Abstract:
There is disclosed an apparatus for efficient and faithful encoding of color image data, by dividing color image data into blocks, extracting luminosity information, structure information and color information from each block, and for each block, individually encoding or individually quantizing the color image the structure information and the luminosity information. The signal system may be one employing signals of low mutual correlation, such as an L*a*b* signal system. An orthogonal transformation of the L* components of the color image data for the respective blocks may be performed.

Description:
This application is a continuation of application Ser. No. 066,119 filed June 25, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a color image data encoding apparatus, for encoding color image data obtained by a color original reader or a color television camera. 
     2. Related Background Art 
     In storage or transmission of image information, it has been customary to reduce the redundancy by suitable encoding in consideration of the efficiency. Such encoding has been principally developed for binary black-and-white or color image information. 
     However, there have been recently developed finer image information with multiple levels, including multi-level color image information. 
     Consequently, as encoding process has become necessary also for such multi-level color image information. So far, such encoding has been made by applying the conventional black-and-white encoding process to each of three primary colors of red, green and blue, by digitization utilizing correlation of three primary colors in each pixel. 
     However the former process mentioned above is naturally inefficient and often causes errors in color registration. The latter process is relatively free from errors in color registration, but cannot be efficient because it strongly relies on the correlation of the primary colors. 
     On the other hand, in certain equipment such as facsimile, electronic file or copying machine, it is often required to effect a conversion of luminosity or color on the data transmitted or stored in a buffer memory or the like. In such case, a conversion on the encoded data is desirable in consideration of efficiency. However the conventional encoding processes do not allow direct conversion of the encoded color image data. 
     For example in a facsimile system, such requirement has been resolved by sending the information on such conversion from the receiver to the transmitter, which in response resends the data after such conversion. Such solution naturally requires an increased time and complicates the mutual communication between the transmitter and the receiver. Also in an electronic file or the like, in which such solution is not available, the encoded data have to be decoded before the required conversion can be made. Such process results in deterioration of image quality due to the repeated processes and requires an additional working memory, so that the advantage of employing encoded data cannot be fully exploited. 
     SUMMARY OF THE INVENTION 
     In consideration of the foregoing, an object of the present invention is to avoid the drawbacks in the above-explained conventional technologies and to achieve compression encoding of color image data with a high efficiency and with satisfactory color reproducibility. 
     Another object of the present invention is to provide a color image data encoding apparatus capable of achieving a high-speed processing and minimizing the deterioration of the image quality. 
     Still another object of the present invention is to provide a color image data converting and processing apparatus capable of a luminosity conversion or a color conversion of encoded color image data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a data format after encoding; 
     FIGS. 2A, 2B and 2C are views showing the mode of block definition and signal conversion; 
     FIG. 3 is a block diagram of an encoding circuit embodying the present invention; 
     FIG. 4A is a block diagram of an L*a*b* converting unit; 
     FIG. 4B is a block diagram of an orthogonal converting unit; 
     FIGS. 5 and 6 are views of portions of data processing in luminosity conversion and color conversion; 
     FIGS. 7 and 8 are block diagrams of circuits for luminosity conversion and decoding; 
     FIG. 9 is a chart showing an example of luminosity conversion; 
     FIG. 10 is a view showing the structure of color information C; 
     FIGS. 11A and 11B are charts showing the relationship between hue θ and saturation h; 
     FIGS. 12 and 13 are block diagrams of circuits for color conversion and decoding; and 
     FIGS. 14A and 14B are charts showing examples of color data conversion. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now the present invention will be clarified in detail by preferred embodiments thereof shown in the attached drawings. 
     In the following there will be explained the outline of color image data encoding according to the present invention. At first three primary colors or red (R), green (G) and blue (B) representing a full-color image are converted into signals of weaker mutual correlation allowing separation into luminosity information and color information, and said signals are divided into small blocks. Then said signals are encoded, for each block, into a format containing luminosity information indicating the brightness in said block, structure information indicating for example an edge present in said block, and color information indicating color in said block. 
     FIG. 1 shows an example of the basic concept of said color image data encoding. The signals R, G, B are converted into L*a*b* signal of CIE 1976 uniform color space, as an example of a signal system with reduced mutual correlation, and the information in each small block is encoded into a format consisting of three elements of luminosity (L), structure (S) and color (C). 
     FIG. 2A illustrates the mode of conversion from the RGB signal system to the L*a*b* signal system in an image, and of definition of square blocks of 4×4 pixels each as an example of the small block. On an original image 201, there are formed blocks 202 of 4×4 pixels each from a corner of the original image. 203 indicates a block containing an image edge therein. 
     FIG. 2B illustrates the distribution of color image data of three primary colors R, G, B constituting the block 203 in case the original image contains a red letter. The edge appears only in the red image. 
