JPS63220672A - Compression-encoding method for half tone picture signal - Google Patents

Abstract

PURPOSE:To easily encode a dither image and with a high compressing ratio as much as possible by adding and encoding '1' to run-length value at the time of encoding and subtracting '1' from the decoded run-length value at the time of decoding and restoring a binary picture signal. CONSTITUTION:A run-length counting device 2 counts the continuous number of white or black dots from the binary signal of each one dot read successively from a line memory 1, and gives the result to an adder 3. The adder 3 adds '1' to the run-length of a white or black dot given from a run-length counting device 2 and gives the result to an MH encoder 4. An MH decoder 5 converts the inputted code to a run-length value and gives the result to a subtracter 6. The subtracter 6 subtracts '1' from the run-length value given from the MH decoder 5, and gives the result to a picture signal reproducing device 7. Thus, by using an one-dimensional encoding method, a high compressing ratio can be attained with a comparatively easy constitution.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a method for further compressing and encoding a binary image signal obtained by converting an image expressed in halftones, such as a photograph, into a dither pattern. Regarding.

[Prior art]

2. Can express halftones in a pseudo manner in order to send and receive halftone images such as photographs using facsimile devices.
A dither method is known as a value conversion method. In this dithering method, for example, when reading a document image at 16 levels, a threshold value of 16 levels from 0 to 15 is set for each dot of a Bayer type 4 x 4 dot dither matrix as shown in Figure 1. The 4×4 dots of the read 16-gradation image signal are converted into a binary image signal using each of these darkness values.

For example, as shown in FIG. 2, the density is 5.5 in 16 gradations.
If a halftone image in the range of 7, 8.6 is to be read, if it is binarized using the dark value of the dither pattern shown in Fig. 1 above, a binary image signal as shown in Fig. 3 will be obtained. It will be done. In the example shown in FIG. 3, for example, on the mth line, one black dot and one white dot are repeated one after another.

The binary image signal obtained in this way is converted into a conventional encoding method in which the amount of data is compressed by representing the number of successive identical white or black image signals, that is, each length value, with a unique code. For example, when using the Ml+code table shown in Table 1, "010" + "000111"
Therefore, for the repetition of 1゛ and °0° of the original binary image signal, for a combination of one black dot and one white dot, after encoding from 2 bits of the original image signal, The amount of data increases to 9 bits, that is, 7 bits.

Table 1: As shown in Table 1, the properties of dithered images are as follows: run lengths are subdivided, and there is almost no correlation between the preceding and succeeding lines.
If a one-dimensional encoding method such as the l+ encoding method is applied, the amount of data will increase as described above, and the VR
When a two-dimensional compression encoding method (eg, MRiJj encoding method) that compresses the amount of compressed code data based on the correlation between adjacent lines is applied, the amount of data may increase significantly.

In view of these circumstances, various solutions have already been proposed.

For example, taking advantage of the fact that in a dithered image, there is a high probability that a certain Pi1 pattern will appear repeatedly in the - dimension direction due to the periodicity of its dark values, the length (length, ``PatternLengLh Cod
in(4” (IEICE Technical Report IE80-42) l or the value of the current pixel is predicted from the values of surrounding pixels that have already been encoded based on the 7th turn of the dither dark value and is encoded. [Ml+coding of R narrow-scale intertone images (IEICE Technical Report IE80-42) J, etc. have been proposed.

[Problem that the invention seeks to solve]

However, in the first method described above, codes for specifying patterns are required in addition to the pattern length, and as the number of patterns increases, the number of codes for specifying them also increases. Therefore, it is difficult to expect a compression ratio that high overall.

Further, in the second method, a considerably high compression ratio is achieved, but the configuration of the circuit device for prediction may become complicated.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a compression encoding method for halftone image signals that can encode dithered images as simply as possible and at a high compression rate. .

[Means for solving problems]

In the present invention, in the Ml+ encoding method used as a normal one-dimensional encoding method, the run length value ゛1゛
Focusing on the fact that the code representing 2゛ has a smaller amount of data (number of bits) than the code representing ``2゛'' in both cases, we added 1 to the actually obtained run length value during encoding. '' is added for encoding, and during decoding, 1° is subtracted from the run length value obtained by decoding to restore the binary image signal.

The present invention converts a binary image signal obtained by dithering a halftone image signal into a dither pattern when the run length value representing the number of consecutive white or black signals is 2'' than when the run length value is I''. In the compression encoding method that encodes with a large number of bits, when encoding, 1゛ is added to the run length value, and when decoding, 1゛ is added from the decoded run length value. The method is characterized in that a binary image signal is restored by subtraction.

[Effect]

In the present invention, since 1° is added to the run length value "1" that often appears in dithered binary image signals, the actual run length value "1" is expressed by a shorter number of pips. It is encoded as the run length value ゛2゛.

[Principle of the invention]

First, the principle of the present invention will be explained below.

Table 1 mentioned above is a code table used for the l encoding method, which is a general one-dimensional encoding method. For run length values of black dots: °2゛ and 3”
are the minimum number of bits, respectively.

On the other hand, when a dithered image is binarized, if the run length value is 1 for both white and black dots,
In other words, white and black dots often appear repeatedly and alternately. However, the sign for the run length value °1 of the white dot is “000111°, which is 6
This is the data amount of 3 bits, and the sign for the run length value '1' of the black dot is 010°, which is the data amount of 3 bits.

