Abstract:
A method for detecting skin color in a digital image having pixels in an RGB color space generally includes the steps of performing statistical analysis of the digital color image to determine the mean RGB color values; then, if the mean value of any one of the colors is below a predetermined threshold, applying a transformation to the digital image to move skin colors in the image toward a predetermined region of the color space; and employing the transformed space to locate the skin color pixels in the digital color image. More specifically, if the mean value of any one of the colors is below a predetermined threshold, a non-linear transformation is applied to the digital image to move skin colors in the image toward a predetermined region of the color space. Then, depending on the preceding step, either the digital image or the transformed digital image is converted from the RGB space to a generalized RGB space to produce a gRGB digital image; skin color pixels are detected within the gRGB digital image; a first skin color image mask is formed based on the detected skin color pixels; a masked gRGB image is generated using the first skin color image mask; and finally the skin color image mask is employed to locate the skin color pixels in the digital color image.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Reference is made to commonly assigned copending application Ser. No. 09/692,929, entitled “Method For Blond-Hair-Pixel Removal in Image Skin-Color Detection”, in the names of Shoupu Chen and Lawrence A. Ray, and filed on even date herewith. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to digital image processing methods, and more particularly to such methods for detecting skin color in a digital image. 
     BACKGROUND OF THE INVENTION 
     In digital image processing it is often useful to detect the areas in the image that are skin color. This information is used, for example, to adjust the skin colors in the image to be pleasing. The location of skin color is also used in face detection and recognition algorithms, automatic image retrieval algorithms, and red-eye correction algorithms. For instance, U.S. Pat. No. 4,203,671, issued May 20, 1980 to Takahashi et al., discloses a method of detecting skin color in an image by identifying pixels falling into an ellipsoid in red, green, blue color space or within an ellipse in two dimensional color space. The problem with this method is that it works well only when an image is properly balanced. For an over- or under-exposed image, the technique is not reliable. Furthermore, the technique does not work well for those skin colors that deviate from the chosen norm. For example, when the detection method is adjusted to detect light Caucasian skin, it fails to properly detect dark skin. 
     There is a need therefore for an improved skin color detection method that avoids the problems noted above. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method for detecting skin color in a digital image having pixels in an RGB color space generally includes the steps of performing statistical analysis of the digital color image to determine the mean RGB color values; then, if the mean value of any one of the colors is below a predetermined threshold, applying a transformation to the digital image to move skin colors in the image toward a predetermined region of the color space; and employing the transformed space to locate the skin color pixels in the digital color image. 
     More specifically, if the mean value of any one of the colors is below a predetermined threshold, a non-linear transformation is applied to the digital image to move skin colors in the image toward a predetermined region of the color space. Then, depending on the preceding step, either the digital image or the transformed digital image is converted from the RGB space to a generalized RGB space to produce a gRGB digital image; skin color pixels are detected within the gRGB digital image; a first skin color image mask is formed based on the detected skin color pixels; a masked gRGB image is generated using the first skin color image mask; the masked gRGB image is converted to a hue image; possible blond hair color pixels are removed from the hue image to produce a modified hue image; a second skin color image mask is formed based on the skin color pixels in the modified hue image; if the second skin color image mask is smaller than the first skin color image mask by a predetermined amount, then the first skin color image mask is selected, otherwise, the second skin color image mask is selected; and finally the selected skin color image mask is employed to locate the skin color pixels in the digital color image. 
     The advantage of the invention is that it works well even when an image is not properly balanced, and furthermore works well for a variety of skin colors. 
    
