JP2682986B2 - Image processing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method used in digital printers, digital facsimiles, and the like, and particularly to inputting image data having a plurality of one pixel levels and making an error in the image data. The present invention relates to an image processing method for binarizing binary data by a diffusion method. [Prior Art] Conventionally, there is an error diffusion method as a binarization method for reproducing a halftone in a digital printer, a digital facsimile, or the like. This method is a method of calculating the density difference between the image density of the original document and the output image density for each pixel, and dispersing the error resulting from the calculation after giving a specific weight to the peripheral image. This error diffusion method is described in reference R.
W. Floyd and L. Steinberg “AnAdaptive Algorithm for
Presented in “Speatial Gray Scale” SID 75 Digest (1976). Since this method has no periodicity, it has a moire phenomenon which is a problem in the dither method and the density pattern method of other binarization methods (when the original is a halftone image such as printing, the copied image is not printed on the original. The phenomenon that a periodical striped pattern that does not exist appears) does not occur, but there is a defect that a unique striped pattern is generated in an output image in an image such as a photograph in which the change in shade is small. [Purpose] The present invention eliminates the above-described drawbacks of the prior art, and an error is generated by filtering error data generated during binarization processing by the error diffusion method so as to suppress the occurrence of a stripe pattern. An object of the present invention is to provide an image processing method capable of suppressing the occurrence of stripe patterns and obtaining a high-quality image while maintaining high gradation and high resolution in the diffusion method. Embodiments Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention.
The input sensor unit 10 includes a photoelectric conversion element such as a CCD and a driving device for scanning the photoelectric conversion element, and performs scanning of a document. In the input sensor unit 10, data via a communication line,
Data from a storage medium such as a disk tape may be input. Image data or various data of the document read by the input sensor unit 10 is sent to the A / D converter 11. Here, the data of each pixel is converted to 8-bit digital data and
It is quantized to data having the number of levels of gradation. Next, in the correction circuit 12, shading correction for correcting unevenness in sensitivity of the CCD sensor and unevenness in illuminance caused by the illumination light source is performed by digital arithmetic processing. Reference numeral 13 denotes a threshold processing unit that performs binarization processing of the data from the correction circuit 12. An image display unit 19 for displaying a binary image is composed of a CRT, a printer and the like. Next, the threshold processing unit 13 will be described in detail. Reference numeral 14 is an adder for adding the gradation image data 20 corrected by the correction circuit 12 and the dither signal 21, and outputs the corrected pixel value 22 to the binarization circuit 15 and the error calculation unit 16. Reference numeral 15 denotes a binarizing circuit, which has a predetermined threshold T (for example, T = 12
The modified pixel value 22 is binarized using 7) and the binary image data 2
3 is output to the image display unit 19 and the error calculation unit 16. The error calculator 16 obtains a binarization error E from the binary image data 23 and the corrected pixel value 22 and outputs it to the error storage processor 17. However, the binarization error E is Is required. Here, P is the corrected pixel value 22 and T is the threshold value (12
7) L and U are binary image data 23, L is the lower limit value (L = O) of the dynamic range of pixels, and U is the upper limit value (U = 255). The error storage processing unit 17 stores the binarization error E in the error buffer memory 26 as shown in FIG.
Then, the data stored in the error buffer memory 26 are multiplied by the weighting coefficients shown in FIG. 3, and the sum is output to the error data processing unit 18. Error buffer memory
The white circles in 26 and Fig. 3 * show the pixel position currently being processed. The weighting factor for the pixel that is close to that position, that is, l, h is 1/3, and the weighting factor for g, i farther from that position is 1/3. Is 1/6. The error buffer memory 26 has a plurality of lines of line buffer memory, and the one shown in FIG. 2 shows a part of the window. The binarization error 24 stored in the error buffer memory 26 is the binarization error before the data currently being processed. FIG. 4 is a diagram showing another example of the weighting coefficient. If this coefficient is used, errors in more peripheral pixels can be considered as compared with the case described above. The error data processing unit 18 in FIG. 2 is a filter provided for removing the stripe pattern generated in the error diffusion method, and processes the data 25 output from the error storage processing unit 17. FIG. 5A shows the amplitude spectrum of the error data generated in the error diffusion method, and the bumps a superposed on the spectrum cause the unique stripe pattern in the error diffusion method. Therefore, a filter having a characteristic (FIG. 5B) for removing this bump is used as an error data processing unit.
