JPH0237154B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for generating pseudo halftone image data, and in particular, to thinning and reducing an image using pseudo halftone data input from an image input device to an image processing device. The present invention relates to a method of generating pseudo halftone image data suitable for cases where

[Background of the invention]

The principle of pseudo halftone is explained in "Digital signal processing of images (pp236)" published by Nikkan Kogyo Shimbun on May 25, 1980 (written by Hikotaka Fukieda), and detailed explanation will be omitted here. However, Figures 6 to 9
The disadvantages of pseudo halftones seen in reduced images will be explained with reference to the drawings.

FIG. 6 is a block diagram of a pseudo halftone image data generation circuit, FIG. 7 is a diagram showing a pseudo halftone slice in which a plurality of slice levels for binarization are arranged periodically, and FIG. The figure shows the relationship between an analog image signal and a plurality of slice levels of pseudo halftone slices, and FIG. FIG. 2 is a diagram illustrating an output that has been reduced by two decimations. In FIG. 6, the analog image signal A as the image input signal is
The A/D converter 1 converts the signal into a multi-value digital image signal, which is input to one side of the comparator 2. The output of the slice level generation circuit 5, which generates slice levels using the outputs of the horizontal period counter 4 and the vertical period counter 3 as addresses, is the other input of the comparator 2, and the digital image signal is compared with a plurality of slice levels. It is binarized and output as pseudo halftone image data D. The pseudo halftone slice is a pattern in which slice levels of 16 values are periodically arranged, as shown in FIG. 7, for example. Further, the relationship between the analog image signal A and the slice level of the pseudo halftone slice is as shown in FIG.

Now, if the halftone image input as an analog image signal increases in density linearly in the horizontal scanning direction as shown in FIG. 9, the slice levels of the first scanning line are L ₀ , L ₈ , L ₂ , L ₁₀ , L ₀ , L ₈ ,
L ₂ , L ₁₀ . . . are repeated, and the shaded portion in FIG. 91 is outputted as black in the dithered image output, and the other portions are outputted as white. Similarly, the slice levels of the second scanning line are L ₁₂ , L ₄ , L ₁₄ , L ₆ , L ₁₂ ,
L ₄ , L ₁₄ , L ₆ ... is repeated, and the slice levels of the third scanning line are L ₃ , L ₁₁ , L ₁ , L ₉ ,
L ₃ , L ₁₁ , L ₁ , L ₉ ... are repeated, and the slice levels of the fourth scanning line are L ₁₅ , L ₇ , L ₁₃ ,
L ₅ , L ₁₅ , L ₇ , L ₁₃ , L ₅ ... are repeated.
The slice level of the fifth scan line is the same as the slice level of the first scan line, and the same is repeated periodically thereafter. If the dithered image output of FIG. 9 1 is thinned down to 1/2, it will become as shown in FIGS. 2 and 3, which is significantly different from FIG. 9 1 and difficult to be considered as a halftone image. In other words, with the data in Figure 9 2, the image becomes extremely black, while with the data in Figure 9 3
data, the image becomes extremely white. It is clear that the image quality of the reduced images obtained by thinning out is significantly lower than that of the pseudo halftone image before reduction.

[Purpose of the invention]

An object of the present invention is to provide a new method for generating pseudo halftone image data that improves the extreme deterioration in image quality of pseudo halftone images that occurs when such conventional dither images are thinned out and reduced.

[Summary of the invention]

In the present invention, when converting to pseudo halftone image data, conditions for thinning and reduction are specified in advance,
This method attempts to select and apply a suitable pseudo halftone slice from among a plurality of pseudo halftone slices prepared in advance. This reduces the difference between the pre-reduction pseudo halftone image and the reduced pseudo halftone image after thinning out, thereby improving the image quality of the reduced image.

In the present invention, for example, as a pseudo halftone slice applied to 1/2 decimation reduction, FIGS.
By preparing slice levels with periodicity as shown in the figure, pseudo halftone image data to which the slice levels marked with a circle are applied can be obtained from the 1/2 thinning reduction data.

