JPS60244160A - Image signal encoding device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention provides an image transmission device such as a facsimile, an image storage device such as an image database, etc., in which binary images and halftone images consisting of characters, line drawings, etc. The present invention relates to an encoding device that encodes and compresses mixed image signals.

[Prior art]

Conventional facsimile machines mainly send binary images to
When inputting a pre-halftone image on the basis of a transition to λ, the statistical properties of the image signal are generally very different from those of a binary image, and the efficiency of encoding becomes extremely low. I end up.

For this reason, some encoding devices for halftone images have been considered, but this device is intended for halftone photographs that display lines, slopes, etc. using a specific screen. It cannot be used for devices that target point photographs. - A conventional image signal encoding device will be described below with reference to the drawings. FIG. 1 is a diagram showing an example of reference pixel positions used for predictive encoding in a conventional image signal encoding device. In the figure, X is a coding pixel that is the pixel to be coded, A and I are pixels on the same line as the coding pixel X, and B, C, and D are pixels on the line immediately before the coding pixel X. Further, R, is the main scanning direction, and R1 is the sub-scanning direction. In predictive encoding, the encoded pixel signal is
It is predicted based on the preceding neighboring pixel signal, and the predicted value, the actual signal, and the p difference signal (prediction error signal) are encoded.

FIG. 2 is a table showing an example of a prediction method in the image signal encoding device of FIG. 1. As shown in Fig. 1, the reference pixels in this example are the pixel immediately preceding one encoded pixel X on the same line, and the three pixels B, C, and D on the line immediately before encoded pixel X. It is used. FIG. 3 is a diagram showing an example of an opaque code for encoding a prediction error signal in the image signal encoding apparatus of FIG. 1.

In the figure, m is an integer of 0 or more and 3 or less, n is an integer of 0 or more, and Xx is a 2-bit binary representation of m. FIG. 4 is a block diagram showing a conventional image signal encoding device, and FIG. 5 is a diagram showing examples of codes for explaining the operation of the image signal encoding device shown in FIG. In each figure, 1 takes as input an image signal indicating a coded pixel This is a predictor that creates the number X. 2 is an exclusive OR (EX-OR) circuit that creates a prediction error signal Y based on the image signal and the prediction signal X; 3 is a sequence of the prediction error signal Y;
This is an encoder that performs encoding based on the code example shown in FIG.

Next, the operation of the conventional image signal encoding apparatus shown in FIG. 4 will be explained. a-1 and ao shown in FIG. 5 are respectively n-1 lines (line immediately before the encoding line)
and an image signal of n lines (encoded lines). One line has 24 pixels, and they are numbered 411 to #24 in the order of scanning. When scanning the encoded line of ao, at each position, each reference pixel A, B, C, D shown in FIG.
Each of them looks like C shown in FIG. Here, each reference pixel.

If B, C, and D are outside the effective screen range, they are set to 0, and are shown enclosed in parentheses in FIG. The predictor 1 shown in FIG. 4 creates a predicted value ΔX shown by Kd in FIG. 5 in accordance with the prediction method table shown in FIG. The exclusive OR circuit 2 uses this △ prediction signal X and the actual image signal (a in FIG. 5) to calculate
A prediction error signal Y shown as e in FIG. 5 is obtained. In the encoder 3, encoding is performed based on this prediction error signal Y according to the code example shown in FIG. 3, resulting in the encoded sequence shown by f in FIG. This signal is virtually added to indicate the end of the line, and the code for this signal is also added to the encoded sequence f. Fifth
In the case of an image signal as shown in the figure, 4# of encoding line a0
Since there is an approximately periodic signal with a 4-pixel interval after 11 (such signals often occur in a halftone image with a horizontal halftone period of 4 pixels), the total code bits of one line are 3.
1, and the amount of data has increased. This is because prediction for a normal binary image as shown in FIG. 2 is also performed for the halftone image portion, so the prediction matching rate is reduced.

