JPH02239380A - Picture element density converting device - Google Patents

Abstract

PURPOSE:To excellently convert the picture element density of an indefinite picture at an arbitrary multiplication factor by calculating the mean density of the converted picture element by means of a projecting method, and binarizing it based on a quantization error and the mean density corresponding to the binarization of the peripheral picture elements. CONSTITUTION:The mean density of the converted picture elements by the projection method is operated in a picture element density arithmetic part for the original picture, and outputted to the binary processing part. The picture element density inputted from the arithmetic part passes through one picture element delay elements 51a to 51e, a delay element 53 which is three picture elements fewer than one line, and adders 52a to 52d. Further the quantization error generated by the binarization of the peripheral picture elements during the passage is obtained by a binary error calculating part 55, and the errors distributed to errors e1 to e4 by an error distribution processing part 56 is added. The density value including the error is binarized by a constant threshold by a binarizing part 54, and made into the converted output. Thus the picture element density of the undefined picture can be excellently converted at the arbitrary multiplication factor.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a pixel density conversion device, and particularly to a pixel density conversion device for a binary image in which a pseudo-halftone-processed image and characters or line drawings coexist. ．． [Prior Art] In facsimile communications, etc., when the resolution within an image reading device or image printing device is different from the resolution during communication, or when image data is enlarged or reduced using an image editing device, etc. Requires conversion processing of image pixel density.

Conventionally, pixel density conversion methods for such binary images include the spc method for characters and line drawings. Sum-of-disputes method. 9 division method. Projection method, linear interpolation method. Various methods have been proposed, such as inversely proportional distance. In addition, for images that have been subjected to pseudo-halftone processing using the dither method, etc., the original multivalued image is estimated from the binary image using an average value filter, etc., and the multivalued image is converted and then re-binarized. Some methods have been proposed, such as methods to perform block-by-block thinning of the dither matrix for the dither method. [Problem N to be solved by the invention] However, when conventional conversion methods for characters and line drawings are applied to images that have been subjected to pseudo-halftone processing using the dither method, etc., in any method, integer multiples and l/
In conversion that is not an integer multiple, image quality deterioration such as moire and periodic lines occurs is significant. On the other hand, when a conversion method for images subjected to pseudo-halftone processing is applied to characters or line drawings, there is a significant deterioration in image quality such as a decrease in resolution or disappearance of thin lines.

Furthermore, in the method targeting dithered images, the dither matrix used for binarization must be known, for example, in order to estimate the original multivalued image (Japanese Patent Laid-Open No. 62-15746).
No. 13) cannot be applied to processing unspecified images. As described above, with the conventionally proposed methods, it is difficult to successfully convert the pixel density of an unspecified image in which a pseudo-halftone-processed image is mixed with text or line drawings. The present invention has been made in view of these points,
It is an object of the present invention to provide a pixel density conversion device that can satisfactorily convert the pixel density of an unspecified image containing a pseudo-halftone-processed image and characters or line drawings at an arbitrary magnification.

[Means for Solving the Problem] In order to solve this problem, the pixel density conversion device of the present invention converts the pixel density of a binary image in which a pseudo-halftone-processed image and characters or line drawings are mixed. A pixel density conversion device, comprising a conversion pixel calculation means for calculating the density or brightness of a converted pixel from the binary image by a projection method, and converting the average density or average brightness of the converted pixel, which is the calculation result, into a binary value. When converting to
A binarization means is provided which performs binarization based on the average density or average brightness and a quantization error accompanying the binarization of surrounding pixels that have already been binarized.

[Operation] In such a configuration, the density or brightness of converted pixels is calculated using a projection method that is said to cause less image quality deterioration due to pixel density conversion for characters and line drawings, and is applied to images that have undergone pseudo halftone processing. In order to suppress moiré and periodic lines that occur when the calculation is performed, when the average density or average brightness of the converted pixel, which is the calculation output, is binarized, it is necessary to Binarization is performed using the error diffusion method or the average error minimization method based on the quantization error and the average density or average brightness. [Examples] The principles and examples of the present invention will be explained below with reference to the drawings. FIG. 1 is a block diagram showing a pixel density conversion device according to this embodiment. The density of the converted pixel is calculated for the original image by the pixel density calculation unit 11 using the projection method, and the calculation result (n
bits) are binarized by the binarization processing unit 12 using the error diffusion method to obtain a converted output image whose pixel density has been converted.