    
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an image processing system useful in practicing the present invention. 
     FIG. 2 is a flow chart illustrating the method stages of the present invention. 
     FIG. 3 is a detailed flow chart illustrating the skin color detection step shown in FIG.  2 . 
     FIG. 4 shows an example of an ellipse classifier in two dimensional color space. 
     FIG. 5 shows a two dimensional color space having a plurality of skin color classifier regions, as used in the present invention. 
     FIG. 6 shows a two dimensional color space having skin color and blond hair color regions. 
     FIG. 7 is a schematic diagram describing the process used to train the ellipse classifier stage shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Because image processing algorithms and systems are well known, the present description will be directed in particular to algorithms and systems forming part of, or cooperating more directly with, the method in accordance with the present invention. Other aspects of such algorithms and systems, and hardware and/or software for producing and otherwise processing the image signals involved therewith, not specifically shown or described herein may be selected from such systems, algorithms, components and elements known in the art. In the following description, a preferred embodiment of the present invention will typically be implemented as software or a computer program. Those skilled in the art will readily recognize that the equivalent of such software may also be constructed in hardware. Given the method as described according to the invention in the following materials, software not specifically shown, suggested or described herein that is useful for implementation of the invention is conventional and within the ordinary skill in such arts. 
     Still further, as used herein, the computer program may be stored in a computer readable storage medium, which may comprise, for example; magnetic storage media such as a magnetic disk (such as a floppy disk or a hard drive) or magnetic tape; optical storage media such as an optical disc, optical tape, or machine readable bar code; solid state electronic storage devices such as random access memory (RAM), or read only memory (ROM); or any other physical device or medium employed to store a computer program. 
     Referring first to FIG. 1, an image processing system useful in practicing the present invention includes a color digital image source  10 , such as a film scanner, digital camera, or digital image storage device such as a compact disc drive with a Picture CD. The digital image from the digital image source  10  is provided to an image processor  12 , such as a programmed personal computer, or digital image processing workstation such as a Sun Sparc  20  workstation. The image processor  12  may be connected to a CRT display  14 , and an operator interface such as a keyboard  16  and a mouse  18 . The processed digital images are sent to an output device  20 , such a hard copy printer, a long-term image storage device, a connection to another processor, or an image telecommunication device connected for example to the internet. 
     Referring to FIG. 2, the processing performed according to the present invention by the digital image processor  12  will be described. The digital image, expressed in a device independent RGB color space such as sRGB color space is first rank order filtered in a rank order filter step  22 . Denote the input RGB image by I={C i }, where i=1,2,3 for three color-channels, and C i  is a matrix with elements of C i (m,n), where m=0, . . . M−1 and n=0, . . . N−1. The rank-order filtering is defined as 
     
       
           C   i ( m,n )| C     i     (m,n)ε{haeck over (C)}     t   =median( C   i ( s,t )ε Ĉ   i )| s,mε(0,M−1);n,tε(0,N−1);s≈m;t≈n   (1) 
       