Similarly, in the case of 1/3, 1/4 reduction, etc.
A pseudo halftone slice suitable for the reduction ratio is prepared in advance.

[Embodiments of the invention]

An embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of the configuration of a pseudo halftone image data generation circuit. The image input signal A is quantized to 4 bits and 16 levels by the A/D converter 11 and becomes one input of the digital comparator 12.
On the other hand, the outputs of the horizontal period counter 4 consisting of an 8-bit binary counter that determines the period of the slice level in the horizontal direction scan and the vertical period counter 3 consisting of the 8-bit binary counter that determines the period of the slice level in the vertical direction scan are as follows. It is input to the memory 15 as an address of the memory 15. The output of the memory 15 becomes the other input of the digital comparator 12 as a slice level. The memory 15 stores in advance pseudo halftone slice level data as shown in FIG. 5, for example, and the slice level data shown in FIG. 5 contains the values shown in FIG. 4. The period of the pseudo halftone slices shown in FIGS. 3 and 4 is changed so that when the slices are thinned down to 1/2, data to which the slice level marked with a circle is applied is output. It was decided. This period was designed so that when the image is thinned down to 1/2, it becomes close to the pseudo halftone image before reduction.

A plurality of sets of these slice level data are stored in advance in a fixed memory such as a ROM, and can be easily addressed using signals S ₁ and S ₂ that specify the reduction ratio. The circuit of FIG. 1 is synchronized by a horizontal clock _T1 and a vertical clock _T2 shown in FIG. Horizontal clock _T1 and vertical clock _T2 can be easily generated from a pulse oscillator, although not shown. Further, address information based on the signals S ₁ and S ₂ specifying the reduction ratio can be easily generated using an operation switch or the like.

Now, when the signal S ₁ =0 and the signal S ₂ =0, the address of the memory 15 where the pseudo halftone slice shown in FIG. outputs an “H” level signal (black signal) when the signal is higher than the slice level from the memory 15;
At other times, an "L" level signal (white signal) is output.

Since the pseudo halftone data output from the digital comparator 12 is data to which the pseudo halftone slice shown in FIG. 3 or 4 has been applied, when this is thinned out to 1/2, The slice level marked with a circle in FIG. 3 or 4 is applied to the data, and a reduced image close to the pseudo halftone image before reduction is obtained. As described above, according to the embodiments of the present invention, it is possible to create a pseudo halftone image that can avoid significant deterioration in image quality when thinned out and reduced.

〔Effect of the invention〕

According to the present invention, it is possible to generate pseudo halftone image data that can prevent deterioration in image quality when a pseudo halftone image is reduced.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a pseudo halftone image data generation circuit according to an embodiment of the present invention, and FIG.
3 and 4 are diagrams illustrating a pseudo halftone slice according to an embodiment of the present invention, and FIG.
The figure shows how the pseudo halftone slice of Fig. 4 is stored in memory, Fig. 6 is a block diagram showing a conventional pseudo halftone image data generation circuit, and Fig. 7 shows a conventional pseudo halftone image data generation circuit.・A diagram showing slices, FIG. 8 is a diagram showing the relationship between analog image signals and slice levels, and FIG. /2
FIG. 3 is a diagram showing a thinned-out and reduced output; 11... 4-bit A/D converter, 12... Digital comparator, 13... Vertical period counter, 14... Horizontal period counter, 15... Memory.

Claims (1)

[Claims] 1. A method of converting an analog image signal into a multivalued digital image signal, and converting the digital image signal into pseudo halftone image data using pseudo halftone slices in which a plurality of slice levels are arranged periodically, , the pseudo halftone slices are provided in the memory corresponding to the reduction ratio of the image, and each pseudo halftone slice is composed of a plurality of periodic slices that display pseudo halftones of the image according to the reduction ratio. A pseudo halftone slice having an array of slice levels, and selecting a pseudo halftone slice corresponding to a reduction ratio from among the plurality of pseudo halftone slices when generating pseudo halftone image data of an analog image signal. How to generate halftone image data.