Color image information processing of electronic color separation machine

The color image information processing of the electronic color separation machine is accomplished by a dedicated color image processing system, a color computer, based on the above principles and algorithms, combined with the process requirements of color image reproduction and people's artistic expectations. Including color correction, level correction, black version of the calculation, UCR and PCR, and subtle levels of emphasis and other five parts, which are independent of each other, but also mutual constraints.

First, color correction

Color correction refers to the correction of the color deviation in the manuscript and its reproduction process in the electronic color separation according to the purpose of reproduction, ie correcting the color difference and achieving correct color reproduction. The method of color correction depends on the image acquisition system and the color recognition method of the electronic color separation machine and is divided into analog color correction and digital color correction.

1. The necessity of color correction

The print copy process of color manuscripts can usually be expressed as a combination of two processes of color decomposition and color reproduction of image information. The so-called color decomposition of image information refers to a color plate composed of three primary colors of subtractive color such as Y, M, and C, which is intended to be decomposed from an original by image conversion processing, and the principle of color reproduction of image information is to seek processing through image conversion. The color separation images consisting of the three primary colors of subtractive methods such as Y, M, and C are stacked on the same substrate to reproduce the color shadow and tone of the original image. In the actual copying process, due to the unsatisfactory of various parts, the existence of color difference is inevitable, and the color difference mainly comes from three aspects. One is the color shift caused by the originals themselves due to the photographic process and materials and the discoloration (color attenuation) of the color medium, as shown in Figure 3.28. The second one is the color difference in the color decomposition process, mainly the light source and the lens. Errors such as color filters, photomultiplier tubes, and photosensitive materials are shown in Figures 3.29 and 3.30. The third is the color difference during the color reduction process, which is mainly due to paper and ink errors. As shown in Figure 3.31.

The electronic color separation machine is an important part of color reproduction, and its own sampling is a color decomposition process. At the same time, it needs to make up for the color error in the process such as ink recovery. Therefore, the color difference of the color separation signal input to the image information processing system (color computer) is inevitable, as shown in Fig. 3.32. Therefore, it is indispensable to adopt necessary correction means.

Many engineers and technicians found through experiments that the color difference in color image reproduction is not only significant, but also very significant and has a certain regularity. When comparing the black standard with 100% and the white standard with 0%, you can get:

Y version of the basic color Y, G, R its color separation density is only black standard 64 an 80% contrast color B, C, M its color separation density 17 to 36% higher than the white elevation

The basic colors M, B, and R have a color separation density of only 67% and a color of 74%. Contrary colors C, G, and Y. The color separation density is 10 to 36% higher than the white color.

The basic colors C, G, and B have a color separation density of only black mark 59 to 83%. In contrast to R and M, the color separation density is 17 to 20% higher than the white mark. Only the Y color is ideal.

Therefore, color reproduction will inevitably result in the lack of basic color. In contrast, the color is excessive, and color correction is to increase the basic color and reduce the opposite color.

2. Analog color correction

(1) Photographic masks and electric masks

In the photolithography process, the photomask is the best and most effective means of color correction. The so-called photomask refers to a mask made of a photographic method and having an appropriate density range and tone, which is the same as the original or the color separation sheet. After the photosynthesis or copying is performed, the basic color of the color separation sheet and the density and tone of the opposite color are changed to achieve the purpose of color compensation.

From the above, we can see that the principle of the photographic mask is that there is a common color error in the negative film obtained after color separation. The basic color of the negative film is high on the color separation negative film, and higher than the black label, when the copy is drawn into a positive image. ,, it seems that the basic color is insufficient. The opposite color on the color separation negative film on the low density, and lower than the density of white negative film, when copying the positive image, it appears that the opposite color excessive. However, according to the manuscript and its Y, M, C, and BK color separation films, they are appropriately photographed to create suitable masks. After the masks are masked with them, the basic colors and opposite colors of the negative color films can be made. The density ratio changes, that is, increase the basic color and reduce the opposite color. There are many types of masks and their production methods are different. Here we take M as an example to introduce a simple example.

