JPS61169086A - Encoding system of half tone image - Google Patents

Abstract

PURPOSE:To attain redundancy-suppressed encoding with high efficiency by area- dividing the image according to its content, and allocating an independent code to respective area, thereby transmitting with priority in order the information high in necessity. CONSTITUTION:An input picture signal is stored in a memory 101. In an memory 104, the said signal is compared with the values in the memory area of the same block in which the said signal is to be stored, and the maximum value of each block is renewed depending on its result. In an memory 105, the minimum value of each block is likewise renewed. An comparing circuit 107 compares the difference (x) be tween the said max. and min. values with a prescribed value P1 or P2(P1<P2). If x<P1, a low tonal difference area is decided on, if P1<=x<=P2, an intermediate tonal difference area is decided on, and if P2<=x, a high tonal difference area is decided on, and the result is stored in an memory 108. A signal conversion circuit 109 performs degeneracy processing of the picture signal in the memory 101 as utilizing the area signal in the memory 108. A border detecting circuit 110 decides respective status S1-S3 based on the border in the corelation between reference scanning line and encoding scanning line, of which result is stored in a memory 114, and at the same time, by the said result, an encoding signal is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Description of the field to which the invention pertains The present invention relates to a halftone transmission system in which an image signal or a color image signal having one tone information is encoded with redundancy suppression and transmitted.

(2) Description of the Prior Art Digitally transmitting an image signal such as a facsimile while maintaining halftones requires a long transmission time. For example, since 4 bits are required to represent 16 gray levels, the transmission time in this case is approximately four times that in the case of black and white binary. Furthermore, storing two image symbols also requires a large storage capacity for the same reason. Therefore, it is desirable to shorten transmission time and reduce storage capacity by suppressing the redundancy of one image symbol.

Images with halftones are divided into multiple areas, considering the quality of the image signal, such as areas with gradual or rapid changes in density.The information to be transmitted is not necessarily the same for each area. Transmitting unnecessary information increases the amount of code, lengthens the transmission time, and requires a large storage memory capacity.

On the other hand, if you omit the necessary information without transmitting it.

This will lead to deterioration in image quality.

As an example of the conventional method, as described in "Japanese Patent Application No. 31932/1984", the boundaries of the quantized gradation levels on the encoded scanning line are different from those of the reference scanning line (usually the previous scanning line). There is a method that encodes and transmits how it changes.

This method can achieve a high redundancy suppression effect in areas where there are few changes in gradation levels that represent shading, but in areas where density changes are large, there are many boundaries between one gradation level, so redundancy suppression is significant. No effect will be obtained.

(3) Purpose of the invention The present invention divides two images into regions according to their contents.

It is characterized by assigning an independent code to each individual area, and its purpose is to achieve more efficient redundancy reduction coding by preferentially transmitting the most necessary information for each area. It's about doing.

(4) Explanation of the structure and operation of the invention According to past studies by the applicants, good image quality can be obtained even if the number of gradation levels that can be expressed is small in areas with large degree changes; It is known that for regions with small density changes, a gradation level of at least 16 gradations or more is required.

The present invention utilizes this property to divide five screens into blocks each consisting of multiple pixels, and when the density difference within each block is small, the number of gradation levels that can be transmitted is large, and when the density difference is large, the number of gradation levels that can be transmitted is large. Preprocessing for signal conversion is performed for each block so that the number of possible gradation levels is reduced.

(5) Examples The drawings will be explained in detail below.

FIG. 2 shows nine example images for explaining the method of the present invention, in which each square separated by a broken line represents each pixel.

Numbers indicate gradation levels. The gradation is white at 0. It is expressed in 16 levels, with black being 15. Also, the large squares 1, 2, 3...10 surrounded by solid lines are.

Each block is shown.

