JPS6351631B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color image reproduction method for performing pseudo halftone reproduction of color images in facsimile and the like. In pixel-type recording such as facsimile, in order to reproduce a color image, it has been common practice to separate and read the three primary color components of an original, control the recording density for each color, and perform overlapping recording. Facsimile recording methods include thermal transfer, in which solid ink on an ink film is transferred onto recording paper by heating; electrophotography, in which a latent image formed on a photoconductive material by light exposure is developed in each color and transferred onto recording paper; Alternatively, there are various methods such as the inkjet method, which records with a nozzle for each color.
The reproducibility of halftones was not always fully satisfactory. Also, depending on the recording method, it is difficult to superimpose colors, and if each color is recorded one after another, the color recorded first will overlap with the color recorded later, so color reproduction may not always be sufficient. It was hot. The inkjet method uses liquid ink, so each color ink mixes on the recording paper.
Mixed colors are relatively easy to reproduce, but when three colors of ink are applied to one pixel in layers, the amount of ink becomes too large, causing problems such as the ink not penetrating into the recording paper. As a countermeasure to these problems, the present inventors have proposed
As proposed in No. 56-153977 (see Japanese Unexamined Patent Publication No. 58-56572), there is a method of changing the color recorded for each dot so that multiple dots can be seen as a mixed color at once. . However, this method treats multiple pixels at once, resulting in poor resolution. As a result, the image quality of areas that require high resolution, such as text areas, deteriorates. In order to eliminate these drawbacks, the present invention has developed a system that separates parts that require high resolution, such as characters, for each block, and parts that require color changes and gradation reproduction, such as color photographic images with shading. separate,
It is characterized by performing separate processing, and its purpose is to
The objective is to reproduce color images without deteriorating character quality. In a normal natural color photograph (hereinafter referred to as a photographic region), the brightness of the hue changes gradually. on the other hand,
In parts such as characters and figures (hereinafter referred to as character areas), the hue and brightness change rapidly. On the other hand, in photographic areas where the hue and brightness change slowly, high resolution is not required. Further, in a text area, although resolution is necessary, it is not necessary to be able to express subtle changes in hue or brightness. This invention utilizes this property to detect the hue and brightness of each block and separate it into a photographic area and a text area, and the former can be expressed with good color and gradation reproducibility.
The latter provides expression with good resolution. Changes in hue and brightness can be easily detected by separating the screen into the three primary colors. This will be explained with reference to FIG. In FIG. 1, the horizontal axis shows the wavelength of the visible light spectrum, and the vertical axis shows the amount of light absorbed by coloring materials such as ink and toner. In other words, yellow (Y) absorbs blue light with a short wavelength, and cyan (C) absorbs red light with a long wavelength.
Magenta (M) absorbs green light of intermediate wavelength.
