The best reproduction of tone (level) (3)

Third, the best tone reproduction of color images

In terms of color images, the reproduction of gradation during reproduction is extremely important, and improper reproduction of gradation is the most important factor affecting the quality of color reproduction. The principle of achieving the best gradation reproduction of a color image is roughly the same as the principle of optimizing the tone reproduction of black and white, but color image copying involves more parameters and variables. Many of the printing characteristics that affect tone reproduction duplicate the gradation reproduction characteristics of a color image as in the case of black-and-white image copying: solid density, number of screen lines, paper properties, dot shape, and ink supply, dot gain, overprint, weight The variables involved in printing processes such as shadows, dot distortion, etc. As long as the printing conditions are stabilized and under control, the tone reproduction of the color image can be optimized by analyzing the printing characteristics.

1. Why do color image replication pursue gray balance? In general, the use of yellow, blue, black and black ink printing grayscale ruler next to the printed matter to study the tone reproduction of the color image and adjust the printing characteristics of the color image. Any factors affecting the neutral reproduction will affect the color image. Graded copy.

Gray balance and tone reproduction are interrelated, but the two are not equivalent concepts. It is important to understand the difference between the two.

The so-called gray balance is the use of a densitometer with a visual filter to measure a color block, the color block produces a defined, neutral density with a certain visual density, even if the measured area is yellow with different coverage Magenta and blue dots do not appear as colored tones.

Tonal reproduction refers to the visual and electro-optical measurement relationships existing between the neutral hair on the manuscript and the neutral gray of the print reproduction.

Gray balance is a major requirement in color image reproduction. Neutral gray scales on the original must reproduce neutral grey when printed with yellow, magenta, and cyan inks. There are two reasons to reproduce a grey ladder ruler as a neutral color. First, there are grey or neutral faces on the manuscript. They must be copied to become neutral with printing. If the substrate is not white paper, the gray balance will not become a serious problem. If there is no other reference point in the observation field, there will be a certain degree of compensation for the neutral deviation of the human eye and brain dialogue or gray. However, when the reference surface in the observation field is a blank paper that surrounds the image, then white and gray deviating from the neutral becomes visually perceptible and harmful defects. The second reason for pursuing gray balance is even more important. When an equal amount of red, green, and blue light is reflected from a surface, the eye sees a neutral color when radiating with a balanced white light and observing a gray gradation scale. At the time, the gray ladder rule reflects or transmits the same amount of red, green and blue violet light into the viewer's eyes (Figure 3-7). Because the printing ink is not pure, when the cyan, magenta, and yellow inks are printed in equal proportions on white paper, the printing surface does not reflect the same amount of red, green, and blue violet light, and thus is not a neutral gray but brown. In order to compensate for this deviation, the neutral ash on the original is rendered neutral gray on the printed image, and the three primary color inks must be printed on the white paper in different proportions. In order to find the correct proportion relationship, a set of color separations can be made for the gray ladder ruler so that the yellow, magenta, and cyan have different dot coverages, and then print copying, if the three inks are combined after printing, from the entire ash ladder Reflecting neutral colors on the ruler, the ratio of yellow, magenta and cyan is correct. If the three primary inks for printing gray scales are neutrally balanced, that is, if the originals and the copied gray scale reflect or transmit the same amount of red, green and blue violet, then the relative point between the printed matter and the original can be guaranteed. The accuracy of color reproduction.

2. A tool for investigating gray balance—the gray ladder ruler. The gray ladder is a useful tool for studying gray balance. One of the advantages of using a gray ladder ruler as a reference is that it allows you to easily and intuitively compare the three-color balance. Another advantage is that by observing the neutrality of the two gray scales, it is much easier to compare the reproduction accuracy than to reproduce the colors by comparing the ink with other colors in the original. Because a certain color difference occurs on the neutral surface, the eye can detect it quickly, and the eye is the most sensitive instrument for detecting gray balance.

A gray scale can be used to detect tone reproduction and can also be used to detect gray balance. In order to detect the accuracy of gradation reproduction, the gradation value of the copied gray scale can be compared with the gradation value of the original; in order to detect the accuracy of the gray balance, only the neutrality of the gray scale to be copied is required. Compare with the original gray scale. There seems to be no relationship between tone reproduction and gray balance. In fact, problems in one area will affect accuracy in the other. When making level adjustments on the color separation machine, it is adjusted individually for each color of gray. If these adjustments do not compensate for the characteristics of each ink and the printer and paper variables are not taken into account, then the gray balance effect will be non-uniform, and parts of the entire gray scale or gray scale will produce color differences, one or more. More colors may lose balance. This would require re-adjusting the gradation value of the three colors after evaluating the color-poor surface on the gray scale.

3. Grey balance evaluation and detection method. Evaluation of the neutrality of the gray ladder ruler from the brightest highlight to the darkest shade, should be observed under standard light sources. Although the eye is a “precision instrument” for evaluating gray balance, it is sometimes due to fatigue or other reasons. You can't rely on your eyes to make precise comparisons. What's worse is that you know that the eye is the most unreliable memory device for color or tone. If the eye first looks at a ruler and then another gray scale, it is impossible for the eye to make an accurate comparison of the neutrality of the two, so the copy and the manuscript should be placed in a viewable position at the same time. Compare. The neutrality of the comparison manuscript and the replica ash ladder can also be determined by densitometer detection. The densitometer is used to measure readings at several points on the copied gray scale with a densitometer through red, green and blue-purple filters, as long as the densitometer is used. The scale is calibrated using a grey scale or an original escalator scale. The readings measured by different filters at the same point on the grey scale should be equal. However, only high quality densitometers with calibration adjustments can give such results.

