. Again, however, the threshold value for this determination may vary depending on the system parameters of the apparatus. 
   In yet another embodiment of the analysis performed at processing segment  108 , the hue variances are calculated based on the subdivision of the subframes into macropixels (smaller subframes). Each subframe for which variance of hue is to be computed is subdivided into a small number of areas (subsubframes or macropixels). Instead of calculating the hue value for each pixel, the color averages are computed for each macropixel as a whole to obtain a single value of hue for each of the macropixels. In Eqs. 11 and 17, therefore, N represents the number of macropixels and the mean hue is based on the values for the macropixels rather than the pixels. This further reduces the computational complexity of the method of the present invention. 
   In a further embodiment of the analysis performed at segment  108 , the mean value of hue in each subframe is calculated only for the first few lines of the subframe. The estimated mean hue value is then used in either Eq. 11 or Eq. 17 instead of the true mean hue value. This allows for real-time single pass image processing without the need to compute the mean values on a first pass and then the variances on a second pass. In the embodiments described above, it is first necessary to determine the hue for each pixel or macropixel in the subframe in a first pass to determine the mean hue, and then to calculate the difference between the hue of each pixel and the mean in a second pass, to determine the standard deviation. In contrast, in this embodiment, by estimating the mean hue using only the first few lines in the subframe, the standard deviation can be calculated upon determining the hue value for each of the remaining pixels by determining the difference between each value and the estimated mean in the same pass. 
   Since a monochromatic subframe will have approximately the same mean value of hue whether a few lines are considered or the whole subframe is considered, the driving concept behind the invention is still utilized while significantly reducing the amount of calculation necessary for the implementation. As a result, this embodiment of the present invention can be implemented in real-time systems without a frame memory because it operates only on the incoming data stream. 
   In the example shown in  FIG. 2 , each subframe marked with an “X” is determined to be monochromatic, and is hence excluded from the white balancing operation. Subframes having two or more colors, such as the edge regions of the displayed shapes, are not eliminated, as well as the subframes which are indicated as being substantially grey in color. Since the hue variances for the subframes which are substantially grey in color are undefined, they will not be less than the predetermined threshold value for Eqs. 11 and 17 above. However, the three subframes which include a very thin grey region and form the right side of the grey rectangle are eliminated because those subframes are determined to be substantially monochromatic (blue). 
   An example of an imaging apparatus  200  incorporating the features of the present invention discussed above is shown in  FIG. 4 , and includes a lens system  202  for directing light from an object to be imaged to the image sensing unit  204  including an image sensor; an analog-to-digital converter  206  for converting the image signals received at the image sensing unit  204  into digital signals; the image/color processing unit  208  for performing image correction processes including a circuit  100  for performing the automatic white balancing as described above and also for performing other processes such as data correction for defective pixels, color interpolation, sharpness filtering, etc.; an output format conversion/compression unit  210  for converting the image data into an appropriate file format for being outputted or displayed to the user; and a controller  212  for controlling the operations of the entire imaging apparatus  200 . The image sensor in the image sensing unit  204  is preferably constructed as an integrated circuit which includes pixels made of a photosensitive material such as silicon. The image sensor may be formed as a CMOS sensor and combined with a processor, such as a CPU, digital signal processor or microprocessor, in a single integrated circuit. Alternatively, the image sensor in the image sensing unit  204  may be constructed as a charge coupled device (CCD). 
   Without being limiting, such an imaging apparatus  200  could include a computer system, camera system, scanner, machine vision system, vehicle navigation system, video telephone, surveillance system, auto focus system, star tracker system, motion detection system, image stabilization system and data compression system for high-definition television, all of which can utilize the present invention. 
   A typical processor system  400 , shown in  FIG. 5 , such as a computer system, for example, generally comprises a central processing unit (CPU)  444  that communicates with an input/output (I/O) device  446  over a bus  452 . The imaging apparatus  200  communicates with the system over bus  452  or a ported connection. The processor system  400  also includes random access memory (RAM)  448 , and, in the case of a computer system, may include peripheral devices such as a floppy disk drive  454  and a compact disk (CD) ROM drive  456  which also communicate with CPU  444  over the bus  452 . 
   Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.