As mentioned above, in the conventional image signal encoding device, even when a normal binary image and a halftone image are mixed,
Since encoding was performed for value images, efficient data compression was not possible. Also, since the image signal obtained by reading the halftone image was simply encoded into a binary signal,
When the period of halftone dots and the pixel interval are close, moiré occurs and the image quality becomes extremely low. A low-pass filter that performs average value processing in a predetermined area near the pixel for an image signal containing a mixture of images and halftone images, and a maximum value of the output of the low-pass filter at the pressure within the area. The binary image and the halftone image are separated using a comparison circuit that detects and compares the difference between Prediction of a predetermined number of preceding coded pixels using each of the predictors for value images and dithered images - predictive conversion of coded pixels by selectively using each of the predictors By encoding, it is possible to compress data with high efficiency, and also for halftone photographs using various screens.
An object of the present invention is to provide an image signal encoding device capable of reproducing images of good quality without moiré.

[Embodiments of the invention]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 6 is a block configuration diagram showing an image signal encoding device which is an embodiment of the present invention, and FIG. 7 shows pixel positions where average value processing is performed in the image signal encoding device of FIG. 6. 8 is a diagram showing a dither threshold matrix in the image signal encoding device of FIG. 6, and FIG. 9 is a diagram showing a prediction method for dithered images in the image signal encoding device of FIG. 6. FIGS. 10 and 11 are diagrams each showing examples of codes for explaining the operation of the image signal encoding device shown in FIG. 6. In FIG. 6, 4 is a low-pass filter that performs average value processing within 3×3 pixels shown in FIG. An area maximum/minimum difference counting/comparing circuit 6 counts the difference between the maximum value and the minimum value within the area shown in FIG. 7, and determines whether the value is 279 or more.
A dither circuit 7 systematically dithers the image signal output from the low-pass filter 4 using a 2×2 dither threshold shown in FIG. a binary signal switching device that selects either the output signal of the dithering circuit 6 or the binary input image signal according to the image signal selection signal that is the output signal of the circuit 5;
1a and 1b are used for binary images (according to the prediction method shown in FIG. 2) and for dithered images (according to the prediction method shown in FIG. 9), which predict the output signal of the binary signal switcher 7, respectively. ), each of the predictors 2a and 2b performs an exclusive operation between the binary and dither prediction signals, which are the outputs of the binary image predictor 1a and the dithered image predictor 1b, respectively, and the encoded image signal. The exclusive OR circuits 8a and 8b for binary images and dithered images that take logical sums are respectively the exclusive OR circuit 2a for binary images and the exclusive OR circuit 2b for dithered images. Based on the binary and dithered prediction error signals that are each output, count the number of prediction errors within the three pixels preceding the encoded pixel X. Each prediction for binary images and dithered images - count The circuit 9 is the output of each prediction-number counting circuit 8a and 8b for binary images and dithered images.
10 is the output of the prediction-several comparison circuit 9, which compares the dithered prediction error number signal and the dithered prediction error number signal to determine whether the dithered prediction error number signal is larger than the binary prediction error number signal; A prediction error signal selector 3 selects either a binary prediction error signal or a dither prediction error signal based on a prediction error signal selection signal 3, which is a prediction error signal selected by the prediction error signal selector lO. This is an encoder that encodes a coded prediction error signal).

Next, the operation of the image signal encoding apparatus shown in FIG. 6, which is an embodiment of the present invention, will be described. a+g+h, 1+J+k+A! shown in FIGS. 10 and 11.
are distinguished by adding a subscript to the line number, with O being the encoded line (n-th line). Also, d,
For e and o, set F for binary and D for dither.
A subscript is added. Further, in i or p, D and F represent selection of dither and binary image signals or prediction error signals, respectively. The right side of #11 of the image signal a shown in FIG. 10 includes a halftone dot image with a horizontal halftone dot period and an interval of four pixels. Here, al and ao are the same as those used in the explanation of the operation of the conventional apparatus shown in FIG. 5 above. Regarding each pixel signal of the image signal in the figure, the average value of the nine pixels shown in FIG. 7 is obtained by the low-pass filter 4 as its output g. Here, if a pixel for which the average value should be calculated is outside the effective screen range, the pixel value was set to 0. Next, by subtracting the difference between the maximum value and the minimum value within the region shown in FIG. 7 for the output gK of the low-pass filter 4, an intra-block difference signal is obtained. In the area maximum/minimum difference counting/comparison circuit 5,
Obtain the image signal selection signal i based on the intra-block difference signal (dither D at h%, binary F at h < 2/9)
.