     
     where {haeck over (C)} i  is the set of C i (m,n) whose value exceeds a predefined threshold T Rank  while Ĉ i  is the set of the remaining C i (m,n) of I. This operation is similar to the traditional process of trimmed median filtering. Notice that the purpose of this rank-order filtering is not to improve the visual quality of the input image as traditional image processing does; rather, it is to reduce the influence of the pixels that have very high intensity values on the subsequent statistics gathering stage  24 . For instance, in situations such as over exposure, or back lit exposure, pixels with values close to 255 will be altered or excluded in the mean value calculation. The resultant image of the rank-order filtering can be denoted by I R ={Ĉ 1 , {haeck over (C)} i }, where the elements of Ĉ i  have the original values, and the elements of {haeck over (C)} i  have the values computed using Equation 1. For simplicity, rewrite the rank-order filtered image as I R ={{tilde over (C)} i } where {tilde over (C)} i =Ĉ i ∪{haeck over (C)} i . 
     The next step is to compute the color mean-statistics in a statistics step  24  for each color channel using I R . This computation produces a set of mean values, that is, {m i }=mean(I R ); where m i =mean({tilde over (C)} i ). A pre-processing decision step  26  is made upon evaluating the mean statistics {m i }. If there exits m i &lt;T M , then the pre-processing decision is ‘Yes’, where the threshold T M  is an experimentally determined value; for example, for an 8-bit image, a value of 100 has been found to be acceptable. 
     If the pre-processing decision is ‘Yes’, then the process will go to a non-linear processing step  34 . The input to the non-linear processing step is the original RGB image I. The non-linear processor employed in this invention is color histogram equalization, which is an image processing technique well known to those of skill in this art and thoroughly described in the literature (e.g., see “Digital Image Processing”, by Gonzalez and Woods, Addison-Wesley Publishing Company, 1992). The input image I is first converted to YIQ space to separate the luminance component and chromaticity components using the standard formula:                [         Y           I           Q         ]     =       [         0.299       0.587       0.114           0.569         -   0.274           -   0.322             0.211         -   0.523         0.312         ]          [           C   1               C   2               C   3           ]               (   2   )                                
     The histogram equalization process is applied to Y only, resulting in an equalized luminance Y′. The reverse mapping from the YIQ to RGB is done by                [           C   1   ′               C   2   ′               C   3   ′           ]     =       [         1.000       0.956       0.621           1.000         -   0.273           -   0.647             1.000         -   1.104         1.701         ]          [           Y   ′             I           Q         ]               (   3   )                                
     An important issue in color histogram equalization is saturation clipping. Remember that the equalization result Y′ could cause C′ i  to either exceed 255 or becomes less than 0. To prevent this from happening, a clipping procedure is used on C′ i . The pseudo code for clipping saturation is as follows:                         if                   C   1   ′       &gt;   255     ;       k   1     =     255   /     C   1   ′                
              else                 if                   C   1   ′       &lt;   0     ;       C   1   ′     =   0     ;          
              if                   C   2   ′       &gt;   255     ;       k   2     =     255   /     C   2   ′                
              else                 if                   C   2   ′       &lt;   0     ;       C   2   ′     =   0     ;          
              if                   C   3   ′       &gt;   255     ;       k   3     =     255   /     C   3   ′                
              else                 if                   C   3   ′       &lt;   0     ;       C   3   ′     =   0     ;          
          k   =     min                   (       k   1     ,     k   2     ,     k   3       )              
              C   1   ′     =     k                   C   1   ′         ,       C   2   ′     =     k                   C   2   ′         ,       C   3   ′     =     k                   C   3   ′                   (   4   )                                  
     where k, k 1 , k 2  and k 3  are initialized to zero. 
     If the pre-processing decision is “No”, the original image I is sent to the gRGB conversion step  32 . 
     In summary, the initialization stage  21  sends an image to a skin-color detection stage  35  depending on the pre-processing decision; either the original image I or the histogram-equalized image I′ is sent to the gRGB conversion step  32 . To unify the notation, define I RGB ={C i }, where i=1,2,3 for three color-channels, and C i  is a matrix with elements of C i (m,n) , where m=0, . . . M−1 and n=0, . . . N−1. Both the original image and the histogram-equalized image are denoted by I RGB  from now on. 
     Still referring to FIG. 2, the input RGB image I RGB  is converted in the gRGB Conversion step  32  to a generalized RGB image, I gRGB , which is defined as I gRGB ={c i }, and c i  is a matrix with elements of c i (m,n), where m=0, . . . M−1 and n=0, . . . N−1. The conversion uses the formula:                           c   j          (     m   ,   n     )       =             C   j          (     m   ,   n     )           ∑   i                       C   i          (     m   ,   n     )                           j   ∈   i       =     [     1   ,   2   ,   3     ]         ;          
            m   =     [     0   ,       …                 M     -   1       ]       ;     n   =     [     0   ,       …                 N     -   1       ]                 (   5   )                                  
     where C i  is the individual color channel (R, G, or B) of the input image. This conversion operation is not valid when              ∑   i          C   i       =   0     ,                          
     and the output will be set to zero. The resultant three new elements are linearly dependent, that is              ∑   j          c   j       =   1     ,                          
     so that only two elements are needed to effectively form a new space (gRG plane) that is collapsed from three dimensions to two dimensions. In most cases, c 1  and c 2 , that is, the generalized R and G, are used in skin color analysis and skin color detection. The skin colors in the image I gRGB  are detected in a skin color detection step  30  that receives multiple ellipse vectors from an ellipse classifier  31 . The ellipse classifier  31  is trained on a large population of images beforehand, which will be discussed below with reference to FIG.  7 . The detected skin colors may then be optionally segmented in a segmentation step  28 . 
     The skin-color detection step  30  is shown in more detail in FIG.  3 . The generalized RGB image I gRGB  is first projected on to the gRG plane in the in a projection step  36 . This projection produces a gRG image I gRG . The projection is affected by simply removing the component c 3  from the gRGB image. The gRG image I gRG  with two components c 1  and c 2  is then passed to a first (I) skin color classifier  38  which receives a first ellipse vector (ellipse vector I) as shown in FIG.  3 . FIG. 4 illustrates an example of an ellipse  58  in gRG color space  56  that is used to designate a region in which skin color pixels are located. The dark region  60  in FIG. 4 is the actual skin-color pixel area. Parameters a, b, (o 1 ,o 2 ), and θ are the constants of the ellipse and define the ellipse vector produced by the ellipse classifier  31 . Parameters a and b are the ellipse axes length, (o 1 ,o 2 ) is the center coordinates of the ellipse in the (c 1 ,c 2 ) space  56 , and θ is the angle between the c 1  axis and the longer axis of the ellipse. 
     For the first skin-color classifier  38 , define an intermediate skin-color mask P I ={p I (i,j)} M×N , that is, an M×N matrix with elements p(i,j). For each pixel of the generalized RG image I gRG , define an evaluation function E(c 1 (i,j), c 2 (i,j)), where i=0, . . . M−1; j=0, . . . N−1, c 1 (i,j) ε[0,1]; c 2 (i,j) ε[0,1]. The classification is simply performed as:                  p   I          (     i   ,   j     )       =     {         1           if                   E        (         c   1          (     i   ,   j     )       ,       c   2          (     i   ,   j     )         )         &lt;   1             0       else                   (   6   )                                
     where “1” indicates a skin-color pixel and “0” a non-skin color pixel, and 
     