In the image in Figure 2, when the difference in gradation levels within a block is 0 to 3, it is a low gradation difference area, when it is 4 to 7, it is a medium gradation difference area, and when it is 8 to 15, it is a high gradation difference area. Then, blocks 4.8.9 correspond to high tone difference areas, block 3.5 corresponds to medium tone difference areas, and other blocks correspond to low tone difference areas.

Since blocks 4, 8, and 9 have a large difference in one gradation level, the number of gradations to be expressed in two may be small. Therefore, the gradation levels 0 to 3 are degenerated to 0, the levels 4 to 7 to 5, the levels 8 to 11 to 10, and the levels 12 to 15 to 15. In the middle gradation area, levels 0 to 1 are set to O, levels 2 to 3 are set to 2, levels 4 to 5 are set to 4, etc., 12
The level ~13 is degenerated to 12, and the level 14-15 is degenerated to 15.

As a result, the gradation level of each pixel is changed as shown in FIG.

FIG. 4 shows boundaries between pixels having nine different gradation levels after the gradation levels have been replaced as described above.

That is, the blocks 3.4, 5, 8.9, etc. shown in Fig. 3 have 16 levels per gradation level in the low gradation difference area and 8 levels in the middle gradation difference area compared to the blocks in Fig. 2. , the number of tone reproductions is reduced in blocks with a large one tone difference, such as 4 levels in a high tone difference area.

As a result, the boundaries between pixels having different gradation levels shown in FIG. 4 are smaller than in the case where the gradation levels are the same as shown in FIG. 2.

In order to suppress the redundancy of image information, it is sufficient to encode information representing each boundary position shown by broken lines in FIG. 4 and the gradation level of each area divided by each boundary. As this means, for example, various known methods can be applied.

As an example, the sequential processing method described in Japanese Patent Application No. 54-31932 will be explained. In this case, the correlation between the reference scanning line B, whose image signal is known in advance, and the encoded scanning line A for one image is as follows:
Depending on the boundary state, it is classified into the following three states.

(1) The first state (hereinafter referred to as S1 state) is on the reference scanning line with respect to the boundary Q on the encoding scanning line.

■The boundary to the left of Q and closest to Q, or the boundary to the right of Q and closest to Q (Q on the reference scan line)
If there is a boundary directly above the boundary, then this boundary).

(2) The gradation level on a predetermined side of the boundary (here, the left side) is the same as the gradation level on the same side of the boundary Q on the encoded scanning line.

■A state in which there is no boundary on the encoded scanning line between the boundary and the boundary Q (although the condition of ■ is not necessarily required,
Notes are added for ease of understanding).

In this case, information indicating that it is in the Sl state, Q and P (
For Q, only the relative distance (X+) of the boundary on the reference scanning line that satisfies the above condition needs to be given. It is convenient to display 1111 by the number of pixels (run length) between P and Q. Also, when Q is to the left of P, i
<0. When it is on the right, it is set as 1, >Q (this may be determined in the opposite way).

Hereinafter, this information will be indicated as S+ (1+). This situation occurs when the same gray level is connected from the encoded scan line to the reference scan line. Therefore, the boundaries of gradation levels are also connected.

FIG. 5 shows the upper two scanning lines of FIG. 4, with the first scanning line being the reference scanning line B and the second scanning line
When encoding the th scanline.

Pi Gh, Pi Gh, Pa Qa, Ps −
Qb + P? The boundary represented by -Qt is the SI state, and 1. are -1, 0, 0, 0, 0, respectively.

(2) Second state a (hereinafter referred to as S2 state) A state in which there is no boundary on the encoded scanning line that satisfies the first state with respect to the boundary P on the reference scanning line.

This condition occurs when the gray level on the reference scan line B is not connected to the encoded scan line A.

In the example of FIG. 5, the boundary P l + P 1' represents the S2 state.

Note that in the +St state, it is sufficient to provide only the information that the S2 state exists.

(3) Third state (hereinafter referred to as S state) A state in which there is no boundary on the reference scanning line that satisfies the first state with respect to the boundary Q on the encoded scanning line.