On the other hand, the output g of the low-pass filter 4 is dithered by the dither circuit 6 using the dither threshold matrix shown in FIG.
and 0 otherwise).

Here, the dither threshold is 1/8 for the #1 pixel of the n-th line.
shall correspond. Low-pass type filter I 4
(D Output g K Corresponding dither threshold wo, Fig. 10 K
Indicated by j. The binary signal switch 7 appropriately selects the human image signal a and the dithered signal k based on the image signal selection signal i to obtain an encoded image signal l. Here, the image signal corresponding to the reference pixel shown in FIG. 1 is processed by the binary image predictor 1a as shown in FIG. A binary prediction signal indicated by dF is obtained, and a binary prediction error signal eF is obtained by the binary image exclusive OR circuit 2a. Further, the dithered image predictor 1b creates a dithered prediction signal shown as KdD in FIG. 11 based on the encoded image signal l according to the prediction method shown in FIG. In the prediction method shown in FIG. 9, not only the reference pixel signal but also the dither threshold value of the pixel
The information (equivalent to the location in the matrix of X2) is also taken into account. This is because in a dithered image, the properties of the pixel vary greatly, such as whether it is easy to change to 0 or easy to change to 1, depending on the threshold value of the pixel, so taking the threshold value into account can improve predictions. It is from. Furthermore, 1 is added as the reference pixel because its threshold is equal to the encoded pixel X and the correlation is stronger than that of each pixel B or C. Dither prediction signal dD
A dither prediction error signal eD is obtained by the exclusive logic circuit 2b for Fi dithered images in the same way as for binary images. The two types of binary and dithered prediction error signals ep and eD described above are applied to the corresponding encoded pixel
Number of binary and dither prediction errors in a pixel. , and OD are counted. The prediction-number comparison circuit 9 calculates the number of binary and dither prediction errors. ,as well as. . By comparing , a prediction error signal selection signal p is obtained (here, p is set to dither D if OF>oD, and set to binary F otherwise). The prediction error signal selector 10 selects binary and dithered prediction error signals ep and eD based on this prediction error signal selection signal p to obtain a coded prediction error signal e. Finally 1 encoder 3
Thus, encoding is performed according to the code example shown in FIG. 3, and a coded sequence f is obtained. Further, the decoding device decodes the coded sequence f according to the code example shown in FIG. 3 to obtain a coded prediction error signal e. Thereafter, by preparing two sets each of a predictor, an exclusive OR circuit, and a prediction-multiple counting circuit at the same time as the encoding device, two types of prediction errors (for the three pixels preceding the decoding pixel) are calculated. Create a signal. Furthermore, at the same time as the first encoding device, a prediction-multiple comparison circuit determines which of the two types of prediction signals (of the decoded pixel) was used in the encoding device, and a predicted value is selected based on that. The coded image signal 10 is obtained by exclusive ORing the decoded coded prediction error signal e.

Note that in the above embodiment, a case was explained in which a further binarized input image signal a was used.
A multi-gradation signal (or analog signal) may be used, and a binarization circuit may be provided immediately before one of the two inputs of the binary signal switch 7 (for a binary image).

In addition, in the above embodiment, a case was explained for a so-called predictive coding method, but a prediction error signal sequence is divided into multiple sequences using reference pixels or dither threshold information, and a code suitable for the statistical properties of each sequence is assigned. It is also possible to adapt to an encoding method based on the so-called Marco model.