       
           E ( c   1 ( i,j ), c   2 ( i,j ))=x ij   2   /a   2   +y   ij   2   /b   2   (7) 
       
     
     and 
     
       
           x   ij =( c   1 ( i,j )− o   1 )cos(θ−π/4)−( c   2 ( i,j )− o   2 )sin(θ−π/4) 
       
     
     
       
           y   ij =( c   1 ( i,j )− o   1 )sin(θ−π/4)−( c   2 ( i,j )− o   2 )cos(θ−π/4)  (8) 
       
     
     The intermediate skin-color mask, P I , is passed to an evaluation step  40 . The evaluation procedure simply computes the ratio, γ I , of the number of detected skin pixels to the image size. The evaluation step  40  has two branches. If γ I ≧T γ , that is, the evaluation result is ‘Good’, then the process branches to a skin color region masking step  42 . In this case, the output from the first skin color classification step  38 , P I , is used for masking the I gRGB  in the skin color region masking step  42 . The output of the skin color region masking step is the masked generalized RGB image: Î gRGB =I gRGB ∩P I . If γ I &lt;T γ , then the evaluation result is ‘Not good’, and the process branches to a second skin color detection classification step  44 . While it should be clear that T γ  may take on a range of different values depending on the application, a preferred value for T γ  is 0.12. 
     The structure and operation of the second (II) skin color classification step  44  is the same as that of the first skin color classification step  38  except it employs a different ellipse vector which determines the position, size and orientation of the ellipse in the (c 1 ,c 2 ) space. As shown in FIG. 5, there are several possible ellipses E 1 -E 7  that can be generated by the ellipse classifier  31 . The idea of having multiple ellipses is to reduce false positives while providing a mechanism for detecting skin colors that deviate from a main skin color region, for example E 1  in the (c 1 ,c 2  ) space  62 . As an illustrative case, the use of only two such ellipses (I and II) is described with reference to FIG. 3, but it will be understood that a larger number of skin color classifiers and ellipses can be employed in the present invention. 
     There may be cases in which switching from the main skin color classifier (the first classifier step  38 ) to another classifier results in even fewer skin color pixels being detected. Therefore, the results from the skin color classification steps  38  and  44  are compared in an evaluation and selection step  46 , even though the result from the first classifier  38  was not satisfied in the previous evaluation step  40 . In a more general case, the selection decision is made as following:                     γ   =       max     ∀   i                       (     γ   i     )              
            if                 γ     =     γ   i            
            then                 P     =     P   i               (   9   )                                  
     where P is the first skin-color mask defined as P={p(i,j)} M×N , and P i  is the intermediate skin-color mask from Skin color Classification I, II and so on if more than two classifiers are used. This first skin-color mask, P, will be used in the skin color region masking step  42 . 
     If the result from the main skin color detector (the first classifier  38 ) is satisfied, then P I  will be directly used as the first skin color mask in the subsequent skin color region masking step  42 . If not, then the process branches to another classifier, the second classifier  44  for example, and both the new result and the result from the main classifier (the first classifier  38 ) will be evaluated and the better (or the best) one will be selected as the first skin color mask to be sent to the masking step  42 . 
     Notice that there are feedback paths  39  and  41  for the skin color classifiers  38  and  44  respectively in FIG.  3 . These feedback paths provide the detection result itself to the classifier for evaluation. The evaluation process is to compute the ratio of the number of detected skin pixels to the image size. If the ratio is smaller than a predefined threshold, the classifier ellipse is then expanded, for instance, by 30% or 40% more, to include more colors. 
     There are two outputs generated from the skin color region masking step  42 . One is the masked gRGB image and the other one is the skin color mask P itself. These two outputs will be used in the subsequent optional blond-hair-color pixel removal that is described below. 
     Referring to FIG. 6, we have discovered that blond hair colored pixels  72  occur in the same region where the majority of skin colored pixels  71  reside, but next to the skin color region  70  that belong to the people having blond hairs. Most currently existing skin detection algorithms do not take the blond hair colored pixels into account. This can cause a problem for image understanding applications that are sensitive to the size of the area of skin color, such as redeye detection algorithms which rely on detecting the actual size of the classified skin color region. A technique for removal of blond hair colored pixels is described in the aforementioned copending Ser. No. 09/692,929, entitled “Method of Blond-Hair-Pixel Removal in Image Skin-Color Detection”, which is incorporated herein by reference, and briefly summarized below. 
     It has been shown that difficulties arise when dealing with images having faces associated with blond hairs. In these cases, the conventional skin-color detection process fails to produce satisfied or desired results that would give help in redeye detection procedure. It is desirable to have blond-hair-color pixels removed from the masked skin-color image obtained by the steps described in the previous sections. 