In this case, information indicating that the status is +53.

It is sufficient to provide information indicating the position of Q and the gradation level m of the area to the left of Q (each expressed as 13.rn).

l, can be expressed as a run length from a known boundary, but in the example below, the immediately preceding 81 on the encoded scan line
Alternatively, it is expressed as a run length from the boundary position in the S3 state or the boundary position in the immediately preceding St state.

m may be given in a format that does not confuse the nine gradation levels with other codes.

In the example of FIG. 5, IQa and Q6' are in the S state and +
1'3 is "is"7 when expressed as a run length from the immediately previous state (S, state Qs).

The result of sequentially encoding the encoded scanning line A in FIG. 5 from left to right is expressed in symbols.

sz, sz, 5l(-1), 5l(0), 5l(0
).

s+(o), 53(7,0)+5s(2,2),3
It becomes 1 (0). That is, after the code for identifying the SI, St, and Ss states, information indicating the boundary position S may be added for the SI*33 state, and information providing the gradation level may be added for the SI*33 state, and the codes may be sequentially encoded.

To transmit or store this information digitally, binary codes of "1" and "0" are typically used.

Table 1 shows an example of the code structure in this case.

In general image signals, l is often small in the +Sl state, so short codes are assigned to these.

In Table 1, +SI state 11,1 is 3 or less, but if IIl+1>3, this can be expressed as S, state.

In FIG. 6, the encoded scanning line A of FIG.
The result of encoding is shown below. The third scanning line is the second scanning line,
The third scanning line may be encoded in the same manner by sequentially referring to the second scanning line.

An example of a circuit configuration for realizing the above functions is shown in FIGS. 1 and 7.

FIG. 1 shows an example of the configuration of an encoding circuit, in which 101 is an image signal memory corresponding to 4 scanning lines, and 102 is a comparison output circuit that compares two numbers and outputs the larger one.

103 is a comparison output circuit that outputs the smaller one, 104 is a maximum value memory, 105 is a minimum value memory, 106 is an arithmetic circuit, 107 is a comparison circuit, 108 is an area information memory, 109
110 is a signal conversion circuit, 110 is a boundary detection circuit, 111 is a counter, 112 is a code generation circuit, 113 and 114 are boundary information memories, and 115 is a switching circuit.

To operate this, first, image signals (representing gradation levels) inputted in the order of scanning lines are manually input to the two-image signal memory 101 and simultaneously inputted to the comparison output circuits 102 and 103.

A comparison output circuit 102 receives an input image signal.

Compare the signals output from the maximum value memory 104.

The larger one is written into the maximum value memory 104 as new data.

The maximum value memory 104 has a memory area for each block of the 9-picture signal, and compares the values stored in the memory area of the same block for both input signals and sequentially selects the larger value for each block. Rewrite it as the maximum value. At the stage when the comparison of the last image signal of the block of the fourth scanning line is completed.

The maximum value memory 104 stores the maximum value of the image signal in that block.

Similarly, the comparison output circuit 103 compares the input image signal with the signal output from the minimum value memory 105 and writes the smaller one as new data to the minimum value memory 105, so that the fourth scanning stripe When the comparison of the last image signal of the block is completed, the minimum value memory 105 has the following information:
The minimum value within that block is stored.

When processing of one block is completed, the maximum value memory 104 and minimum value memory 105 are reset.

Memorize the value of the next block.

The arithmetic circuit 106 stores a maximum value memory 1 for each block.
The difference is obtained from the output of 04 and the output of minimum value memory 105.

The comparison circuit 107 sets the difference X between the maximum value and the minimum value to a predetermined value P+
, Pz (P+ <Pz), X<P,
Then, if +PI≦x<P, it is determined to be a middle tone difference area, and if P2≦X, it is determined to be a high tone difference area, and this result is stored in the area information memory 108. Note that when dividing into two areas, a low tone difference area and a high tone difference area, only one P is required.