     FIG. 2C illustrates signals L*, a*, b* obtained by conversion from the R, G, B signals shown in FIG. 2B. X 11  -X 44  represent components of the signal L* in the block. 
     The conversion from the RGB signal system to the L*a*b* signal system is achieved according to the following equations: ##EQU1## wherein Xr, Xg, Xb, Yr, Yg, Yb, Zr, Zg and Zb are constants. 
     From these equations there are derived: ##EQU2## wherein X 0 , Y 0  and Z 0  are values corresponding to reference white light. 
     FIG. 3 shows an embodiment of the circuit for achieving the encoding shown in FIG. 1. A 4-line buffer 301 is used for temporarily storing the R, G and B signals entered line by line for example from a color scanner, for the purpose of defining the aforementioned 4×4 blocks. The signals of 4 lines stored in said 4-line buffer 301 are read in a size of 4×4 pixels to define a block of 4×4 pixels to define a block of 4×4 pixels. An L*a*b* converting unit 302 performs the conversion from the RGB system to the L*a*B* system for each pixel according to the above-explained converting equations. 
     FIG. 4A shows an example of said L*a*b* converting unit 302, wherein the conversion is achieved by a look up table process utilizing table memories 401, 402, 403, storing conversion table corresponding to the above-mentioned equations, accessible by the R, G, B signals. In this manner the R, G, B signals are converted, in each pixel, into the L*, a*, b* signals or reduced mutual correlation. An L* signal 303 is released, in the order of X 11 , X 12 , . . . , X 21 , . . . , X 44  in the L* block shown in FIG. 2C, from the L*a*b* conversion unit 302. An orthogonal conversion unit 304 performs orthogonal conversion of the L* signal, for example by Hadamard conversion or scattered COS conversion. Said orthogonal conversion is conducted for each block for extracting edge forms contained in the block, thereby improving the efficiency of quantization to be conducted afterwards. 
     In the following there is shown a second-order Hadamard Conversion, as an example of the orthogonal conversion: ##EQU3## wherein: 
     X: original m×n matrix 
     H: Hadamard matrix 
     H T  : inverted matrix of H 
     Y: m×n matrix after conversion. 
     Adopting the structure of L* shown in FIG. 2C for X: ##EQU4## the above-explained equation can be written as follows in the present embodiment wherein m=n=4: 
     
         Y.sub.16 =1/4×H.sub.16 X.sub.16 
    
     wherein 
     X 16  =[X 11 , X 12 , . . . , X 22 , . . . , X 44  ] T   
     Y 16  =[Y 11 , Y 12 , . . . , Y 22 , . . . , Y 44  ] T   
     H 16  :16×16 Hadamard matrix 
     so that ##EQU5## 
     FIG. 4B shows an example of the circuit employing an orthogonal conversion unit 304 for Hadamard conversion, wherein provided are an Hadamard matrix address generator 410 for generating addresses in the row direction in the matrix calculation; circuits 411, 412, 413 for calculating Y 11  from X ij  in the foregoing equation, wherein 411 is a look-up table for multiplying the input values X ij  with coefficients of the Hadamard matrix, 412 is an adder for cumulatively adding values sequentially output of the look-up table 411, and 413 is a 1/4 divider for multiplying the output of the adder 412 with 1/4; and circuits 415-417 for similarly obtaining Y 44  from X ij . There are in fact provided 16 sets of circuits for obtaining Y 11  -Y 14 , Y 21  -Y 24 , Y 31-Y   34  and Y 41-Y   44  from X ij  to respectively execute following 16 calculations: ##EQU6## 
     In FIG. 3, a signal 305 corresponds to an output Y 11  of the orthogonal conversion unit 304, representing a DC component close to the average value of Y ij  of the block, and being a coefficient indicating the luminosity or light level of the block. A quantizer 307 quantizes said value Y 11  of 10 bits into 8 bits to obtain light information L 308. 
     Other fifteen coefficients Y 12  -Y 44  (306) represent the edge structure present in the block and are encoded by a digitizer 309 into 12-bit structure information S 310 corresponding to one of 4096 patterns determined in advance. Thus the structure information 310 represents the form of the edge contained in each block. 
     Averaging circuits 311, 312, composed of adders and dividers, respectively calculate average values a*, b* of the signals a*, b* for each block, supplied by the L*a*b* conversion unit 302. 
     A quantizer 313 collectively quantizes the block average values of a* and b* into a 12-bit signal, thus providing the color information C 314 of each block. 
     The digitizer 307, 309, 313 can be effectively composed of vector digitizers. 
     A multiplexer 315 collects the luminosity (L) information 308, structure (S) information 310 and color (C) information 314, obtained in the above-explained manner, into a code 316, as shown in FIG. 1, for each block. 
     In this manner the R, G, B signals supplied for example from a color scanner are converted, for each block of a predetermined size, into the L*, a*, b* signals of low mutual correlation, and the color image of each block is represented by the luminosity information, structure information and color information, based on said signals. 
