JPS63275274A - Compression method for dot picture data - Google Patents

Abstract

PURPOSE:To efficiently compress dot picture data at high speed so as to transmit it by using data in which a same theoretical value continues from the leading end and/or the tailing end of dot data which can be obtained by means of displacement as the number of units which are counted at every prescribed unit. CONSTITUTION:An area unit which is to be compressed and coded is set to integer multiplied dot area and threshold matrixes in respective picture elements are displaced in a picture element unit in accordance with a prescribed reference. Data in which the same theoretical value continues from the leading end and/or the tailing end of dot data which can be obtained by the displacement are counted at every prescribed unit and they are displayed as the number of units from the leading end and/or the tailing end, whereby data excepting said data are used as they are. Thus, a high regulating ratio can be obtained and as to hardware, they can be address-converted, data-converted and coded in a comparator, whereby high speed compression can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to a compression method for efficiently compressing halftone image data.

(Technical Background and Problems to be Solved) Compression technology for pseudo-halftone images that express halftones using binary data is used to transmit halftone images and newspaper image data by facsimile. This is because image data is enormous, and it is inefficient to transfer the data as it is or store it in memory. Facsimile data transmission uses compression technology to efficiently transmit data, and conventionally the MR method (one-dimensional compression method; Mo
defined lluffman Coding) and M
R method (two-dimensional compression method; Modified-READ)
is being used. However, applying the Mll method and the MR method to pseudo halftone images requires rearranging the image data depending on the thickness and depth of the dither matrix threshold value to generate longer runs and efficiently transmit character images. It is not easy to select a data rearrangement method that reduces small runs of about 1 to 3 bits, and this method is inappropriate for data compression of halftone images.

There is also a predictive encoding method that considers the position of the current pixel in the dither matrix as well as the states of the preceding m reference pixels, but increasing the number of m states makes it difficult to implement into an actual device. Even if the above-mentioned Mll method and MR method were applied as they were, there was a drawback that the compression ratio was poor.

(Object of the invention) This invention was made due to the above-mentioned circumstances,
SUMMARY OF THE INVENTION An object of the present invention is to provide a method for compressing halftone image data for efficiently compressing and transmitting halftone image data at high speed.

(Means for Solving the Problems) The present invention provides a network that compares an image signal with n threshold values (an integer of 3 or more) per pixel and generates digital halftone data of n bits per pixel. This invention relates to a method for compressing point image data, and the above object of the present invention is to compress and encode the area unit to be an integer multiple of the halftone area, and to set the threshold matrix for each pixel in accordance with a predetermined standard in the pixel unit. This is achieved by rearranging the halftone dot data obtained by rearranging and using the data in which the same logical value continues from the beginning and/or the end as the number of units counted for each predetermined unit. Further, the area unit to be compressed and encoded is an integral multiple of the halftone dot area, and the threshold matrix for each pixel is rearranged in the pixel unit according to a predetermined standard, and then the threshold matrix in each pixel is sorting in a predetermined order, and unitizing the bit-wise halftone dot data obtained by the sorting into units of bits that are integral multiples of n, and forming units in which the same logical value continues from the beginning and end of the bit-wise halftone dot data. This can also be achieved by finding two group parts, shortening the intermediate data between the two group parts according to a code table, and using this shortened data as halftone dot data.

(Operation of the invention) In this invention, the threshold matrix for converting human image data into halftone data is rearranged in advance according to a predetermined standard, and the logical value is 1'° from the beginning or end of the halftone data.
Or, by outputting 0°° continuously, and by unitizing the halftone data in predetermined bit units and counting logical values or continuous units from the beginning and end, the halftone image can be efficiently generated. Data compression has been achieved. Furthermore, by rearranging the unitized bit-by-bit threshold matrices in a predetermined order, the intermediate halftone dot data is shortened according to the coat table, thereby further improving the degree of compression. That is, in the present invention, compression of halftone image data is realized by utilizing redundancy based on spatial correlation of images and correlation of halftone dot patterns.