If the X and P, , P are compared in parallel at the time when the image signal of the fourth scanning line is being input, then when the image signal of the fourth scanning line is stored in the image signal memory 101, each block The region determination result is stored in the region information memory 108.

Next, when image signals are taken out in the order of scanning lines from the single image signal memory 101, the empty memory area is filled with the following information.

The image signal of the next input scanning line is stored), and the signal conversion circuit 109 degenerates to 8 gradations or 4 gradations as described above according to the area signal stored in the area information memory 108. Perform processing. This completes the image signal conversion for performing redundancy suppression processing.

Next, this image signal is input to the counter 111.

A boundary detection circuit 110 detects a boundary where one gradation level changes while counting run lengths in which the same gradation level continues.
Enter.

When a boundary is detected by the boundary detection circuit 110, a signal indicating boundary detection, a counter output, and a new gradation level after the boundary are input to the code generation circuit 112.
Counter 111 is reset.

When the boundary is detected, the code generation circuit 112.

The state is compared with the reference scanning line information stored in the boundary information memory 113 to determine which state is s, s, , Sl. Further, the result is stored in the boundary information memory 114, and an encoded signal is generated and output.

When the encoding for one scanning line is completed, the switching circuit 115 causes the boundary area memory 11 to
3 and 114, and the content of the boundary information memory 114 is referred to as reference scanning line information during the encoding process of the 9th scan line, and the boundary information of the new encoded scan line is stored in the boundary information memory 113.

FIG. 7 shows an example of the configuration of a decoding circuit for restoring an original picture signal from a signal encoded in this manner.

In Fig. 7, 121, 122, 123 are S
+, St, Ss boundary state detection circuit; 124 is an O count circuit for determining l when in S state; 125 is a level detection circuit for detecting level m in S3 state;
126 is an image signal control circuit, 127 and 128 are boundary information memories, 129 is a switching circuit for the boundary information memories 127 and 128, and 130 is an image signal generation circuit.

To operate this decoding circuit, first, encoded signals are input to the boundary state detection circuits 121, 122, and 123.

If a signal representing the S1 state is received, the fact that it is in the S state and the difference 11 are determined as the S1 state or S.

If the status is received, this fact is transmitted to the 9-picture signal control circuit 126.
Enter.

S. When receiving the status reception notification, the 9-picture signal control circuit 12
6 stops the operation of the boundary state detection circuits 121, 122, and 123, counts the continuously sent "O" sequence, learns the value of jl, and receives the subsequently sent code m. Know the tone levels.

Upon receiving the information of 221m, the boundary state detection circuit 12
1, 122, and 123 are operated to receive the primary boundary state. The image signal control circuit 126 is.

When receiving any of the S+, St, and Ss status signals, the content of the image signal on the encoded scanning line is known by comparing the content with the boundary information of the reference scanning line stored in the boundary information memory 127.

Based on this, the boundary information memory 128 is stored as the boundary information of the current scanning line, and the 9-pixel signal generation circuit 13
0, a decoded original image signal is generated.

When the decoding for one scanning line is completed, the switching circuit 129 causes the boundary information memory 127 to
and 128 and when encoding the primary scanning line,
The content of the boundary information memory 128 is referred to as reference scanning line information, and the boundary information of the encoded scanning line is stored in the boundary information memory 128.
Memorize to 7.

By the way, there are cases where a large number of pixels with different density levels exist near the boundary where two gradation levels change due to noise when reading the original. As a countermeasure for this, when the gradation level difference within a block is small and less than a predetermined value,
Alternatively, when the gradation levels of a predetermined number N or more pixels have the same value, the gradation level of each pixel in the block is unified to the gradation level close to the average value in the block, and the gradation level in the block is Just make it uniform.