     In reproducing the color image by decoding the above-explained code, each area defined by the edge, defined by the structure information, in each block is painted with a color represented by the luminosity information and the color information. In this manner the color original image can be satisfactorily reproduced. 
     In the present embodiment the R, G, B signals are converted into the L*, a*, b* signals, but there may also be employed a L*a*υ* signal system, a YIQ signal system for NTSC format, a PAL signal system or a YUV signal system. 
     Also for the orthogonal conversion there may be employed a scattered COS conversion or a slant conversion instead of the Hadamard conversion. 
     Furthermore, the quantizers are not limited to the aforementioned vector quantizers, and the bit allocations of the L, S and C information are not limited to the foregoing explanation. 
     Furthermore the input signals are not limited to red, green and blue but may be composed of yellow (Y), magenta (M) and cyan (C), for example, according to the image sensor employed. 
     Also the values a*, b* may be preserved in more precise manner, instead of the average value thereof. 
     As explained in the foregoing, this encoding process allows efficient encoding, as the original color image data are converted into signals of lower mutual correlation, then said converted signals are separated into three elements respectively representing the luminosity, structure and color, and said three components are independently encoded as shown in FIG. 1. 
     If a conversion is required on the color image data encoded as shown in FIG. 1, there may be conducted a conversion on the light information only as shown in FIG. 5 if the conversion on the luminosity only is required, or a conversion on the color information only as shown in FIG. 6 if the conversion on the color only is required. 
     In the following there will be explained the conversion in luminosity and in color, in the above-explained encoding process. 
     FIG. 7 shows a circuit for achieving a conversion in luminosity. 
     In FIG. 7 there are shown a transmission channel or a memory 320; a demultiplexer 321 for performing a function inverse to that of the multiplexer 315 shown in FIG. 3, for parallel conversion of the luminosity, structure and color information of the serial encoded data shown in FIG. 1; a conversion unit 322 for light or luminosity conversion; a 8-bit input signal 323 to said conversion unit 322 for controlling the level of said conversion. FIG. 8 shows an example of the circuit when said conversion unit 322 is composed of a look-up table (LUT). The signal L is converted, by said luminosity conversion, into a signal L&#39;. FIG. 9 shows an example of conversion of the 8-bit luminosity signal, in a range from 0 to 255. Thus there can be made variations of 256 levels, in which a value 128 is a default value corresponding to no variation. The control signal 323 may be manually varied by the operator through keys or levers provided on an operation unit, or automatically varied for example according to an output equipment. 
     In FIG. 7 there are further shown an inverse orthogonal converter 325, which can be composed of a hardware same as that of the orthogonal converter 304 shown in FIG. 3 and employed in encoding; a decoding unit 326 provided with a look-up table for decoding the structure information S; an output signal 327 thereof; decoding units 328, 329 provided with look-up tables for respectively decoding the color information a*, b*; decoding L*, a* and b* signals 330, 331, 332; decoding units 333, 335, 337 respectively for decoding the R&#39;, G&#39; and B&#39; signals; and output lines 334, 336, 338 for the decoded R&#39;, G&#39; and B&#39; signals. In this manner the conversion of the luminosity can be achieved in a simple manner. 
     FIG. 10 indicates the data format of the color information C, which is represented by hue θ and saturation h in an a*-b* space. FIG. 11A shows the relationship between the hue θ and the saturation h. FIG. 11B shows the mode of division on the a*-b* space, wherein a lattice point, as indicated by 501, is selected as a representative color by the quantizing, circuit 313 shown in FIG. 3. 
     FIG. 12 shows an embodiment of a conversion-decoding circuit for effecting a color conversion combined with decoding. Said circuit is different from the luminosity converting circuit shown in FIG. 7, in that the luminosity conversion unit 322 is replaced by a color data conversion unit 100. A signal instructing the mode of color data conversion is supplied to said color data conversion unit 100 through a color conversion control line 101. Said color conversion provides converted color information C&#39;. FIG. 13 shows an example of the circuit in which the color data conversion unit 100 is composed of a look-up table. 
     The color conversion is conducted as shown in FIGS. 14A and 14B. In the 8-bit control signal, the upper 4 bits define the amount of variation in the hue, while the lower 4 bits define the amount of variation in the saturation h. Said signal 101 may be varied by the operator or according to the output equipment. 
     As explained in the foregoing, the color image data separated into three elements respectively representing luminosity, structure and color can be easily subjected to a conversion process of color information or luminosity information in encoded state. 
     Arbitrary conversion on luminosity and color information is rendered possible by employing both the luminosity conversion unit 322 shown in FIG. 7 and the color conversion unit 100 shown in FIG. 12. 
     Though the present invention has been explained by preferred embodiments thereof, it is not limited to such embodiments but is subject to various modifications and variations within the scope and spirit of the appended claims.