Key Technology Analysis of Color Processing in Prepress System

(A) Introduction to the Technology of Color Processing

The reproduction of a color image is a process of image quality that aims to satisfy a certain need. The purpose of color image processing is to perform various transformations on the grayscale and coordinates of the input image to remove various interferences and

distortion.

Color image processing can be divided into two methods, one of which is the use of analog methods, which is to use analogous electronic circuits to process the analog electrical signals of the image, and the other is to use digital methods, that is, to use computer software to image information. The digital signal is processed.

Regardless of the method used to process color images, the three major elements to be processed are unchanged, which is color, level, and clarity. What we want to discuss is the processing of the most important colors.

(1) Basic principles of analog processing

The first method of analog image processing was to adopt a photoengraving method. This was a very complicated process. Many fields and various equipment and raw materials were not standardized, resulting in a non-linear transfer of replication, making it difficult for us to grasp. The law of true change requires revisionists to have high technology.

The analog electronic color separation machine uses the electronic method to simulate the photoengraving process. Its principle layer converts the image information into an analog electrical signal, and according to the requirements of the replication, a corrective power is generated on the basis of the original electrical signal. Signal, through the superposition of two electrical signals to achieve image correction.

The production of color images from the beginning of the scan to the final color separation contains several processing modules.

Scanning is to convert the optical signal in the image field into an analog electrical signal. Only after scanning can we obtain the analog voltage signal we need, and the processing that we will later carry out is based on it. here has

Two things are worth noting: First, after scanning, we changed the continuous optical signal into a discrete electrical signal (analogue). Second, since we are dealing with color images, we need to perform color separation when scanning. That is, we need to obtain at least 3 analog electrical signals for each scanning point. These three analog electrical signals (Vy, Vm, VC) are linearly related to the original density and are the analog voltage signals of the original density.

These three analog voltage signals (which also generate a virtual light mask signal Vus in the actual power division) are the basis of our work, and several of our processing module developments are based on them. Based on the processing requirements we mentioned before, it is generally divided into the following modules: the color processing module, the gradation correction touch module, the black version calculation module, the background removal module, and the subtle emphasis module. The color correction processing module can be expressed as a function:

∑ni=1 KyiYi (Vy,Vm,Vc)

Among them, Kyi in the above equation is a coefficient, where y represents the processing of the yellow channel signal, and also the processing of the product (m) signal and the cyan signal (c). The above function is also called the color correction parameter.

From the above number of parameters, the independent variables that can express each parameter are three analog voltage signals. In other words, if I want to process the electrical signal of the yellow channel to produce a superimposed signal, it is not enough to use only the original yellow signal, and the color processing of any color plate involves three color-coded electrical signals. In other words, these three signals are related.

(2) The acquisition of analog voltage signals

We talked about obtaining three basic analog voltage signals Vy, Vm, Vc. How do these three signals get?

In fact, the scanning part of the electric extension is equivalent to a color density meter with three color filters as red, green and blue color filters, and the three letter wave color chips come out of the optical signal is three analog voltages. The basics (behind the photoelectric conversion), in which the red filter corresponds to the cyan version is to generate the Vc signal, the green filter corresponds to the product version that is Vm, and the blue filter corresponds to the yellow version that is Vy.

The size of the electrical signal (ie, the photoelectric conversion) is linear with the density, rather than a linear relationship with the light intensity. The density and light intensity are in a logarithmic relationship, ie, D=log(1/T), where D Force density, T is light transmittance. It can be seen from the above that if the light transmittance is large, the density is small, whereas if the light transmittance is small, the density is large. For example, if the light transmittance is 0.01, the density is D=2.

3) Digital processing

The principle of digital processing is to quantify the analog electrical signal to a numerical value on the basis of simulation. The advantage of this processing is that the digital data can be input into the computer for processing, that is, image processing is performed by computer software.

The advantage of digital image processing is that analog circuits are no longer used to generate and superimpose correction signals. In the previous analog processing methods, both the color processing and the hierarchical processing involved a large number of electronic circuits. The design of the circuits was complicated and the functions were complicated. Limited, but in the numerical image processing, as long as the software design can carry out a variety of processing, some of the processing (such as filter processing) is the analog circuit is difficult to achieve.

In addition, the processed image can be displayed on the screen, and the effect can be seen and processed to achieve what you see is what you get. There are also numerical figures that can be saved on the outside or on the Internet.

With the development of digitization, more and more electric extensions are being reconstructed. Actually, the signal acquisition and signal output parts of analog extensions are exactly the same as those of digital extensions. However, the signal processing methods are not the same, and one type is analog electronics. Lines, one is using computer software.

