JPH0526389B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit that adaptively converts image signals into pseudo-halftones.

Halftone processing is widely used as a means of expressing continuous gradation in a concise manner. For ordinary photographs that consist of continuous gradation, halftone processing provides sufficient gradation reproduction and gives a natural feel without the appearance of false contours. However, when line drawings such as characters, which are originally binary image signals, are converted into halftone dots, the lines become thick in the center of the halftone dots, and the lines are cut off in the periphery, resulting in a decrease in resolution and small characters. Even that makes it difficult to decipher. On the other hand, if line drawings such as characters, which are originally binary image signals, are binarized using a fixed threshold in order to preserve their contours and reproduce them with good resolution, image signals containing continuous gradation, such as photographs, will be processed. In this area, gradations are not expressed and false contours appear.

An object of the present invention is to provide a circuit that performs adaptive pseudo-halftone that achieves both resolution and gradation expression ability, which have been difficult to achieve with the above-mentioned prior art.

According to the present invention, there is provided a circuit that adaptively converts an image signal including binary and continuous gradations into pseudo-halftones, the circuit comprising: a calculation means for calculating the sum of the absolute value of the difference between the pixel of the line and the pixel of interest and the absolute value of the difference between the pixel preceding the pixel of interest and the pixel of interest; (b) based on the calculated value of the calculation means; The input image signal is adjusted such that when the calculated value is large, the input image signal approaches a fixed threshold, and when the calculated value is small, the input image signal approaches a periodic threshold consisting of a plurality of thresholds arranged in a predetermined order. (c) binarization means to binarize the input image signal using the output of the threshold value determination means. A circuit is obtained.

A detailed description will be given below with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention, and is an example in which contrast is used as a criterion for determining whether or not continuous gradation is used. Image input device 11 equipped with a sensor, analog-digital converter, etc.
A contrast calculation circuit 12 calculates the contrast of the pixel of interest from the digitized image data obtained from the image data. Then, the threshold determining circuit 14 changes the halftone threshold given in advance based on the contrast value and outputs it to the binarizing circuit 15. A binarization circuit 15 compares the image signal obtained from the image input device 11 with the changed halftone threshold value, and outputs a binary image signal.

FIG. 2 shows the contrast calculation circuit 12. The image signal input from terminal 21 is sent to line memory 2.
2, the signal is delayed by one line and is input to the subtracter 23-A. The subtracter 23-A calculates the absolute value of the difference between the pixels of the current line and the pixels of the immediately previous line.

The above contrast value is used to calculate the contrast value between lines, but the 1-pixel memory 24 instead of the line memory 22 and the subtractor 23 are used to calculate the contrast value between lines.
B is also used to calculate the contrast value within the line. The adder 25 outputs the sum of the contrast values between lines and within the lines, that is, the absolute values of the difference values, to the terminal 2.
Output to 2.

In this embodiment, high contrast parts are determined based on the fact that steep edges exist in line drawing parts such as characters, that is, parts with high contrast. In addition to this, it is also possible to determine whether the image signal is continuous tone or not using the spatial frequency of the image signal.

FIG. 3 shows a block diagram of the threshold determination circuit 14. A clock for each pixel is supplied to the terminal 31 from a clock generator (not shown) to drive the pixel counter 38A. Similarly, a clock for each line is supplied to the terminal 32 from a clock generator (not shown) to drive a line counter 38B. Using the values of these two counters as addresses, the threshold table ROM (read-only memory) 35 outputs the halftone threshold T, an example of which is shown in FIG. However, here, instead of writing the threshold value T as it is in the ROM 35, in order to binarize the line drawing portion, a value (T- _{Tl )} obtained by subtracting a fixed threshold value Tl from T is written. Next, in the divider 36, the contrast value inputted from the terminal 22 is normalized, and the value N is used to divide the ROM 3.
Divide the output T- _Tl of 5 to obtain (T- _Tl )/N.
However, N≧1. Next, in the adder 37
Add T _l and output (T-T _l )/N+T _l to terminal 33. This becomes the changed halftone threshold.

As N increases at the edge of the line drawing, the value of (T - T _l )/N decreases, and the threshold approaches T _l at any position of the halftone dot, resulting in an image that looks like it has been binarized with a fixed threshold. can get. On the other hand, when an image with few edges and gradual gradation changes, such as a photograph, is input, N becomes small.
The influence of T _l becomes small and the threshold value is determined by T.
Since this T changes as shown in FIG. 4 depending on the position within the halftone dot, a normal halftone dot image is obtained.

FIG. 5 shows the binarization circuit 15, in which the threshold determined by the threshold determining circuit 14 is inputted from the terminal 43 and compared with the image signal from the image input device 11 inputted to the terminal 51 by the comparator 53. The result is output to terminal 16.

As described above, in the adaptive pseudo-halftoning circuit of the present invention, resolution is preserved because binarization is performed using a nearly fixed threshold in high-contrast areas such as line drawings. In addition, since the portion where the gradation changes gradually is binarized using the halftone threshold, the gradation expressiveness is preserved. Therefore, it is possible to obtain an image in which there is no blurring even in parts such as characters, and there is no false contour appearing in parts such as photographs.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
FIG. 2 is a block diagram of the contrast calculation circuit, FIG. 3 is a block diagram of the threshold value determination circuit, FIG. 4 is a diagram showing a threshold value matrix for halftone dot formation, and FIG. 5 is a block diagram of the binarization circuit. In the figure, 11... image input device, 12...
Contrast calculation circuit, 14...threshold value determination circuit,
15...Binarization circuit, 22...Line memory, 2
3A...Subtractor, 23B...Subtractor, 24...Pixel memory, 25...Adder, 35...ROM (read-only memory), 36...Divider, 37...Adder, 38A... Counter, 38B... Counter, 53... Comparator.

Claims (1)

[Scope of Claims] 1. A circuit that adaptively transforms an image signal including binary and continuous gradations into pseudo-halftones, the circuit comprising: a calculation means for calculating the sum of the absolute value of the difference between the pixel of the previous line and the pixel of interest and the absolute value of the difference between the pixel before the pixel of interest and the pixel of interest; (b) calculating the calculated value of the calculation means; Based on the above, the input image is adjusted such that when the calculated value is large, it approaches a fixed threshold, and when the calculated value is small, it approaches a periodic threshold consisting of a plurality of thresholds arranged in a predetermined order. Adaptive pseudo-halftone characterized by comprising: threshold determining means for outputting a threshold for a signal for each pixel; and (c) binarizing means for binarizing the input image signal using the output of the threshold determining means. circuit.