Used in 18. Since the relationship between the frequency and the amplitude in FIG. 5A is determined according to the weighting number shown in FIG. 3 or 4, it is possible to design the filter using a known method. For example, when it is desired to remove the stripe pattern at the shed portion and the highlight portion, a filter having a characteristic of removing the bump at the frequency is used. Although FIG. 5 shows one dimension for convenience of explanation, since the amplitude spectrum of the actual error data is two-dimensional and cannot be separated into two one-dimensional spectra, the filter used as the error data processing unit is also two-dimensional. Use a dimensional filter. The data filtered by the error data processing unit 18 is added to the gradation image data 20 by the adder 14 as a dither signal 21. The corrected pixel value 22 thus added is binarized by the binarizing circuit 15 and binarized image data 23 is output to the image display unit 19. In this way, by filtering the error data in the binarization method in the error diffusion method, it is possible to prevent the unique stripe pattern while maintaining high gradation and high resolution by the error diffusion method. In the above embodiment, the removal of the fringe pattern by the error diffusion method based on the frequency characteristic is described, but a similar effect can be obtained by providing another mechanism in the error data processing unit. One example is a median filter. The median filter is called an intermediate value filter, and is a filter having a characteristic that when the data is larger or smaller than the intermediate value, the data is processed to have the intermediate value. In the error diffusion method, in particular, in the highlight part and the shadow part, as a result of error accumulation, black and white of pixels are inverted, and a stripe pattern is generated, which may cause deterioration of image quality. The accumulation of this error appears as an inversion of the sign in the error data, and it occurs locally in the highlight part and the shadow part. It is possible to suppress the occurrence of the striped pattern by removing it. Also, in FIG. 1 described above, the gradation image data 20 and the dither value
After the addition of 21, the binarization circuit 15 is configured to binarize with the threshold value T. In this, the dither value number 21 is subtracted from the threshold value T, and the subtracted threshold value T'is used for gradation. Image data 20
It is equivalent to binarizing. [Effect of the Invention] As described above, according to the present invention, the error data generated in the binarization apparatus by the error diffusion method is filtered so as to suppress the generation of the stripe pattern, and thus the higher order in the error diffusion method is obtained. While maintaining tonality and high resolution, it is possible to suppress the occurrence of stripe patterns and obtain high-quality images.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an image processing apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram of an error storage processing unit, FIG. 3, and FIG.
FIG. 5 is a diagram showing an example of the weighting coefficient, and FIG. 5 is a diagram showing an example of the operation of the error data processing unit. In the figure, 10 is an input sensor unit, 11 is an A / D converter, 12 is a correction circuit, 13 is a threshold processing unit, 14 is an adder, 15 is a binarization circuit, 16 is an error calculation unit, and 17 is an error storage process. Reference numeral 18 is an error data processing unit, and reference numeral 19 is an image display unit.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Hidefumi Osawa               3-30-2 Shimomaruko, Ota-ku, Tokyo               Inside Canon Inc. (72) Inventor Hiroshi Hosokawa               3-30-2 Shimomaruko, Ota-ku, Tokyo               Inside Canon Inc.                (56) References JP-A-55-13554 (JP, A)                 JP 62-139472 (JP, A)

Claims (1)

(57) [Claims] In an image processing method of inputting image data having a plurality of pixel levels and binarizing the image data into binary data by an error diffusion method, the error data generated during the binarization processing by the error diffusion method is striped. It is characterized in that filtering is performed so as to suppress the generation of a pattern, the filtered error data is added to newly input image data, and the image data to which the filtered error data is added is binarized. Image processing method.