Abstract:
A method and apparatus for processing image signals in which image sensors produce an image signal of a specific color and an image signal of the complementary color from a color image of an original by means of filters. A white balancing correction is applied to these image signals by selectively adjusting at least one of the image signals so that the image signals are equal to one another for each picture element along the scanning direction, while a white area of the original or a reference area is being scanned. In a preferred embodiment, a digital signal representative of a specific color is obtained by attenuating the white balanced image signal of the specific color and comparing it to the white balanced image signal of the complementary color, whereby faithful color separation of the specific color is achieved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method and apparatus for processing image signals in a color image reader and particularly to a method and apparatus for separating an image signal of a specific color from the image signal of its complementary color wherein a light signal from a color image is optically split into a light signal of the specific color and a light signal of the complementary color by means of a filter and these two light signals are photoelectrically converted and then compared with each other. 
     2. Description of the Prior Art 
     In general, a conventional color image reader is constructed so that a color signal from a color image is optically split into at least two light signals by means of a filter, a dichroic mirror or the like. Thereafter, these light signals are photoelectrically converted and predetermined processing is done on the image signals which were obtained by the above photoelectrical conversion, whereby the image signal of the specific color is separated, in other words, color separation is attained. 
     Color image signal processors for effecting such color separation have heretofore been of the following types: 
     (1) Image signals obtained by the above photoelectrical conversion are converted to binary values, i.e., digitized by comparing them with respective predetermined threshold values, and an image signal of a specified color is separated by executing logical operations on these binary-valued signals. 
     (2) Image signals obtained by the photoelectrical conversion are subjected to analogue operations to form analogue image signals and these analogue image signals are converted to binary values, i.e., digitized, by comparing them with respective threshold values, and an image signal of the specific color is separated by executing logical operations on these binary-valued signals. 
     In each of the above processors, image signals of a desired color are separated by comparing the image signals with certain threshold values. The comparison operation is disadvantageous in that it is difficult to obtain an image signal of a specific color whose luminance is low. 
     Furthermore, because of the variation in brightness of a light source for irradiating an original as well as the variation in sensitivity of the photoelectric conversion for each picture element, the image signals subjected to color separations are not uniform for all the picture elements so that the quality of the image obtained is deteriorated. 
     Accordingly, it is an object of the present invention to provide a method and apparatus for processing an image signal of simple construction wherein white balancing correction is effected, and at the same time, color separation is attained irrespective of the original&#39;s luminance, whereby a highly reliable image information signal is obtained. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an image signal of a specific color and an image signal of the complementary color are obtained by means of filters, a white balancing correction is applied to these image signals, and digitization is carried out on the basis of the extent of the differences in the levels of both image signals for corresponding picture elements, whereby faithful color separation of the specific color becomes possible. 
     More specifically, according to the present invention, there is proposed a method for processing an image signal in which an image signal of a specific color is separated from a light signal containing image information. The method comprises the steps of splitting said light signal into a first light signal containing image information of the specific color and a second light signal containing image information of the complementary color with respect to said specific color; photoelectrically converting said first and second light signals to first and second electrical signals corresponding respectively thereto; effecting a &#34;white balancing&#34; correction by increasing or decreasing at least one of said electrical signals with the correction determined for each picture element in such a manner that both electrical signals are made equal to each other for each picture element along the main scanning direction; and producing the image signal of said specific color from the relationship between two electrical signals to which said white balancing correction has been applied. 
     In the method according to the present invention, the splitting step may be carried out by the use of either a combination of a semi-transparent mirror with filters, or a dichroic mirror. The white balancing correction step may be carried out by attenuating the first and second signals by a first attenuator circuit and a second attenuator circuit, respectively. The attenuation factor of either one of said first attenuator circuit or said second attenuator circuit is set for each picture element in a scanning line such that said first and second signals are made equal to each other with respect to each picture element. The producing step may be carried out as follows: a signal corresponding to said specific color is selected from two electrical signals which have been subjected to said white balancing correction and is attenuated by a predetermined attenuation factor; the signal thus attenuated is compared with the signal corresponding to the color complementary to said specific color thereby separating and producing the image signal of said specific color. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a schematic view illustrating a method for obtaining image signals of a specific color and its complementary color; 
     FIG. 2 is a block diagram illustrating an embodiment of a color image signal separation unit according to the present invention; 
     FIGS. 3-5 are graphical representations showing examples of image signals; 
     FIGS. 6(a)-6(e) are graphical representations showing a method of obtaining a digital representation of a color-separated image information signal; 
     FIG. 7 is a block diagram illustrating the embodiment of an image signal separation unit according to the present invention shown in FIG. 2 being used to obtain an image signal of a complementary color; 
     FIG. 8 is a block diagram illustrating another embodiment of an image signal separation unit according to the present invention; 
     FIGS. 9(a)-9(e) are graphical representations showing a method of obtaining a digital representation of a color-separated image information signal of a complementary color; and 
     FIGS. 10(a)-10(h) are graphical representations showing a method of obtaining digital representations of a color-separated image information signal of a specific color and a complementary color. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, light emitted from a light source 1 and reflected at picture elements on an original 2 is passed through a lens 3, and is divided into two beams by means of a semi-transparent mirror M 1  and a mirror M 2 . Then, these beams are directed to image sensors I 1  and I 2  through filters F 1  and F 2  for the specific color and the complementary color, and are photoelectrically converted in the image sensors I 1  and I 2  to obtain image signals IS 1  and IS 2 , respectively. The image sensors, I 1  and I 2  may be linear multi-element devices, the image signals IS 1  and IS 2  being signals corresponding to a line of picture elements. 
