Method and apparatus for converting gray scale
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention involves a method and an apparatus for converting any M gray levels in a K-gray scale (halftone) image to M gray levels in an N-gray scale image, where K&gt;N≧M. 
     More particularly, the invention is directed to a method and an apparatus whereby image display output data from a personal computer, or the like, adapted to be displayed on one device, such as a CRT capable of displaying an image with K (e.g., 64)-gray scale levels, may also be displayed on another device, such as a plasma display capable of displaying an N (e.g., 16)-gray scale level image, in such manner that M (e.g., 14) gray levels in the K-gray scale level image are selected and converted to the N-gray scale level image so as to be mutually identifiable. 
     2. Prior Art 
     It is known that a dot matrix image having a multi-level gray scale may be displayed in a halftone representation using a dithering technique in order to convert it for display with another multi-level gray scale. However, with this technique, one-dot gray scale is converted to a multi-dot halftone and it cannot be applied to a one-dot to one-dot conversion. 
     Some personal computers can display a multi-level gray scale representation on a CRT, but they are not adaptable to convert that gray scale representation to another type of multi-level gray scale representation. Also, since the output from a personal computer to a printer is usually a two-level gray scale output, white and black, a gray scale conversion capability from one multi-level gray scale to some other multi-level gray scale is not provided. 
     At present there are a number of different display devices with various gray scale capabilities. For instance, an output device such as a CRT may display an image with, for example, a 64-gray scale representation while another display device, such as a conventional plasma display panel (PDP), can only display an image in, for example, a 16-gray scale. Consequently, it is necessary to convert display data outputted from the CRT controller for a 64-gray scale display to adapt it for display in 16-gray scale; or, in other words, if M (M≦16) gray levels in the 64 gray scale outputted from the CRT controller are converted to ones in a 16-gray scale representation, they can be displayed on the PDP. However, if the 64-gray scale representation is converted to a 16-gray scale representation on a proportional distribution basis, that is, for example, if the conversion to the 16-gray scale representation is carried out by extracting the upper y (e.g., 4) bits of the 64-gray scale x (e.g., 6) bit representation (hereinafter such a conversion is referred to as a &#34;natural conversion&#34;), degeneration of the gray levels can occur wherein some gray levels that are different from each other in the 64-gray scale representation are represented as the same gray level in the 16-gray scale representation. Such degeneration of gray levels presents a problem because gray levels which were originally identifiable become unidentifiable and the image represented may appear quite different on different displays. 
     It is thus desirable, and the object of the present invention, to solve this problem by converting first gray level signals to second gray level signals in such manner that, when each of M gray levels represented in a K-gray scale image is converted to a level represented in an N-gray scale image, the conversion is made without causing the degeneration of the gray levels and in accordance with the relative sequence and magnitude of their brightnesses. 
     SUMMARY OF THE INVENTION 
     In order to achieve the desired object with the invention, a first table is created in which the respective M gray levels of the K-gray scale are mapped to different levels in the N-gray scale, which levels have the relative order and magnitude of their brightnesses in the K-gray scale. Further, a second table is created in which the M gray levels in the N-gray scale are mapped to their corresponding M gray levels in the K-gray scale on a one-to-one basis according to the first table, whereby each of the M gray levels in a K-gray scale representation can be converted to an N-gray scale representation using the second table. 
     The first table is a processing table for making the second table, and in this regard, M gray levels in the K-gray scale are mapped into the N-gray scale having the relative order and magnitude of their brightnesses, so that a conversion table for solving the above problem is obtained by mapping the contents of the first table into the second table in a manner corresponding to the M gray levels in the K-gray scale. 
     If the condition K&gt;N≧M is satisfied, any M gray levels in a K-gray scale can easily be converted to ones in an N-gray scale on a one-to-one basis, using the conversion table, and in addition a gray scale conversion is obtained in which the relative order and magnitude of the brightnesses of those M gray levels are maintained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating an arrangement of components, for processing a display output of a personal computer, incorporating a gray scale converter according to the present invention. 