(Embodiment of the Invention) Fig. 1 shows an outline of the method of the invention according to the flow of data, in which image data expressed in 8 bits is halftone dotted by a threshold value matrix (dither matrix), and the halftone The dot data DD is compressed 3 and stored in memory 4, and then the data is expanded 5 to create 4-bit halftone dot data 6.
It is said that In this case, the present invention uses a complete restoration method in which the halftone data DD and the expanded data ED are exactly the same.

Figure 2 shows human image data 1 and threshold matrix 10.
This shows how halftone dot data DD is formed by The image data is compared with four threshold values to generate binary halftone dot data DD of 4 bits per pixel. °゛1°° when higher than the value
(black) and "0" (white) when it is low, and each bit threshold of the threshold value matrix IO is unrelated to the level value of the pixel and depends only on the coordinates of the pixel.

Here, the threshold matrix data CDI in FIG.
As shown in , the size of the encoding block BL is twice the halftone dot data CDI, that is, 2 halftone units, and 1
An encoding method is used in which each pixel is 4 bits, such as PL. Therefore, since the size of the encoded block BL is 2 halftone units, the image has 100 pixels/block. Then, as shown in FIG. 4, image data 1 is binarized for each block using a threshold matrix IO to obtain halftone dot data DD2 as shown in FIG. Next, the halftone image data (4 bits) in the block BL are rearranged in descending order of the four threshold values for each pixel of the threshold matrix IO. These data rearrangements are all fixed types determined by the threshold value matrix lO. In other words, since the threshold value for each bit of the threshold matrix 10 is fixed as shown in FIG. By rearranging the image data, all the image data is rearranged in order of increasing (or decreasing) threshold value as shown in FIG.

Then, the rearranged 400-bit halftone data DD3 is encoded every 8 bits. In this case, the sorted 400
The bit halftone data DD3 is scanned in units of 8 bits from the beginning, and the number of units in units of 8 bits in which all 8 bits are followed by 0°' is determined. This group part is called the white part W, and the length of the white part W is "O゛°~"50'', so 6
It can be encoded in bits. Next, the rearranged 4
The halftone dot data DD3 of 00 bits is scanned in units of 8 bits from the end, and the number of units in units of 8 bits that continue from 8 bits 1 to 1° is determined. This group part is Kurobe B
Since the length of this black part B is also from 0 to 50', it can be encoded with 6 bits. The same is true if the beginning position of part B is the number of units from the beginning of the black part. In addition, the data in the intermediate part other than the white part W and the black part B of the rearranged 400-bit halftone data DO3 is referred to as intermediate data, and this intermediate data is divided into 8-bit units, and as a result of the above rearrangement, 2 as shown in court table 0 to 3 in Figure 7.
There are only five types of patterns, all of which can be fixedly encoded with a 5-bit encoding code. In other words, 8-bit intermediate data can be compressed into 5-bit data. Generally, when .degree. C. is an integer, this encoding is performed in units of .rho..times.n bits, resulting in a (n+1)' type of pattern, which can be replaced by x bits such that 2X-'<(nil)'≦2X. Furthermore, by combining the method of expressing the white part W and the black part B of the rearranged 400-bit halftone dot data DD3 by the number of units (that is, encoding the B part and W part with the above 6 bits), it becomes more efficient. data compression can be performed. When I tried compressing actual image data, I was able to obtain a compression ratio of approximately 177.5.

Furthermore, in the examples of coat tables 1 to 3 in FIG.
01.11010.11011,11100,1110
By assigning data as shown in Figure 8 to 1,11110°11111", we were able to obtain a compression ratio of approximately 178.6! In other words, as long as the above 2x-(nil) is not 0, , (2X-(n+1)) types of codes can be assigned to the specific pattern in the middle part as the part B symbol.