First, the principle of pixel density calculation using the projection method will be explained with reference to FIG. Here, as an example, we will show the case of conversion magnification %. The converted pixel A, which is the pixel of interest, is projected onto the original image, and the original pixels that overlap the pixel surface of the converted pixel A on the projection plane are P, Q.
,R,S. Here, if the areas occupied by the pixel planes P, Q, R, and S in the pixel plane of the projected converted pixel A are SP*So and SR*Sl1, respectively, then the average density of the pixel of interest A is . is expressed by the following equation. (Ip.Io.
In, Is: densities of pixel planes P, Q, R, S) Next, by binarizing the average density IA of this pixel of interest A, a converted output pixel whose pixel density has been converted is obtained. A block diagram of the pixel density calculation section 11 is shown in FIG. In the illustrated example, it is possible to process a range of conversion magnification>station. Reference numeral 41 denotes a pixel extraction unit which extracts original pixels near the position (x, y) of the converted pixel on the projection plane. 42 is a conversion pixel position calculation unit, which performs main scanning. Calculates the position of the converted pixel on the projection plane, which is determined according to the sub-scanning direction conversion magnification. 4
Reference numeral 3 denotes an area calculation unit, which calculates each area according to the position information obtained by the position calculation unit 42 and the conversion magnification. 44
is an arithmetic processing section expressed by equation (1), which calculates the average density of the target converted pixel from the multiplication result of the outputs of the pixel extraction section 41 and the area calculation section 43. It should be noted that the pixel density calculation section 11 is not limited to the above configuration, and generally any processing method may be used as long as the calculation result expressed by the following equation (2) can be obtained.

Z > Area overlapping the pixel surface of a pixel This embodiment shows a case where the number of original pixels K to be referenced is limited to four in order to simplify the circuit. By increasing the number of reference pixels, it is possible to widen the range of conversion magnification that can be processed (minimum magnification when reducing). Further, depending on the configuration of the pixel extraction section 41, it is possible to process sequentially input image data or to process image data stored in a storage device such as a memory. Next, the binarization processing unit 12 using the error diffusion method will be explained. When the projection method is applied to a pseudo-halftone image such as a dither, if the calculation result is simply binarized (that is, binarized with a fixed threshold), moiré or periodic lines may appear due to quantization errors, etc. It is emphasized and degraded. In this embodiment, in order to prevent deterioration of image quality due to such quantization errors, binarization processing is performed using the error diffusion method. Figure 4 shows a block diagram of the binarization processing section 12. The pixel density of the output from the pixel density calculation unit 11 using the projection method is 1
Pixel delay elements 51a-51e. Binarization errors 01 to e4 previously generated in surrounding pixels while passing through the delay element 53 and adders 52a to 52d, which are three pixels less than one line.
is added. The density value including the binarization error of the surrounding pixels is binarized by the binarization unit 54 using a constant threshold value, and the value becomes the conversion output. A quantization error caused by this binarization is calculated by a binarization error calculation section 55, and distributed to e1 to e4 by an error distribution processing section 56. In the binarization error calculation unit 55, the binarization error is E, and the input density to the binarization processing unit is Io (a value obtained by normalizing the maximum value to 1).
．． Set the threshold to T. When the binarized output is set to ``1'' or ``O'', the following calculation is performed. Also, in the error distribution section 56, for example, ei''''e is calculated as follows.
4 is calculated. e. xe4 is distributed to pixels surrounding the pixel of interest as shown in FIG. Note that although the example shown in FIG. 4 is a case where the error is diffused to four surrounding pixels, the present invention is not limited to this, and the decision may be made in consideration of image quality and circuit scale. However, in order to effectively eliminate moiré, it is necessary to diffuse 100% of the binarization error to the surrounding area. That is, erl is determined so as to satisfy Σan=E (n: the number of peripheral pixels to which the error is distributed). Also, the same result can be obtained by using the average error minimization method instead of the pixel diffusion method. As explained above, in this embodiment, by using the projection method to binarize the average density or average brightness of converted pixels using the error diffusion method, an unspecified pseudo-halftone-processed image can be created. Pixel density can be converted to any magnification with the same processing, with less influence on mixed characters and line drawings.

In this way, in this embodiment, pixel density conversion can be performed without deteriorating image quality. Therefore, this embodiment is effective when transmitting an image from a G3 facsimile to a 04 facsimile, and vice versa. [Effects of the Invention] According to the present invention V, it is possible to provide a pixel density conversion device that can satisfactorily convert the pixel density of an unspecified image in which a pseudo-halftone-processed image and characters or line drawings are mixed together at an arbitrary magnification.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the pixel density conversion device of this embodiment. FIG. 2 is a diagram explaining the principle of pixel density conversion using the projection method. FIG. 3 is a diagram showing an embodiment of the pixel density calculation section using the projection method. , Fig. 4 is a diagram showing an example of a binarization processing unit using error diffusion processing, and Fig. 5 is an explanatory diagram of the error diffusion method.

Claims (1)

[Scope of Claims] A pixel density conversion device for converting the pixel density of a binary image in which a pseudo-halftone-processed image and characters or line drawings are mixed, the pixel density conversion device converting the pixel density of a binary image using a projection method. A converted pixel calculation means for calculating the density or brightness of a pixel, and when binarizing the average density or average brightness of the converted pixel which is the calculation result,
A pixel density conversion device characterized by comprising a binarization unit that performs binarization based on the quantization error accompanying the binarization of surrounding pixels that have already been binarized and the average density or average brightness. .