     However, it is not a trivial task to parameterize the sub-regions such as the blond hair color region  72  and the skin color region  70  in the (c 1 ,c 2 ) space  68  so that the hair color can be separated from the face skin color. If the space dimension drops down to one, the separation of blond hair color pixels from the skin color pixels becomes fairly easy. This further reduction of dimension size is realized by converting the masked generalized RGB image Î gRGB  to a hue image H={h(m,n)} M×N  in a converting to hue image step  54 . A typical hue conversion is performed as:                if                   (       c   min     =       min     i   ∈     [     1   ,   2   ,   3     ]              (       c   i          (     m   ,   n     )       )         )       ≠       (       c   max     =       max     i   ∈     [     1   ,   2   ,   3     ]              (       c   i          (     m   ,   n     )       )         )                   do             (   10   )                 if                     c   1          (     m   ,   n     )         =         c   max                   do                   h        (     m   ,   n     )         =       (         c   2          (     m   ,   n     )       -       c   3          (     m   ,   n     )         )     /     (       c   max     -     c   min       )                                   else                 if                     c   2          (     m   ,   n     )         =         c   max                   do                   h        (     m   ,   n     )         =     2   +       (         c   3          (     m   ,   n     )       -       c   1          (     m   ,   n     )         )     /     (       c   max     -     c   min       )                                       else                 if                     c   3          (     m   ,   n     )         =         c   max                   do                   h        (     m   ,   n     )         =     4   +       (         c   1          (     m   ,   n     )       -       c   2          (     m   ,   n     )         )     /     (       c   max     -     c   min       )             ;                               h        (     m   ,   n     )       =       h        (     m   ,   n     )       *   60      °       ;                                 if                   h        (     m   ,   n     )         &lt;     0.0                 do                   h        (     m   ,   n     )           =       h        (     m   ,   n     )       +     360      °         ;                                            
     In a blond-hair color detection step  52 , a predefined partition parameter T H =15 is used to determine if an element h(m,n) is a skin pixel or a blond hair pixel. A second mask, {tilde over (P)}={{tilde over (p)}(i,j)} M×N , is formed. If h(m,n) is a skin pixel, then the corresponding element {tilde over (p)}(m,n)=1, else {tilde over (p)}(m,n)=0. In some cases, the blond-hair-color pixel removal may take away true skin-color pixels and the resultant second skin-color mask shrinks to an unusable small region. Therefore, the first skin-color mask P will be called back and the second the skin-color mask {tilde over (P)} is discarded. This action is performed in an evaluation and selection step  50  following the blond-hair color detection step  52  as shown in FIG.  3 . If the second skin color image mask is smaller than the first skin color mask by a predetermined amount, the first skin color image mask is selected, otherwise, the second skin color image mask is selected. The masked RGB color image Î RGB  is the result of an AND operation of the selected skin color image mask and the original RGB color image I RGB . This operation is performed in a skin color region masking step  48 . 
     Referring to FIG. 7, a large image pool  74  containing over a thousand images from different sources is prepared for sample skin color patch collecting. It is not necessary to collect skin patches from all prepared images. A set of randomly selected images should be representative enough for the whole image pool, in theory and in practice as well. A set of uniformly distributed indices is used to retrieve the images from the image pool and form a set of random selected images  76 . Sample skin color patches  78  are collected manually from the retrieved images. The collected skin color patches are then converted from the RGB space to the gRGB space  80  using Equation (5) above. The gRG data of the gRGB image is then evaluated in term of its distribution density  82  in the gRG plane  84 , that is, the (c 1 ,c 2 ) space as described above. In practice, the distribution density is approximated with the histogram of the gRG data. The distribution density is further projected onto the (c 1 ,c 2  ) space after eliminating some of its elements whose height is less than 5% of the distribution density peak. The projected cluster  88  forms approximately an ellipse region in the gRG plane  86 . The final step of the ellipse classifier training is to find the parameters for the ellipse region. The best-fit ellipse  92  in the gRG plane  90  is computed on the basis of moments. An ellipse is defined by its center (o 1 ,o 2 ), its orientation θ and its minor axis a and major axis b (see FIG.  4 ). The center of the ellipse region is computed by 
     