In the above embodiment, the halftone pixel data (4 bits) in the block BL is sorted by the average value of the 4-bit threshold values for each pixel in the threshold matrix IO.
The 4-bit threshold values may be sorted in descending order of minimum value, or may be arranged in descending order of maximum value. Also, when the threshold value of the threshold matrix lO is equal to or higher than 128° (intermediate value), the halftone pixel data (4 bits) in the block is divided into 4 bits for each pixel of the threshold matrix lO. Sort the bits in descending order of the maximum threshold value, and when the threshold matrix value is less than '127', sort them in descending order of the minimum value of the 4-bit threshold value for each pixel in the threshold matrix. Also good. Also, in the above, the block is 45°
2M4-dot units are used, but it can also be applied to any halftone dot unit, and data compression in the intermediate area is not necessarily necessary. Furthermore, the above-mentioned '1'',
The data of "Ooo" may be reversed, and the logical conversion of °°1°" and °"o" with respect to the threshold position is also arbitrary.

(Effects of the Invention) As described above, according to the data compression method of the present invention, a high compression rate can be obtained, and even with regard to hardware, encoding can be performed using address conversion, data conversion, and a comparator, so high-speed compression can be achieved. be able to.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the method of this invention according to the data flow, FIG. 2 is a diagram showing the relationship between human image data of this invention and halftone data, and FIG. 3 is an example of halftone data of this invention. Figures 4 to 6 are diagrams for explaining the method of this invention, Figure 7 is a diagram showing a data compression code table used in this invention, and Figure 8 is an extended encoding code table.
, zo. 1... Image data, 2... Halftone conversion, 3... Compression,
4... Memory, 5... Extension, lO... Threshold matrix. Applicant's agent Yuzo Yasugata 3rd figure sub, 1-1- / DD2 5th box, 6th figure

Claims (8)

[Claims] (1) In a method of comparing an image signal with n threshold values (an integer of 3 or more) per pixel to generate n-bit digital halftone data per pixel, the unit of area to be compressed and encoded is a halftone dot. In addition to making the area an integral multiple, the threshold matrix for each pixel is rearranged in pixel units according to a predetermined standard, and the same logical value continues from the beginning and/or end of the halftone dot data obtained by this rearrangement. A method for compressing halftone image data, characterized in that data is counted in units of predetermined units and displayed as the number of units from the beginning and/or from the end, and data other than the above is used as is. (2) The method for compressing halftone image data according to claim 1, wherein the predetermined standard is an average value of n-bit values of each pixel. (3) The method for compressing halftone image data according to claim 1, wherein the predetermined criterion is the maximum value of n-bit values of each pixel. (4) The method for compressing halftone image data according to claim 1, wherein the predetermined criterion is the minimum value of n-bit values of each pixel. (5) The method for compressing halftone image data according to claim 1, wherein the unit unit is a bit unit that is an integral multiple of the n. (6) If the predetermined standard is such that the average value of the n-bit values of each pixel is larger than the standard value, the order is in the order of the maximum value of the n-bit values, and the average value is larger than the standard value. 2. The method for compressing halftone image data according to claim 1, wherein if the value is smaller than the minimum value of the n-bit value. (7) In a method of comparing an image signal with n threshold values (an integer of 3 or more) per pixel to generate n-bit digital halftone data per pixel, the area unit to be compressed and encoded is a halftone dot. The area is an integral multiple of the area, and the threshold matrix for each pixel is rearranged in pixel units according to a predetermined standard, and then the threshold matrix within each pixel is rearranged in a predetermined order, and the threshold matrix is obtained by the rearrangement. The bit-wise halftone data is divided into units of bits that are an integer multiple of n, and two group parts of the units in which the same logical value continues from the beginning and end of the bit-wise halftone data are obtained, and the two group parts are obtained. A method for compressing halftone image data, characterized in that intermediate data between group parts is shortened according to a code table, and the shortened data is used as halftone data. (8) A method for compressing halftone image data according to claim 7, wherein a look-up table is used as the code table.