The image information of each pixel is indicated by giving a density level to each of yellow, magenta, and cyan components. The concentration level of each of these components is
“0” is white (or the absence of that hue component)
The larger the value, the greater the color component of the light reflected from the document surface. This yellow,
The magenta or cyan density level is determined by detecting the reflected light from the document surface through blue, green, and red filters. For example, when passing through a blue filter, contributions from long wavelengths and intermediate wavelengths of the illumination light are removed, and only the amount of reflection of short wavelengths is detected. By subtracting this amount of reflection from the amount of incidence or the amount of reflection from the white portion, the amount of absorption of short wavelength light, that is, the yellow (complementary color of blue) density level of the document surface can be determined. The yellow, magenta, and cyan regions Y, M, and C are shown in FIG. 1 because, when the absorption is large in these spectral regions, the yellow, magenta, and cyan color components become large, respectively. That is, the magnitude of the absorption amount in each spectral region indicated by arrows 1, 2, and 3 represents the density level of yellow, magenta, and cyan, respectively. In FIG. 1, the amount indicated by arrow 7 corresponds to the amount of reflection from the white portion of the document, and in this specification,
Absorption refers to the amount of extra absorption in the colored areas of the document compared to the amount of reflection in the white areas. Since the absorption component corresponding to arrow 4 is absorbed over all wavelengths, it can be regarded as a black component. Further, since the part indicated by arrow 5 has reflection over all wavelengths, it can be regarded as a white part. Therefore, the remaining portion indicated by arrow 6 is considered to represent the hue. 1st
In the illustrated example, among the three colors, the yellow component is the largest, the magenta component is the second largest, and the cyan component is completely absent except for the part that contributes to black. The hue and brightness of the document surface are expressed by the sizes of these three primary color components. A large change in hue or brightness corresponds to a large change in the density level of at least one of these three primary color components. For example, if there is a large change in hue, the density level will decrease significantly for some of the three primary color components and increase for other components, and a large change in brightness from white to black will correspond to a large change in density level for each component. This corresponds to an increase in the concentration levels of both. Therefore, if there is a large change in the density level of at least one component, it can be determined that the area is a character area, and if there is a small change, it can be determined that the area is a photographic area. As a means of determining whether the change in density level is large or small, for black and white images, Japanese Patent Application No. 56-143418 (Japanese Unexamined Patent Publication No. 58-44861)
As stated in the above publication, the screen is divided into blocks each consisting of a plurality of pixels, and the difference between the maximum and minimum density levels within each block is determined.
This can be determined by whether it is large or small. The procedure of one embodiment of this invention will be described below. (1) Divide the screen into blocks consisting of multiple pixels. (2) For each block of the image, the maximum and minimum density levels of one component of the three primary colors of yellow, magenta, and cyan Y _nax , Y _nio , M _nax , M _nio , C _nax ,
Find C _nio . (3) Furthermore, the difference between the maximum and minimum density levels D _Y for each component of yellow, magenta, and cyan
≡Y _nax −Y _nio , D _M ≡M _nax −M _nio , D _C ≡C _nax −
Find C _nio . (4) Compare D _Y , D _M , and D _C with a predetermined standard value P, and all of D _Y , D _M , and D _C
If the block is smaller than the photo area, _DY ,
If at least one of D _M and D _C is greater than or equal to the reference value P, the block is determined to be a character area. (5) For blocks determined to be text areas,
Color reproduction is performed to improve resolution for each pixel as described below. ○…

Claims (1)

[Claims] 1. Divide the screen into blocks consisting of a plurality of pixels, find the maximum and minimum values of the density level for each color component in each block, and calculate the density level of at least one color component. A color that determines a text area when the difference between the maximum and minimum values is greater than a predetermined value, and assigns each pixel in this block a color that is close to the original signal among the colors that can be reproduced for each pixel. Signal processing is performed, and when the difference between the maximum and minimum density levels of each color component is smaller than the predetermined value, it is determined that it is a photographic area, and the hue of the entire block is determined for each pixel in this block. . A color image reproduction method, characterized in that color signal processing is performed to assign a color to each pixel so that the brightness is close to the original signal. 2 Divide the screen into blocks consisting of a plurality of pixels, find the maximum and minimum values of the density level for each color component within each block, and calculate the maximum and minimum values of the density level for at least one color component. If the difference is greater than a predetermined value and the density level is such that each pixel in each block is all white or black, it is determined to be a character area, and the color that can be reproduced for each pixel in each pixel in this block is determined. Among them, color signal processing is performed to assign a color close to the original signal, and in other cases, it is determined to be a photo area, and based on the determination result of the photo area or text area, both outer sides of consecutive 3 or 4 blocks are If the block in the block is a text area, the block in the middle is corrected to be a text area regardless of the above judgment result, and each pixel in the block judged to be a text area is reproduced pixel by pixel. Color signal processing is performed to allocate a color close to the original signal among possible colors, and to each pixel in a block determined to be a photographic area, the color signal processing is performed so that the hue and brightness of the block as a whole are close to the original signal. A color image reproduction method characterized by performing color signal processing to assign a color to each pixel.