(B) Color Correction Method in Image Processing

Color correction refers to the correction of color deviations in the original and copying process according to the requirement of copying. To achieve correct color reproduction, the method can be divided into analog color correction and digital color correction. To clarify how to perform color correction, let's first look at why we need to perform color correction.

The copying process of color manuscripts can be divided into two processes: color decomposition and color reproduction. The color decomposition refers to a color plate composed of three primary colors of subtractive colors such as Y, M, C, and the like. The color reproduction refers to the process of superimposing the color separation images consisting of the three primary colors on the same substrate to reproduce the original image. Color and tone. In the process of copying, due to the unsatisfactory conditions, the presence of color difference is inevitable. There are three major sources of color difference: one is: color shift caused by the photographic process and materials; the second is: color shift in the color decomposition process, mainly light source, lens, color filter, photomultiplier tube and photosensitive material, etc. Error; Third, the color difference in the color reduction process, mainly the error of ink and paper. Because of the color difference in the color reproduction, the color separation density of the basic color of each color plate is not enough, and the color separation density of the opposite color is too large. Therefore, the color correction is to increase the basic color and reduce the opposite color.

1. Analog color correction method:

The method of analog electronic correction is based on the principle of photographic masks. Just turn the density signal into a corresponding electrical signal. We can list the following equation:

V1y=A11Vy+A12Vm+A13Vc

V1m=A21Vy+A22Vm+A23Vc

V1c=A31Vy+A32Vm+A33Vc

Among them, V1y, V1m, V1c are the corrected signals, and Vy, Vm, Vc are the original signals.

We just want to select these 9 coefficients according to the requirements of the process. Different coefficients have different effects on the correction of colors.

1) Basic color correction:

The basic School Bag refers to a basic signal generated by a specific signal generator in order to correct the basic color deficiency and inverse color excess of the three color version.

The premise of basic color correction is to identify the basic color and the opposite color of each color plate first. For example, if we want to divide the yellow plate, the yellow plate has yellow, red, and green colors, and its opposite color is cyan, blue and green. When separating colors, it is first necessary to identify whether the color to be processed is the basic color or the opposite color of the yellow version. If it is the basic color, then the car color correction signal is generated, and if it is the opposite color, the opposite color correction signal is generated.

When the basic color is scanned, the basic color correction signal is not equal to zero, and the higher the saturation of the basic color is, the stronger the signal is. When it is not the basic color, the basic color correction signal is equal to zero, and the opposite color signal is generated at this time. Therefore, only one signal can be generated for each color.

The following is a method of generating a correction signal:

The basic color correction signal is K1Y1(Vy,Vm,Vc)=K1N(Vc-O.95Vm-0.05Vc)

The opposite color correction signal is K2Y2(Vy,Vm,Vc)=K2P(Vy-0.95Vm-0.05Vc)

Among them, K1 and K2 are adjustment coefficients. From the above equation, it can be known that the basic correction signal has no effect on the neutral gray. This is exactly what we expect.

2) Selective color

The basic color correction completes the correction of the basic color and the reverse color of each color plate. However, due to the deviation of the color code itself and the color filter's color filter, a certain color code color correction will occur when the color separation is appropriate. The lack of or excessive color correction of the two color marks requires that we individually calibrate the various solid colors and composite colors according to the requirements of the original and the copying process. This is the selective color correction.

The selective correction signal generator has the following functions: first, it can identify whether the scanning area is the selected color; second, it can recognize the saturation and brightness of the color; thirdly, it can generate the selective color calibration signal according to the saturation and brightness of the color. .

2. Digital color correction

Digital color separation process and simulation is not the same, the simulated color separation is carried out through the color filter, and the digital electrical extension data collected is the original R, G, B tristimulus values, according to three The stimuli are used to calculate the dot. The calculation of the dot is a solution to the equation. This equation is the Neugebauer equation. Our basic correction is done by specifying the parameters of the equation. The parameters are based on the properties of paper and ink. This separation process also includes the basics. School color. This function has been very good in the image software. We only need to determine the properties of materials such as paper and ink, and then fill in a few parameters.

For selective color correction, we only need to select the color to be corrected, and then we can change the selected color as we like. Compared with the simulated color correction method, the method is simple, the correction range is large, and we can look at us. The corrected color effect cannot be performed in simulation mode for special effects processing. With the development of the times, the analog electronic color separation method has been systematically replaced. Most of the old electronic color separation systems have already undergone high-end networking transformation. Digital image processing is simple, intuitive, easy to grasp and other characteristics are emerging from the traditional printing field, is no longer a patent of professional and technical personnel, more and more advertising companies and even families can enjoy the benefits of digital image processing.