〔Effect of the invention〕

As explained above, the present invention is an image signal encoding device that performs average value processing on a predetermined area near the pixel for an image signal containing a mixture of binary images and halftone images. The binary image and the halftone image are separated using a comparator circuit that detects and compares the difference between the maximum and minimum values of the outputs of the pass filter and the low pass filter within the area, and Then, the output of the low-pass filter is dithered (by a dithering circuit), and a predetermined number of preceding encoded pixels are predicted using predictors for binary images and dithered images. By using several predictors, each of the predictors is used interchangeably to predictively transform and encode the encoded pixel, making it easy to perform encoding adapted to each pixel.
In addition to making it possible to compress data with high overall efficiency, it is also possible to reproduce high-quality images with no moiré and extremely good gradation reproducibility even for halftone photographs using various screens. This has an excellent effect.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of reference pixel positions used for predictive encoding in a conventional image signal encoding device, and FIG. 2 is a diagram showing an example of a prediction method used in the image signal encoding device of FIG. Figure 1 shows an example of the table. FIG. 3 is a diagram showing a code example when encoding a prediction error signal in the image signal encoding device of FIG. 1, and FIG. 4 is a block diagram showing a conventional image signal encoding device. FIG. 5 is a diagram showing each code example for explaining the operation of the image signal encoding device shown in FIG. 4, and FIG. 6 is a block diagram showing the image signal encoding device which is an embodiment of the present invention. 7 is a diagram showing the pixel positions where average value processing is performed in the image signal encoding device of FIG. 6, and FIG. 8 is a diagram showing the dither threshold value in the image signal encoding device of FIG. 6. 9 is a table showing a prediction method for dithered images in the image signal encoding device of FIG. 6, and FIGS. 10 and 11 are diagrams showing the image of FIG. 6. FIG. 3 is a diagram showing examples of codes for explaining the operation of the signal encoding device. In the figure, 1...Predictor, 1a...Predictor for binary images, 1b...Predictor for dithered images, 2...Exclusive OR circuit, 2a...For binary images Exclusive OR circuit, 2b... Exclusive OR circuit for dithered image, 3...
- Encoder, 4 - Low-pass filter, 5... Area maximum/minimum difference counting/comparison circuit, 6... Dithering circuit,
7... Binary signal switcher, 8a... Prediction matching rate counting circuit for binary images, 8b... Prediction-number line counting circuit for dithered images, 9... Prediction-number line comparison circuit, 10...
This is a prediction error signal selector. In each figure, the same reference numerals indicate the same or equivalent parts. Agent Masuo Oiwa Figure 1 Figure 2 Figure 3 Figure 4 0 RJfOLJ t) Ql h Figure 8 Figure 9 Procedural amendment (voluntary) 1. Indication of the case: Japanese Patent Application No. 59-101205 2, title of the invention Image signal encoding device 3, person making the amendment Relationship to the case Patent applicant address: 2-3, Maruno Gakko-chome, Chiyoda-ku, Tokyo Name:
(601) Mitsubishi Electric Corporation Representative Hitoshi Katayama 4, Agent address 2-3-5 Maruno Gakko-chome, Chiyoda-ku, Tokyo, Subject of amendment: ``Detailed Description of the Invention'' column of the specification. 6. Contents of the amendment (1) "Shi-sama" in the 12th line of page 3 of the specification is amended to [sho-J]. (2) In the same book, page 12, line 20, “1 is J,” “I is
Correct it with ノ.

Claims (1)

[Claims] A predictive encoding device that predicts an image signal in which a binary image consisting of characters, line drawings, etc. and a halftone image are mixed, based on the values of preceding adjacent pixels, and encodes the prediction error signal. , a low-pass filter performs average value processing in a predetermined region near the pixel, and a dither converts the output signal from the low-pass filter into υ binarization by dithering processing using a variable threshold. detecting the maximum and minimum values of the outputs of the low-pass filter corresponding to pixels in a predetermined area in the vicinity of the pixel, and whether the difference therebetween exceeds a predetermined value. a comparison circuit that determines whether the first and second predictors that perform prediction for the binary image and for the dithered image signal;
, first and second prediction coincidence rate counting circuits that count prediction coincidence rates in a predetermined number of preceding pixels based on output signals from each @1 and second predictor; Based on the output signals of the first and second prediction matching rate counting circuits,
An image signal encoding device comprising: a prediction error signal selector that selects one of the two types of prediction error signals.