The specific preparation method of the mask is as follows: 1) The original is photographed with R-color filter, developed to a normal density of 1/3 to D; then the positive mask is registered to the color document; 2) with G - The color filter performs color separation exposure, and controls the density of the dichroic sheet in an appropriate range (D); 3) The document is photographed with a B-filter and developed to 1/10 of the normal density, that is, 1/10D, 4 ) The two negative films obtained in steps 2 and 3 can be turned over to obtain the ideal color separation sheet. Other color printing processes are similar. The results shown in Table 3-2 can be obtained by measurement. In the table, the basic color is indicated by X. √ is the opposite.

From the above process, the following mathematical expressions can be summarized:

The above formula is further promoted in the photolithography method to obtain the following equations:

When expressed in a matrix, it can be expressed as:

Equations (3-65) and (3-66) are the mask correction equations and the mask correction matrix. The most attention in the equation is the color correction coefficient Aij(i,j=1,2,3), and the Aij is The process is different and different.

The analog electronic color separation machine performs various processes with an analog voltage signal V that is linearly related to the density of the original document. Similarly, the following equation can be obtained:

Formula (3-67) is an electronic masking equation for the color correction of the electronic color separation machine. It also shows the correct choice of the color correction coefficient Aij for the color correction implementation.

In addition to the selection of three coefficients, there are also color corrections using neutral gray with two correction coefficients, ie:

For neutral gray:

Therefore, the color correction of the electronic color separation machine has two correction methods of two color correction coefficients and three color correction coefficients.

(2) Color correction signal and color correction signal generator

The analog color correction of the electronic color separation machine is realized by the color correction signal obtained by the color correction signal generator and the superimposition of the master signal. Color correction signals are generally divided into two types: one is a basic color correction signal, and the other is a selective color correction signal.

(i) Basic color correction and acquisition of color correction signals

The basic color correction refers to the basic color correction signal made by a specific signal generator for correcting the three-channel basic color and the reverse color. Its purpose is to correct the basic deficiencies caused by Y, M, C color plates in color reproduction or Contrary color excessive color reproduction error.

The basic color correction adopts the electric mask of the analog photographic mask method, and its function expression is:

Its color correction equation is:

From equation (3-68), the basic color correction consists of two parts: the basic color correction and the reverse color correction. The key is to obtain the correct color correction signal or to determine the correct color correction coefficient.

The basic color correction signal generator is a special circuit capable of generating a color correction signal, and has two functions of (1) identifying whether a scanning part is a basic color or a reverse color, and (2) generating a corresponding color correction signal.

It can be seen from Fig. 3.33 that when the scanning part is the basic color, ΔV≠0, and ΔV increases with the color saturation of the scanning area. If the scan is not a basic color then ΔV=0. Similarly, when the scanning position is the opposite color, ΔV≠0, and Δv increases as the color saturation of the scanning area increases, and if the scanning position is not the opposite color, ΔV.

The electrical principle of the color correction signal generator is shown in Figure 3.34. It can be seen from the figure that the basic color correction signal generation is based on the response voltages of V, V and V to six solid color standards, and is obtained by appropriate mathematical transformation. of. When the V, V, V response voltages to the six solid color patches are different, the coefficients of their mathematical transformations are also appropriately changed. The ultimate goal is to generate the correct basic color calibration signals.

Figure 3.35 is a schematic diagram of the basic color correction circuit of the Hell Corporation DC-300. This is a typical basic color correction circuit. It can be seen from the figure that the mathematical expression of the basic color correction signal is:

The mathematical expression of the inverse color correction signal is:

K and K are adjusted by the basic color correction signal strength controller.

When scanning the neutral grey area of ​​an original,

Therefore, the basic color correction signal has no effect on the neutral gray region.

Similarly can be obtained:

From the above relationship, we can find the results of Table 3-3, and from this we can draw the following conclusions:

1) Each color version of the basic color and the reverse color correction signal voltage to identify the role of the recognition of the original document, the scanning part is the basic color, or the opposite color, it is to identify the original scanning area hue sign, and the voltage value is larger, the original color The greater the saturation.

2) Six Flag Signal Voltages As independent signal voltages, it is possible to adjust the correction values ​​of the three colors of the basic color and the opposite color, respectively.

3) The basic color or reversed color correction voltage of each color separation plate has different response values ​​to the three basic colors or opposite colors in the color plate, that is, different identification sensitivity, and one color matching school