     FIG. 2 is a schematic representation of the conversion table in the gray scale converter of FIG. 1. 
     FIGS. 3a-3e are schematic representations showing the operational stages of the processing table in the gray scale converter of FIG. 1. 
     FIGS. 4a-4c are flow charts illustrating the algorithm used in creating the conversion table of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an example of an application of a gray scale conversion apparatus in accordance with the present invention. Data outputted from the control unit 10 of a personal computer, or the like, is converted by a CRT controller 20 for display on a CRT (not shown) which is capable of displaying 640×400 dots. This display data specifies gray levels in 64-gray scale with x (e.g.,6) bits representing each of 640×400 dots. The data is converted to analog data by a digital-to-analog converter (DAC) 30 and outputted to the CRT, on which it is displayed in the 64-gray scale. 
     This invention enables the display data output by the controller 20 to be displayed on another output device, for instance, on a plasma display panel (PDP), as well as on the CRT. As previously noted, at present, a conventional PDP can only display each dot in a 16-gray scale, so that, unlike the CRT, it cannot directly display the data outputted from the CRT controller 20 in a 64-gray scale. Consequently, it is necessary to convert the output data to adapt it for display in 16-gray scale. Although the CRT can display in the 64-gray scale, the CRT controller 20 can use only 16 gray levels at most of the 64-gray scale. Therefore, if M (M≦16) gray levels in the 64 gray scale outputted from the CRT controller 20 are converted to ones in a 16-gray scale representation, they can be displayed on the PDP. However, if the 64-gray scale representation is converted to a 16-gray scale representation on a proportional distribution basis, that is, for example, if the conversion to the 16-gray scale representation is carried out by extracting the upper 4 bits of the 64-gray scale representation 6 bits (&#34;natural conversion&#34;), degeneration of the gray levels can occur wherein some gray levels that are different from each other in the 64-gray scale representation are represented as the same gray level in the 16-gray scale representation. Clearly, such degeneration of gray levels is undesirable because gray levels which were originally identifiable become unidentifiable in the converted image. 
     This invention overcomes the above drawback by carrying out a gray scale conversion satisfying the following three conditions: 
     (1) The M gray levels in the K-gray scale, when converted to levels in the N-gray scale, must correspond to each other in the N-gray scale on a one-to-one basis, that is, the original M gray levels must be identifiable also in the N-gray scale; 
     (2) The M gray levels in the K-gray scale, when converted to levels in the N-gray scale, must maintain the relationship of the relative magnitudes of their brightnesses when in the N-gray scale; and 
     (3) The relative brightness of dots in the K-gray scale must be also kept in the N-gray scale; however, if this is in conflict with the conditions in (1) and (2), an optimum gray level in the N-gray scale is to be selected. 
     As seen in FIG. 1 this conversion is implemented by a gray scale conversion apparatus 40, in which the invention is incorporated, that receives the output of the CRT controller 20 and includes a control circuit 42 for creating a conversion table 50 (shown in detail in FIG. 2). The conversion table 50 comprises a table Tin 44, for representing any selected M gray levels in the K-gray scale, and a table Tout 46, for representing gray levels in the N-gray scale corresponding to those M gray levels, and, as will be apparent from the following description, any of the M gray levels in the K-gray scale can be converted to ones in the N-gray scale by using these tables. 
     The algorithm for making such a conversion table will now be described with reference to the processing tables shown in FIGS. 3a-3e and the flow charts shown in FIGS. 4a-4c. 
     (a) Initialization Stage 
     The input representing any of the M gray levels in the K-gray scale is Vin, and the output representing the gray level in the N-gray scale corresponding to the Vin is Vout. By way of example, it will be assumed that K=64, N=16 and M=14. In this case, the Vin is an integer in the range from 0 to 63, the Vout is an integer in the range from 0 to 15, and zero means the lowest brightness or black. 
