JPH0142189B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an apparatus for encoding an image signal obtained by scanning an image, such as a newspaper or weekly magazine, in which normal binary images such as characters and figures and halftone photographs are mixed. The image signal obtained by scanning a halftone photograph has very different characteristics from the image signal of a general binary image (hereinafter simply referred to as a binary image), and it is composed of pixels separated by the period of the halftone dot (called in-phase pixels). ) is known to have a strong correlation. Therefore, in predictive coding, which predicts the pixel signal of interest by referring to the values of neighboring pixels and encodes the prediction error signal, in the case of an image in which a binary image and a halftone photograph are mixed as described above, , the efficiency of encoding can be improved by using not only pixels close to the pixel of interest but also in-phase pixels and their neighboring pixels as predictive reference pixels. FIG. 1 shows the position of a reference pixel with respect to a pixel of interest; FIG. 1A shows the position of the reference pixel in a normal binary image, and FIG. In addition, in the following explanation, the period of halftone dots is
The explanation will be given by taking as an example a halftone photograph using a 45° diagonal mesh having 10 pixel intervals in the scanning direction. Table 2 shows the conditional entropy per pixel measured when the pixels shown in Figure 1 a and b are used as reference pixels for a halftone photograph with a halftone dot period of 10 pixels in the scanning direction. The results are shown below.

[Table] This value indicates the target value of compression (target value of average code length per pixel) by the predictive encoding method. As is clear from this table, if the reference pixels shown in FIG. 1b are used, the amount of code can be reduced to about 1/3 compared to the normal binary image encoding method. Therefore, conventionally, the reference pixel pattern shown in FIG.
_It is very approximated _{by a} pixel portion Xo _′ and three portions X _{1 ,} As shown in Figures a and b, if the periodic pattern is considered to be the same as the halftone dot period, these are considered to be halftone areas and the prediction signal for the halftone photograph is used. In this case, a method of encoding using a binary image prediction signal has been considered. This conventional encoding device will be explained below with reference to the drawings. FIG. 3 shows a block diagram of a conventional encoding device. In the figure, 1 is a memory that stores image signals for 6 lines, 2 is an area judger that inputs the reference pixel signal shown in FIG. 1b, 3 is a binary image predictor, and 4 is a halftone photograph. 5 is a prediction signal selector that selects either a predicted value for a binary image or a halftone photograph, and 6 is a prediction error that calculates the exclusive OR of this prediction signal and the true value of the image signal. signal generator,
7 is an encoder. Next, the operation will be explained. The image signal a input to the encoding device is temporarily stored in the image signal memory 1, and then read out and used as the reference pixel signal b. This reference pixel signal b is input to a region determiner 2 and two predictors 3 and 4.
The area determination unit 2 inputs the pixel signal at the position shown in FIG.
If it is determined that it is a value image area, "0") is output.
On the other hand, the binary image predictor 3 inputs the image signal at the position shown in FIG.
The prediction signal d of the pixel of interest Xo for the value image is output.
Further, the halftone photo predictor 4 similarly receives the image signal at the position shown in FIG.
A predicted signal e of Xo is output. And these 2
One of the types of prediction signals d and e is selected by the selector 5 according to the area determination signal c. The prediction signal f selected by the selector 5 and the image signal a are input to the prediction error signal generator 6, where an exclusive OR is taken to generate the prediction error signal g, and the code is Sent to vessel 7. These processes are performed every time the image signal a is sent for one pixel. The encoder 7 inputs this prediction error signal g and generates code data h. As the code used at this time, the code used in facsimile can be used. Table 3 shows an example. This code is called a local code, and the symbol 1
If the value of M is selected according to the probability of occurrence of (prediction mismatch), high coding efficiency can be obtained.

【table】

Claims (1)

[Scope of Claims] 1. In an image signal encoding device for encoding an image in which a normal binary image such as a character or a figure and a halftone photograph are mixed, a nearby pixel within a predetermined range around a pixel of interest an area determination means that uses pixels to determine whether or not the pixel of interest is in a halftone photographic area; and an area determination signal that predicts an area determination signal of the pixel of interest with reference to an already encoded area determination signal. a predictor, a region determination prediction error signal creator for creating a prediction error signal from a region determination signal from the region determination means and a prediction signal from the region determination signal predictor, and a region for encoding the prediction error signal. A judgment signal encoding means, and a binary image and a halftone photograph for outputting prediction signals for a binary image and a halftone image of a pixel of interest to be encoded by referring to image signals of already encoded pixels. a prediction signal selector that selects either of the two prediction signals from the two image predictors based on the region determination signal based on the region determination signal from the region determination means; An image signal prediction error signal generator that creates a prediction error signal from a prediction signal and an image signal of the pixel of interest; and an image prediction error signal encoding unit that encodes the image signal prediction error signal; An image signal encoding device characterized in that an output signal from a signal encoding means is outputted prior to an output signal from an image signal prediction error signal encoding means for the pixel of interest. 2. The image prediction error signal encoding means has a divider that outputs the image signal prediction error signal to either a binary image encoder or a halftone photograph encoder according to the area determination signal. An image signal encoding device according to claim 1, characterized in that: 3. It is characterized in that the encoded signal of the region determination signal for one line is output as the encoded signal of the region determination signal for multiple lines prior to the encoded signal of the image signal prediction error signal for multiple lines. An image signal encoding device according to claim 1 or 2. 4. The region determination signal output from the region determination means has an m value of 0 to m-1 (m>3), and the region determination prediction error signal generator combines the region determination signal with the m value and the determination. According to any one of claims 1 to 3, the method is characterized in that a maximum of m-1 binary prediction error signals shown in Table 1 are created from each predicted signal of the signal. The image signal encoding device described above. [Table] 〓: No encoded symbol