Image processing method, image processing apparatus and image forming apparatus

Abstract

An image processing method is provided, which converts image data into image data having spatial frequency components, judges whether the absolute value of the coefficient of a predetermined frequency component of the transformed image data is greater than or equal to a predetermined value, and changes the exchange of the image data according to the judgment result. The coefficients of the components are used to inverse transform the image data with the changed coefficients into image data having spatial coordinate components, compare the tone value of the inversely transformed image data with a predetermined value, and convert it into a tone value corresponding to the comparison result. By performing appropriate image processing according to the characteristics of the image, the characteristic portion of the original image can be kept well, and the number of tones of the image can be reduced.

Description

technical field [0001] The present invention relates to an image processing method for performing image processing by changing the spatial frequency components of image data, an image processing device for executing the image processing method, and an image forming device equipped with the image processing device. Background technique [0002] As a method of reducing the number of tones of an image including halftones, for example, as a method of binarizing an image with 256 tones into a two-tone image, it is known to perform binarization by comparing the tonal value of the image with a predetermined value, that is, a threshold value. method, dithering method, and error diffusion method, etc. (Japanese Patent Application Laid-Opens No.2000-299783, Japanese Patent Application Laid-Opens No.2000-299783, No.H06 -189119(1994) and No.2002-10085). [0003] FIG. 1 is a schematic diagram showing an example of a 4×4 dither matrix used when binarizing a 16-tone image in a conventiona...

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Problems solved by technology

[0009] However, in the dithering method, the dithering matrix of the same pattern is used for binarization, so there is a problem that a texture (texture) unique to dithering, that is, a periodic pattern image is generated in the binarized image.

[0010] On the other hand, in the error diffusion method, since the error is diffused according to the same matrix for each pixel, the quantization error in the highlight part with a large tone value is diffused in a chain, and there is a problem of correlation between pixels.

Specifically, since pixels with different hue values ​​are made to have the same hue value, there is a problem of generating an image in which pixels are partially connected.

[0011] Furthermore, in the invention disclosed in Japanese Patent Application Laid-Opens No. 2002-10085, since predetermined halftone data is used, there is the same problem of texture generation as in the error diffusion method or the dither method.

That is, the halftone processing is only performed in the frequency domain by the same method as the above-mentioned conventional method, so the same problem as above occurs.

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