In addition, in the above explanation, when converting the image signal shown in FIG. 2 to the image signal shown in FIG. Of course it's a good thing.

In the above description, the case where the number of gradations is 16 has been explained, but the present invention can also be applied to cases with other numbers of gradations by adopting a code configuration suitable for the number of gradations. Further, although the case where the block size is 4×4 and 16 pixels has been described, it goes without saying that any configuration such as 4×8, 8×8, etc. can be adopted.

In addition, in the explanations above, we have explained the case of image information that does not include nine color information, but color images also have information for each individual color component of R, G, B or Y, I, Q, or only for a specific color component. If this method is applied to, highly efficient transmission of color images becomes possible.

Note that the code structure shown in Table 1, which encodes boundary information at positions where five gradation levels change, shares part of the code structure with the modified read (MR) code, which is a standard coding method for black and white binary. can be converted into For example, in Table 1, S1 state, S2 state
By making the S and S3 states correspond to the vertical mode, pass mode, and horizontal mode of the MR code, it is possible to share a part of the code table and processing circuit, which is specially required for the nine-line encoding system. Fewer circuits are required.

(6) Description of Effects As explained above, according to the encoding method of the present invention, tone components that have little impact on visual perception are omitted and only components with high importance are encoded, so that the quality of one image is almost reduced. Highly efficient transmission of originals and color images with shading is possible without deterioration.

Furthermore, as the processing circuit and code structure for encoding the boundary information at the position where two gradation levels change, part of the MR encoding for black and white binary can be used as is, so the circuit load can be reduced.

Table 1 L(m) shows m in binary notation and the gradation level, which is expressed in 4 bits.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram of an embodiment of the encoding circuit, Fig. 2 is an explanatory diagram of an example of an image signal, Fig. 3 is an explanatory diagram of an example of an image signal conversion result, and Fig. 4 is an explanatory diagram of an example of an image signal conversion result.
The figure is an explanatory diagram of the boundary state after image signal conversion, Figure 5 is an explanatory diagram of the boundary state, Figure 6 is an explanatory diagram of the encoding example of Figure 5 using the codes in Table 1, and Figure 7 is decoding. FIG. 2 is an example configuration diagram of a circuit. In FIG. 1, 101 is an image signal memory, 102.10
3 is a comparison output circuit, 104 is a maximum value memory, 105 is a minimum value memory, and 106 is an arithmetic circuit. 107 is a comparison circuit, 108 is an area information memory, 109 is a signal conversion circuit, 110 is a boundary detection circuit, 111 is a counter, 112 is a code generation circuit, 113°, 114 is a boundary information memory, and 115 is a switching circuit.

Claims (2)

[Claims] (1) Means for dividing a screen into blocks each consisting of a plurality of pixels, and n types of block groups B_1, B_2, where n is a number of 2 or more, in order of decreasing gradation level difference within the block. ,..., B_n, and the number of gradation levels N_1, N_2,... having the relationship N_1<N_2<...<N_n.
, N_n, the number of gradation levels of pixels belonging to block group B_1 with the smallest gradation level difference is N_1 gradation,
The gradation level difference is jth (j=1, 2,..., n
) A method for encoding a halftone image, comprising a preprocessing means for degenerating the number of gradation levels of a small block group to N_j gradations. (2) Regarding the boundaries indicating the edges of individual quantized tone levels, the first state is connected from the reference scan line to the encoded scan line, and the first state is connected from the reference scan line to the encoded scan line. A second state that is not connected and disappears, and a third state that is not connected to the reference scan line and is newly generated on the encoded scan line, and encodes information indicating these, and In the first state, information on the position of the boundary on the encoded scanning line is added to the code indicating the state, and in the second state,
In the third state, each gradation level is encoded by adding only a code indicating the state to the code indicating the state, and a code indicating the position of the newly generated boundary and the gradation level associated with that boundary. 2. The halftone image encoding method according to claim 1, further comprising means for encoding a halftone image.