     Step 1: For the table Tin, M (14) gray levels among the levels in the K-gray scale are selected as the Vin, for example, 0, 1, 2, 20, 21, 22, 23, 42, 43, 46, 60, 61, 62 and 63, and are entered. 
     Step 2: A processing table Tuse 48, as shown in detail in FIG. 3a, is prepared and filled with blanks. The 16 individual positions in the processing table Tuse are indexed by digits from 0 to 15. 
     Step 3: The contents of the table Tout are made blank at first, but the contents will be filled in and completed in the second stage, as described later. 
     (b) First Stage 
     In this stage the relationship of the relative magnitudes of the brightnesses for the M gray levels to be outputted in the N-gray scale are entered in the table Tuse with a check or &#34;o&#34;, completing the table Tuse in the manner as follows: 
     Step 1: Select one out of the respective elements entered in the table Tin, on which element the processes of the following steps 2-4 have not already been performed, and carry out the steps 2-4 for the selected element. When all elements in the Tin have been processed, the second stage is carried out. 
     Step 2: Make the &#34;natural conversion&#34; of Vin for the element selected in step 1, and define it as value Vj. Vju=Vj is assumed to be the initial value. 
     Step 3: Examine the contents of a position in the table Tuse whose index is Vju, and if it is blank, enter a check or &#34;o&#34; and go to Step 1. If a check has already been entered in that position, assume that Vju=Vju+1. In that case, if Vju&lt;N, repeat Step 3. If Vju≧N, go to Step 4 with an initial value of Vjd=Vj-1. 
     Step 4: Examine the contents of a position in the table Tuse whose index is Vjd, and if it is blank, enter a check or &#34;o&#34; and go to Step 1. If a check has already been entered in that position, assume that Vjd=Vjd -1 and repeat Step 4. Because M≦N, the condition of Vjd&lt;0 never occurs. 
     When the first stage is completed, checks have been entered in the table Tuse in the positions corresponding to the M indices. 
     (c) Second Stage 
     This stage is that in which the table Tout in FIG. 2 is completed by: selecting the positions in the table Tuse which have a check entered and correspond to the respective elements in the table Tin, and entering the indices corresponding to the positions into the table Tout. This is carried out in the manner as follows: 
     Step 1: Select from the table Tin an element on which the processes of the second stage have not been performed, and execute the following Steps 2-4. The second stage will be completed with the completion of the processes of the following Steps 2-4 on all the elements in the table Tin. 
     Step 2: Determine where the value Vin of the element selected in Step 1 is in ascending order of all the elements of the table Tin, and define it as value J. J takes a value in the range of [1, M]. 
     Step 3: Assume as Vout the J-th smallest index of the indices for which the checks have been entered in the table Tuse. 
     Step 4: Enter the Vout obtained in the Step 3 in a position of the table Tout corresponding to the Vin, and go to Step 1. 
     In this second stage, the elements of the tables Tin and Tuse have been counted up from the smallest one, but, alternatively, the same result may be obtained if they are counted down from the largest one. 
     Further, in the above-described first stage, when the checks have been entered in the table Tuse, they are examined in the ascending order using Vju as the initial value and, at the time when N-1 is reached, then the checks are examined in the descending order using Vjd as the initial value. However, even if, to the contrary, the checks are first examined in the descending order using Vju as the initial value and are then examined in the ascending order using Vjd =Vj+1 as the initial value when zero is reached it is apparent that a similar result is obtained. 
     The conversion table 50 of FIG. 2 can accordingly be created as described above. Using this conversion table, a signal Vin representing one of the selected M gray levels in a K-gray scale can be converted to a signal Vout representing a gray level in an N-gray scale. This conversion is on a one-to-one basis preserving the relative order and magnitude of the respective brightnesses. 
     Also, it will be seen that it is easy to implement a gray scale converter practicing this invention by means of software according to the above-described algorithm, and it will be within the purview of one skilled in the art to implement this algorithm in hardware by a logic circuit.