     Referring to FIG. 2, the image signal IS 1  of the specific color and the image signal IS 2  of the complementary color are applied to a color image signal separation unit wherein a white balancing correction is effected and a precise color separation signal with respect to the specific color not influenced by fluctuations in luminance is produced. 
     First, the white balancing correction will be described. 
     Assume that there are n picture elements along one scanning line. When a white portion of the original is scanned, the image signal IS 1  is, for example, as shown in FIG. 3, while the image signal IS 2  is, for example, as shown in FIG. 4. Variations in level of the image signals IS 1  and IS 2  in respect of each picture element are caused by unevenness of brightness of the light source 1, variation in the characteristics of the optical systems, and the nonuniformity of sensitivity of the image sensors I 1  and I 2 . 
     In the present invention a white balancing correction is performed by making both image signals IS 1  and IS 2  coincide with each other, i.e., equal for each picture element. 
     Referring again to FIG. 2, if the attenuation factor of an attenuator A 1  is represented by α 1 , the factor α 1  is selected so that the relationship of IS 1  ×α 1  &lt;IS 2  is established for each picture element. 
     When the signal IS 1  ×α 1  is superimposed on the image signal IS 2 , the graphical representation shown in FIG. 5 is obtained. In FIG. 5, the shaded portion represents the difference between these signals. From FIG. 5, it is apparent that both signals can be made to coincide by attenuating the image signal IS 2  by an amount corresponding to the shaded portion for each picture element or bit. Assuming that the level of IS 1  ×α 1  for the ith bit is ai, and the level of IS 2  is bi (i=1 to n), both image signals can be made to coincide by attenuating the respective bits of IS 2  by the ratio of ai/bi=β i . 
     Since the attenuator A 1  provides a constant attenuation factor α 1  with respect to all the bits, an attenuator of the usual type may be employed. 
     The attenuator A 2  is of a type whose attenuation factor differs for each bit or picture element, and therefore a programmable attenuator whose attenuation factor can be varied in accordance with a digital signal is utilized. For instance, if the attenuation factor of the attenuator A 2  is controlled by a digital signal of 8 bits, 2 8  (=256) attenuator levels can be produced. 
     During a period when the blank space (white part) of the original is being scanned by the optical system, the digital signals for determining attenuation factors with respect to each picture element are determined and stored in a random access memory RAM. The digital signals are decided beginning with the most significant bit. The manner in which the digital signals are determined will be described hereinbelow. 
     Returning to FIG. 2, the random access memory RAM is initially cleared, and an ON signal (e.g., 1) is added to the most significant bit from a control unit CU. In this case, the ON signal is a signal for turning a switching element SW in the programmable attenuator A 2  on, while an OFF signal is one for turning the switching element SW off. The output of the attenuator A 2  becomes IS 2  /2 by the above ON signal, and the output IS 2  /2 is compared with IS 1  ×α 1  in a comparator C 1 . Based on such comparison, either an ON signal (1) or an OFF signal (0) is stored in the most significant bit b 7  of the random access memory RAM when IS 2  /2&gt;IS 1  α 1  or IS 2  /2&lt;IS 1  α 1 , respectively. Such operation as described above is executed for all the picture elements of the blank space along the scanning line. 
     Next, the contents which have been stored in the random access memory RAM as mentioned above are read out, and at the same time, an ON signal (1) is added to the next lower bit from the control unit CU to perform the same operation as that described above, whereby ON/OFF signals are stored in the RAM. 
     In this same manner, all the bits for the digital signals for making both image signal IS, and IS 2  coincide for all of the white picture elements can be determined and stored. 
     In the case where the digital signals are determined beginning with the most significant bit for all the picture elements of one scanning line, as in the example as described above, it is required to scan the entire blank space along the scanning line eight times. However, it may, of course, be possible to determine and store all bits of the digital signal for each picture element at one time by slowing down the scanning speed. 
     Although the above description has been made with reference to the case where white portions of the original are scanned, the present invention is not limited to the above case, but may be carried out by scanning a white body separately provided in the apparatus prior to the scanning of the original. 