       
         
           o 
           1 
           =m 
           10 
           /m 
           00 
         
       
     
     
       
           o   2   =m   01   /m   00   (11) 
       
     
     where the moments are computed as:                m   pq     =       ∫     -   ∞     ∞            ∫     -   ∞     ∞            c   1   p                     c   2   q                     f        (       c   1     ,     c   2       )                            c   1                            c   2                     (   12   )                                
     where f (c 1 ,c 2 )=1, in this application. The orientation θ can be computed by determined the least moment of inertia:              θ   =     0.5                 arctan                   (       2                   μ   11           μ   20     -     μ   02         )               (   13   )                                
     where the central moments are calculated as:                μ   pq     =       ∫     -   ∞     ∞            ∫     -   ∞     ∞              (       c   1     -     o   1       )     p                       (       c   2     -     o   2       )     q          f        (       c   1     ,     c   2       )                            c   1                            c   2                     (   14   )                                
     and finally the length of minor and major axis can be computed as:              a   =     1.0623                     (       A   3     /   B     )     0.125               (   15   )               b   =     1.0623                     (       B   3     /   A     )     0.125                               A   =       ∑       (       c   1     ,     c   2       )     ∈   ellipse_region                         [         (       c   1     -     o   1       )                   sin                 θ     -       (       c   2     -     o   2       )                   cos                 θ       ]     2                               B   =       ∑       (       c   1     ,     c   2       )     ∈   ellipse_region                         [         (       c   1     -     o   1       )                   cos                 θ     -       (       c   2     -     o   2       )                   sin                 θ       ]     2                                                
     The above computation provides an initial set of ellipse vector [a, b,o 1 ,o 2 ,θ]94. In practice, manual adjustment is needed to best fit the final ellipse to the gRG data cluster  88 . 
     The subject matter of the present invention relates to digital image understanding technology, which is understood to mean technology that digitally processes a digital image to recognize and thereby assign useful meaning to human understandable objects, attributes or conditions and then to utilize the results obtained in the further processing of the digital image. 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     PARTS LIST 
       10  digital image source 
       12  image processor 
       14  display 
       16  keyboard 
       18  mouse 
       20  output device 
       22  rank-order filter 
       24  color-mean statistics 
       26  preprocessing decision 
       28  segmentation step 
       30  skin color detection step 
       31  ellipse classifier 
       32  sRGB conversion step 
       34  non-linear processing step 
       35  skin color detection stage 
       36  projection step 
       38  first skin color classification step 
       39  feedback path 
       40  evaluation step 
       41  feedback path 
       42  skin color region masking step 
       44  second skin color classification step 
       46  evaluation and selection step 
       48  skin color region masking step 
       50  evaluation and selection step 
       52  blond hair color detection step 
       54  conversion to hue image step 
       56  sRGB color space 
       58  ellipse 
       60  dark region 
       62  sRG (c 1 ,c 2 ) color space 
       68  sRG (c 1 ,c 2 ) color space 
       70  skin color region for blond hair people 
       71  skin colored pixels 
       72  blond hair colored pixels 
       74  image pool 
       76  random selected images 
       78  sample skin color patches 
       80  sRGB space 
       82  distribution density 
       84  gRG plane 
       86  gRG plane 
       88  projected cluster 
       90  gRG plane 
       92  best-fit ellipse 
       94  ellipse vector