It should be emphasized here that the densitometer is a printed measurement and control instrument that can be used to compare the density of two colors, but is not suitable for color measurements. Color can be measured more objectively with a colorimeter and a spectrophotometer. A colorimeter is a special densitometer with three wide pass filters whose spectral response is close to the spectral response of the human eye. Any densitometer can become a colorimeter by adding such a filter, and the colorimeter can also be used as a special instrument for measuring color. Precise color measurement uses a spectrophotometer, which decomposes the visually visible colors into specific wavelengths. At present, most printing companies in China still use accurate calibrated densitometers to evaluate and compare colors.

4. Color images and best tone reproduction. The basic principles of color halftoning and black-and-white halftone tone reproduction are the same, and the tonality of the original must also be compressed in order to adapt it to the limited density range of the paper and ink. The key decision for both is to determine which gradation areas of the compressed manuscript have a significant effect on the gradation area of ​​the print reproduction.

Although the basic principle is the same, since copying uses four colors instead of monochrome, color copying involves more variables than black and white copying. Factors such as field density, dot shape, number of screens, paper properties, and printing variables, such as dot gain, ghosting, and stencil, are not different from those used for black-and-white copying in color printing, as long as the printing conditions are met. With the optimization, curves can be drawn based on the printing characteristics data, and then a set of numerically ideal color separation sheets can be analyzed and determined.

The steps for determining the color copy tone curve with the conversion graph are the same as those for black and white tone reproduction. First of all, the existing printing conditions should be optimized, but it is not just a negative film like black and white mesh copying, and it determines the network coverage rate for each level of the ladder. Instead, it has to have four separate credits and certainty. The dot coverage values ​​for the four color separations. As mentioned earlier, in addition to the balance control of the three primary color separation films, for each color, the density of the solid ink must also be optimized and remain stable in the printing, too much ink will stick dark The adjustment causes excessive printing dots to increase, but too little ink will result in lack of saturation, giving the picture a feeling of being diluted with water. As long as printing conditions are optimized, the three colors and balance can be determined experimentally or with the aid of specific tools such as Rff's TRAND table and GATF gray balance table. Under the same printing conditions, the optimal ink density at each level of three-color overprinting, four-color overprinting, and monochrome black can be determined. Based on this information, the conversion map can be plotted, and the color separation sheet can be determined as the most suitable color by converting the image. The tone coverage required by the best tone replication curve.

The first step in drawing a transition map is to determine the dot coverage for each step of a color separation positive or negative. This can be measured directly with a dot coverage meter, or it can be measured with a transmission densitometer and the corresponding dot coverage value can be found in the conversion table. Then, the reflection densities at each rung of the original taper ruler and the print densities of four colors of yellow, magenta, cyan, and black were measured with a reflection densitometer. The above-mentioned numerical values ​​are estimated by using the complementary color filter of each color. Theoretically, the red, green, and blue colors reflected by a neutral plane are equal, so when measuring a neutral plane, the densitometer should show equal readings for red, green, and blue purple filters, respectively. When measuring a neutral surface, a high-quality densitometer can be calibrated precisely so that it can get the same readings when measured with three filters.

The format of the conversion chart is shown in Figure 3-8. It has four quadrants. In the II quadrant, the horizontal axis is uniformly distributed with the dot coverage values, and the vertical axis is shared by the I and II quadrants, indicating the density of the printed gray scale. The horizontal axis of the first quadrant represents the continuous density value of the original scale. In the II quadrant, the relationship between the dot coverage values ​​of the yellow, magenta, qingyang, or yin films, and the integrated density value of the printed grey scales is plotted. This quadrant is called the gray balance quadrant. In the first quadrant, first draw a 45° line, which is the ideal replica of the original. The points of intersection between the high density values ​​obtained on the three- and four-color printed sheets and the highest density values ​​found in the original are then plotted. In Figure 3-8, the highest density from the original is 2.80, and the highest densities from the tri-color and four-color prints are 1.20 and 1.50, respectively (through the visual filter on the print Measured on).

Similar to the method of drawing the best tone curve for a black and white manuscript (see Figure 3-8), the best tone reproduction curve is plotted in the first quadrant of the transition graph. The ideal print copy is a 45° line starting from the origin of the image. Because it is impossible to achieve perfect reproduction, the tone reproduction must be compressed to the highest density within the four-color printing. The best tone reproduction curve has more dark shadows than the highlight zone. A point is drawn halfway between the ideal dark spot and the actual four-color highest density point, and a point with a density of 0.25 is selected on the manuscript scale to express the correct high optical density on the original. Connect these two points with a dotted line, this line expresses the best compression of highlights and shadows. This line also serves as the "guiding line" for drawing the final curve. The best tone reproduction curve will extend from the highlight point upward along this dotted line, starting to compress at a mid-adjustment position with a manuscript density of approximately 1.1. The curve, from this point to the highest four-color density point, is connected with a streamlined curve to form a complete four-color best tone reproduction curve, with the mid-dark tone accounting for approximately 0.90 in the density range. From the same point, the streamlined curve is drawn to the highest density of the three colors to form a complete three-color best tone reproduction curve. The difference in density between three- and four-color copies will be produced by the black version. The angle between the straight part of an optimal tone reproduction curve and the horizontal axis is slightly less than 45°. Quadrant IV is the transfer curve. The third quarter quadrant is used to determine the dot coverage curve that is needed to produce the best gradation duplication density in the negative (or positive) part of the mesh. This curve can be obtained by connecting the quadrants in series. Applying these curves can be divided into The color patches determine the composition of the dot coverage at different control points.

The method of drawing the best tone reproduction curve has been described above, but that means only the normal normal manuscript. In fact, the manuscript accepted by color separation