     Furthermore, while the above description is for the case where the ratio between IS 1  ×α 1  and IS 2  (IS 1  ×α 1  /IS 2  =β i ) is within the range of 0 to 1, the ratio β i  is generally around 0.5 to 0.6, so that the ratio β i  may be represented by 256 steps within a range of 0.5 to 1. 
     Now the manner of digitizing will be described hereinbelow. 
     In the digitization method according to the present invention, comparisons of each image signal with respective threshold values are not made. Instead, the variation in the difference of the levels of the image signals of the specific color and the complementary color is examined. For this purpose, the attenuation of the image signal IS 1  of the specific color is significantly increased, i.e., the attenuation factor α 2  is significantly decreased (for example, α 2  =0.1α 1  to 0.2α 1 ). The attenuator A 2  reads out RAM synchronously with the image signal of the complementary color and attenuates it with an attenuation factor β i  for each picture element. 
     Assuming now that the specific color is, for example, red, light beams are separated by means of a red filter and a cyan filter (cyan is the complementary color of red), and, respectively, projected on to the image sensors I 1  and I 2 . The change in the image signal of the complementary color with respect to red picture elements is remarkable while the change in the image signal of the specific color with respect to the red picture elements is small. 
     Referring to FIG. 2, the attenuation A 1  is switched from &#34;white&#34; to &#34;color&#34;, switching an attenuation of α 2  /α 1  in series with the attenuation α 1 , changing the attenuation factor of A 1  from α 1  to α 2 . By comparing IS 1  ×α 2  with IS 2  ×β i  for each picture element, a signal, IO, having a certain nonzero level appears at the output of the comparator C 1  only in the case where the picture elements are red. 
     FIGS. 6(a) and (b), respectively, show the output of the attenuator A 1  when the attenuation factor is α 1  and when the attenuation factor is α 2 . FIG. 6(c) shows the output of the attenuator A 2 . FIG. 6(d) shows curves obtained by overlapping the curve in FIG. 6(b) with that of FIG. 6(c). FIG. 6(e) shows an image information signal IO from the comparator C 1  which has a certain nonzero level only when IS 1  ×α 2  is greater than IS 2  ×β i , i.e., only in the case of red picture elements while having a zero level in the case of other than red picture elements. Even if the luminance of the red color in the original changes, the relative relationship between IS 1  ×α 2  and IS 2  ×β i  does not change, so correct color discrimination can be achieved. 
     Although the above description has been with reference to the case where the specific color is red (whose complementary color is cyan), the invention is also applicable to the other specific colors such as blue (whose complementary color is yellow), and green (whose complementary color is magenta). 
     If the relationship between the specific color and its complementary color is reversed, that is the picture elements are of the complementary color and the image signal of the complementary color is applied to the attenuator A 1  while the image signal of the specific color is applied to the attenuator A 2 , as shown in FIG. 7, a color-separated image signal of the complementary color can similarly be obtained. 
     FIGS. 9(A)-9(e) are graphical representations, like those shown in FIGS. 6(a)-6(e) with respect to FIG. 2, for the situation illustrated in FIG. 7 in which a color-separated image signal of an original containing the complementary color is obtained. 
     Referring to FIG. 8, when a color-separated signal is intended to be obtained for the complementary color as well as for the specific color, the signal IS 2  ×β i  is further attenuated by a factor of α 3  by means of an attenuator A 3 . The attenuation factor α 3  is preferably around 0.1-0.2. In comparator C 2 , IS 1  ×α 1  is compared with IS 2  ×β i  ×α 3 . In this case, for picture elements of the complementary color, the image signal of the complementary color does not change remarkably with respect to the white color part, while the image signal for the specific color changes significantly so that the difference between both image signals is clearly detected by comparator C 2 , to thereby obtain a color-separated signal of the complementary color. 
     FIGS. 10(a) and (b), repsectively, show the output of the attenuator A 1  when the attenuation factor is α 1  and when the attenuation factor is α 2 . FIG. 10(c) shows the output of the attentuator A 2 . FIG. 10(f) shows the output of the attenuator A 3 . FIG. 10(d) shows curves obtained by overlapping the curve in FIG. 10(b) with that of FIG. 10(c) while FIG. 10(g) shows curves obtained by overlapping the curve in FIG. 10(a) with that of FIG. 10(f). FIG. 10(e) shows the output of the comparator C 1  which has a certain non-zero level only when IS 1  ×α 2  is greater than IS 2  ×β i , i.e., only in the case of red picture elements, while having a zero level in the case of other than red picture elements. Similarly, FIG. 10(h) shows an image information signal from the comparator C 2  which has a certain non-zero level only when IS 2  × β i  ×α 3  is greater than IS 1  ×α 1 , i.e., only in the case of cyan picture elements.