The best color reproduction

The tone reproduction is only an important aspect of the color reproduction quality. In the color image reproduction, the copy quality of the color portion is also an important aspect. How to match the color saturation of a manuscript is a key issue in color reproduction. Since existing paper inks and the like cannot yet completely reproduce the color gamut of a natural color original, it is possible to use a method of compressing various colors in equal amounts. In order to balance the saturation of all colors, it is desirable to perform equalization compression. But sometimes it is necessary to strengthen another part of the color on the basis of losing some part of the color in order to favor a particular kind of color. In order to improve the saturation of certain colors, it is also possible to use 5 or 6 inks for printing.

To achieve a balanced compression of color or a certain color, the use of so-called corrected color reproduction technology in the print image reproduction technology, it is also very important to understand the relationship between color correction and actual production conditions.

First, why must be corrected after achieving gray balance

When discussing the issue of tone reproduction in the front, the problem of gray balance was analyzed in detail. It is the basis for achieving the best reproduction of gradation, however there is often a conceptually ambiguous relationship between the relationship between color correction and gray balance. Because the gray balance adjustment is to compensate for non-ideal absorption and reflection due to impure ink, the gray balance can naturally be regarded as a correction made to the ink impure. So people will ask this question: Since the gray balance is to compensate the impure nature of the three primary inks, why do we need to do further color correction? You can answer this question from the following three points:

In a photographic color separation process or an electronic color separation process, a gray gradation scale is used as a manuscript, and a color separation sheet is used as a manuscript. Then, the degree of neutrality of the printing reproduction is checked according to the printed ash ladder rule. The three exposures are adjusted in the photographic color separation, and all the photosensors have the same output through the basic calibration in the electronic color separation. If the exposure is correct or the photocells of the color separation machine have reached an exact balance, then the three light levels from the neutral scale should be equal. In order to get a neutral gray, just adjust the dots of the three color plates. Coverage. However, unlike gray, various colors in an original will produce unequal output through each photoelectric element. When the pigment is brought into balance in order to achieve neutral gray, the pigment corresponding to the color part of the original will not be automatically reached. balance.

2 When the three-color ink reaches the gray balance, the combination of the three will reflect the same amount of red, green and blue violet light into the viewer's eyes, but most of the optical problems of color printing are caused by secondary color reproduction. , which is caused by two ink sets. The unique absorption and reflection characteristics of individual inks, the superposition of halftone mesh dots, and the pattern created by superimposition make secondary colors more problematic. These problems cannot be solved by gray balance correction alone.

3 Because there is an additive failure and a proportional failure in color reproduction, the color reproduced on the printing press lacks saturation compared with the original. In order to obtain better results, the saturation of some spot colors needs to be improved. This improvement requires correction. The adjustment is complete, but it does not affect the tri-color gray balance.

Second, the ideal color space and the actual color space

At present, the technology for correcting the color reproduction has become very mature. The development of this technology is based on the density color space theory. Although this theory is the basis of the color separation computer design for color separation, it should be monitored and controlled from the image quality. The technical point of view understands it.

In halftone print copying, the print physical quantity is represented by a hue value ranging from 0% to 100%, and an ideal three-dimensional color space can be drawn using these halftone values.

The Cartesian coordinate system is selected and the coordinate values ​​are represented by the corresponding primary color symbols. After drawing these tone values ​​along the Cartesian coordinate system, the color space becomes a cubic diagram (Figure 3-9), and the coordinate zero is white (W). In the upper right corner, the black (K) is opposite the white - the lower left corner of the cube, and the line from white to black represents a continuous ash trap.

With the aid of the coordinate axes, a vector of arbitrary colors can be obtained by means of the selected primary colors. A certain color corresponds to a point in the space, because any point in space can be seen as the vertices of the vector drawn from the origin of the coordinates.

It seems that the concept of color vector is unnecessary because the actual concept of color has nothing to do with the directionality of the vector. However, in the color space, the color vector does have directionality. For example, the secondary color red is synthesized from the magenta and yellow dots. If the red color has a blue taste, it is close to the M coordinate axis. If the red color is orange, it is close to the Y coordinate axis. If the red color is unsaturated, Its vector deviates from any angle on the hand.

If you look at the magenta ink in the ideal cube as shown in Figure 3-9, black, blue, red, and magenta are located at the four vertices of the lower cube plane, respectively. The color of the four vertices in the plane on the cube, the print volume of the magenta ink is zero, so at this further vertex, the tune of the magenta will also be zero. The four vertices in the lower plane of the cube are black, blue, red, and magenta all printed in red ink. For the magenta dichroic, these colors in the lower plane can be called black. Conversely, the color in the upper plane is white for the magenta color separation sheet, ie, on the magenta color separation sheet, cyan, yellow, and green are white. The line connecting the black and white points is the gray value represented by the magenta color separation sheet.

However, if the actual density value of the solid ink is used to replace the percentage of the dot value of the solid ink in the color space, the color space expressed using the dot tone value becomes the color space of the density expression, and then the position of the color will change. If you measure the density of magenta, red, blue, and black through a green filter, in the ideal case, the density values ​​should be equal, because these colors contain the same amount of magenta. In fact, the density values ​​measured are unequal because the cyan and yellow inks are impure and have undesirable absorption. The color white in the upper plane is measured when using a green filter. In the case of yellow, cyan, and green, the ideal density value should be zero, but for the same reason, these densities are also different and larger than the ideal value. If the density values ​​of these colors are arranged in the color space coordinates, the ideal three-dimensional color space will become a distorted hexahedron (as shown in Figure 3-10), but the black on the gray diagonal will remain change.

The purpose of color correction calculation is to correct this distortion of the color space and restore it to a cube shape.

Third, according to the material and process conditions to optimize the actual color space

Whether it is the color decomposition error or the error caused by the color reproduction, the color density spatial cube will be distorted. As can be seen from the distortion of the hexahedron in Figure 3-10, as long as there is a representative color point (three primary colors or their color) moving position, it will bring about changes in the color associated with this color component. This is in line with reality. For example, when reducing the color matching ratio of the blue ink, the color of the blue color itself changes most, and the color green and blue-violet color associated therewith produce different degrees of variation, so the color correction is integral. The basic basis for color reproduction is that if the primary colors (yellow, magenta, cyan) and secondary colors (red, green, and blue-violet) of the original are copied, the other mixed colors can also be substantially obtained. Correctly copy. This is in theory consistent with the principle of color synthesis of three primary colors, and has also been verified in the long-term practice of color reproduction.

In order to obtain the best tone and color reproduction, the printing conditions of the factory must be taken into account when separating colors. Using the standard ink used in this factory and the normal printing conditions, the color scale and gray balance scale are used as the standard for the initial setting of color separation equipment. Color patches include yellow, magenta, cyan, secondary color red, green, blue-violet, and three color black and white color blocks.

The color reproduction and detection control methods of the printing process are also based on the foregoing principles, and the front and back process echoes are one of the core contents of quality control.