From the above, when converting the code of the run length value '1' that frequently appears in both white and black dots, the angle 1°
If you add up to 2 degrees, the run-length value will be converted to the code with the minimum number of bits in both white and black cases, resulting in a run-length value of "2 degrees," so a significant amount of data can be compressed. .

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below based on drawings showing embodiments thereof.

FIG. 4 is a block diagram showing a circuit configuration for implementing the method of the present invention, that is, encoding and decoding processing of halftone images based on the above-mentioned principle. There is.

First, the configuration for encoding processing will be explained.

1 in the figure is a line memory, which is read at 16 gradations by an image reading device (for example, a scanner, etc.) not shown, and is a dithered image at the darkness value shown in the 4×4 dot dither matrix in FIG. The converted binary dither image signals are sequentially input line by line. The dither image signal inputted to the line memory 1 in units of one line is once stored in the line memory 1, and then sequentially read out to the run length counter 2 as a signal of one dot, that is, one bit of 1" or 0". be done.

A run length counter 2 counts the number of consecutive white or black dots, that is, a run length, from each one-dot binary image signal sequentially read out from the line memory 1, and supplies the count to an adder 3.

Adder 3 adds 1" to the run length of the white or black dot given from run length counter 2, and
give to

The Ml encoder 4 encodes the run length value given from the adder 3 and outputs it according to the relationship between the run length value and the code for each of the white and black dots in the MH code table shown in Table 1 above.

Therefore, a halftone image obtained by converting a 16-tone digital image into a binary dither image signal is stored in the line memory in units of one line.
After being temporarily held, the dots are sequentially read out to the run length counter 2 dot by dot, and the run length values of white or black dots are counted. Then, the run length value obtained by the run length counter 2 is given to the adder 3 and added by °1", and the run length value after this °1" addition is M
l+ is applied to encoder 4 and encoded.

Therefore, the run length value ゛1゛ of white or black dots that frequently appear in the binary dither image signal is converted to °2° by the adder 3, so the code output from the MH encoder 4 is originally If so, '000111° (for white dots) or 010' (for black dots) is 01.
! 1" or °11', and is compressed by 2 bits or 1 bit.

Note that when the run length value counted by the run length counter 2 is 0 or more than 2 degrees, the adder 3 adds 1 degree, and the MH encoder 4 corresponds to the result.
encoded by

Next, the configuration for decoding will be explained.

In the figure, reference numeral 5 denotes an MH decoder, into which a code signal from another facsimile apparatus received by a receiving circuit (not shown) or the like is input. The H1 summer coder 5 converts the input code into a run length value according to the MH code table shown in Table 1 above, and supplies it to the subtracter 6.

The subtracter 6 subtracts 1' from the run length value given from the Ml decoder 5, and supplies the result to both signal processors 7.

The image signal regenerator 7 generates a bit signal 'O° representing a white or black dot according to the run length value given from the subtracter 6.
Alternatively, '1' is sequentially output to the line memory 8.

The line memory 8 sequentially stores the bit signals given from the image signal regenerator 7, and when the bit signals reach one line, outputs them to, for example, a printer (not shown).

Therefore, the code of the run-length value to which 1° has been added as described above is subtracted by 1° from the run-length value decoded by the Mll decoder 5, and then the image signal is reproduced as before. Since the image signal is reproduced into a binary image signal by the unit 7, the original image signal is reproduced.

In the above embodiment, an example is shown in which the original image is converted into a digital image signal with 16 gradations and this is dithered, but the present invention is similarly applicable to cases other than 16 gradations.

Furthermore, in the above embodiment, the run-length value '1' is encoded as 2'; however, in the inter-encoding method, the code corresponding to the run-length value '2' is either white or black. Therefore, for example, if the code corresponding to the run-length value '3' is the minimum pinot number, or if the code corresponding to the run-length value '2' is the minimum number of bits.
If the number of bits is less than the number of bits encoded by ``, it is sufficient to add 2゛ to the run length value ゛1゛ of the original image signal and encode it, and furthermore, the run length value corresponds to ゛4°4°. Via 1-
If it is a number, the number obtained by subtracting 1 from the run length value may be added to the run length value of the original image signal for encoding.

〔effect〕

As described above, according to the present invention, it is possible to achieve a high compression rate with a relatively simple configuration using the conventionally commonly used one-dimensional encoding method (M11 encoding method). become.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing an example of a 4x4 matrix indicating the threshold value of a dither pattern, Figure 2 is a schematic diagram showing an example of a halftone image, and Figure 3 is a schematic diagram showing an example of a halftone image. FIG. 4 is a schematic diagram showing an example of an image signal binarized using the dark value of the dither pattern shown, and FIG. 4 is a block diagram showing a circuit configuration for use in implementing the method of the present invention.

Claims (1)

[Claims] 1. A binary image signal obtained by dithering a halftone image signal from a case where the run length value representing the number of consecutive white or black image signals is '1'. In a compression encoding method that encodes with a large number of bits, the method in the case of '2' encodes by adding '1' to the run length value, and when decoding, the decoded run length value is A compression encoding method for a halftone image signal, characterized in that a binary image signal is restored by subtracting '1' from a length value.