JPS6156666B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color reproduction method for reproducing pseudo halftones 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. In addition, with thermal transfer methods and electrophotography, it is difficult to superimpose each color, and if each color is transferred sequentially, the color transferred later overlaps the color transferred earlier, so color reproduction is not always satisfactory. 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, a method can be considered in which the color recorded for each dot is changed so that when a plurality of dots are grouped together, they appear as a mixed color. For example, FIG. 1 shows an example in which the dither method for reproducing halftones with black and white binary values is applied to color reproduction, and the marks ◯, △, and □ each indicate the color to be recorded. For example, ○ records in magenta, △ in cyan, and □ in yellow, and the number indicates the threshold level for recording that color. For example, for the pixel indicated by A, if the magenta color density level is "1" or higher, magenta printing is performed regardless of the density levels of other colors, and if the density level is "1" or lower, it is recorded as white. Similarly, the pixel indicated by B is recorded in cyan if the cyan color signal level is "9" or higher, and the pixel indicated by C is recorded in yellow if the yellow signal level is "3" or higher. However, this method basically cannot represent black. Normally, by mixing dye materials of three primary colors, all visible light is absorbed and black is produced, but in this method, each of the three primary colors is recorded in a different position, so when viewed as a whole, much of the incident light is absorbed. It is a light gray color due to reflection. As a countermeasure to this problem, it is necessary to perform four-color recording by adding black to the three primary colors, but it is difficult to satisfactorily reproduce the original image using a designer method while maintaining the color tone and gradation. This invention was developed in consideration of the above points, and divides the screen into blocks each consisting of multiple pixels.
The feature is that the ratio of white, black, and each of the three primary color components is determined for each block, and the number of pixels to be recorded in each color is determined according to the ratio.The purpose is to reproduce each hue and black well. It is in. Another object of the present invention is to enable high-resolution color reproduction without degrading the quality of characters and the like. This invention will be explained in detail below. FIG. 2 is a diagram for explaining the principle of the color reproduction method of the present invention, in which 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. That is, yellow absorbs blue light with a short wavelength, cyan absorbs red light with a long wavelength, and magenta absorbs green light with an intermediate wavelength. The image information of each pixel is indicated by giving a density level to each of yellow, magenta, and cyan components. The density level of each of these components is such that "0" represents white (or the absence of that hue component),
The larger the value, the larger the color component of the light reflected from the document surface. The yellow, 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. 2 because if the absorption is large in these spectral regions, the yellow, magenta, and cyan color components become large, respectively. That is, the magnitude of absorption in each spectral region indicated by arrows 1, 2, and 3 is yellow,
This represents the density levels of magenta and cyan. In FIG. 2, 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 portion indicated by arrow 5 is reflected over all wavelengths, it can be regarded as a white component. Therefore, the remaining portion indicated by arrow 6 is considered to represent the hue. Second
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. Therefore, regarding the hue components, the amount of coloring of yellow, magenta, and cyan may be adjusted depending on the magnitude of absorption of each color. Considering white and black together, this corresponds to allocating the number of pixels according to the size of each of the white, black, and three color components. As a means to do this, the screen is divided into blocks each consisting of a plurality of pixels, and the processing is performed in accordance with the following procedure. From the density levels of yellow, magenta, and cyan hue components given to each pixel in the block, calculate the average density levels S _Y , S _M , and S _C of each of the yellow, magenta, and cyan components in the block as a whole. demand. Among the average density levels S _Y , S _M , and S _C , the level with the smallest value is defined as the black component _SB . In addition, the maximum level is the saturation value S of the density level.
The value subtracted from _nax (corresponding to the size indicated by arrow 7 in FIG. 2) is the white component _SW . In the example of FIG. 2, the white component S _W corresponds to the size shown by arrow 5, and the black component S _B corresponds to the size shown by arrow 4. The number of pixels N in one block is distributed according to the following equation. ○…

Claims (1)

[Claims] 1. Divide the screen into blocks consisting of a plurality of pixels, calculate the average value of the density levels within the block for each hue, and calculate the average value of the density levels within the block for each hue from the minimum value. The number of black pixels in the block is determined, the number of white pixels in the block is determined from the maximum value, and two of the three colors cyan, magenta, and yellow are assigned to the remaining pixels according to their respective density levels. A color reproduction method characterized by: 2. If the average value of the intra-block density level of a certain hue is greater than the average value of the intra-block density levels of the other two hues by a predetermined value k ₁ or more, all pixels within that block have the same intra-block average density level. A color reproduction method according to claim 1, characterized in that reproduction is performed only in high hue and white. 3. A patent characterized in that when the difference in density level between each hue in all pixels in a block is smaller than a predetermined value _k2 , all pixels in that block are reproduced only in white and black. A color reproduction method according to claim 1. 4. In a color image reproduction method in which each pixel is divided into n sub-pixels and one pixel is configured by the n sub-pixels, each pixel is divided from the minimum value among the density levels of each hue corresponding to each pixel. Determine the number of black subpixels among the subpixels that make up the image, determine the number of white subpixels from the maximum value, and assign two of the three colors cyan, magenta, and yellow to the remaining subpixels at each density. A color reproduction method characterized by assigning colors according to levels. 5 If the average value of the sub-pixel density level of a certain hue is greater than the average value of the sub-pixel density levels of the other two hues by a predetermined value k ₁ or more, all pixels within that sub-pixel are 5. The color reproduction method according to claim 4, wherein the color reproduction method is characterized in that reproduction is performed using only hues and white having a high average density level. 6 The density level difference between each hue is set to a predetermined value in all pixels within the subpixel.
5. The color reproduction method according to claim 4, wherein when k is smaller than ₂ , all pixels within the subpixel are reproduced only in white and black.